diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 4ec3e39facc..bf6d3322176 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -400,7 +400,7 @@ impl<'a> AstValidator<'a> {
         if let Defaultness::Default(def_span) = defaultness {
             let span = self.session.source_map().guess_head_span(span);
             self.err_handler()
-                .struct_span_err(span, "`default` is only allowed on items in `impl` definitions")
+                .struct_span_err(span, "`default` is only allowed on items in trait impls")
                 .span_label(def_span, "`default` because of this")
                 .emit();
         }
diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs
index 636c37142ad..fef6602b9cc 100644
--- a/compiler/rustc_error_codes/src/error_codes.rs
+++ b/compiler/rustc_error_codes/src/error_codes.rs
@@ -111,6 +111,7 @@ E0206: include_str!("./error_codes/E0206.md"),
 E0207: include_str!("./error_codes/E0207.md"),
 E0210: include_str!("./error_codes/E0210.md"),
 E0211: include_str!("./error_codes/E0211.md"),
+E0212: include_str!("./error_codes/E0212.md"),
 E0214: include_str!("./error_codes/E0214.md"),
 E0220: include_str!("./error_codes/E0220.md"),
 E0221: include_str!("./error_codes/E0221.md"),
@@ -503,7 +504,6 @@ E0779: include_str!("./error_codes/E0779.md"),
 //  E0196, // cannot determine a type for this closure
     E0208,
 //  E0209, // builtin traits can only be implemented on structs or enums
-    E0212, // cannot extract an associated type from a higher-ranked trait bound
 //  E0213, // associated types are not accepted in this context
 //  E0215, // angle-bracket notation is not stable with `Fn`
 //  E0216, // parenthetical notation is only stable with `Fn`
diff --git a/compiler/rustc_error_codes/src/error_codes/E0212.md b/compiler/rustc_error_codes/src/error_codes/E0212.md
new file mode 100644
index 00000000000..17465414650
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0212.md
@@ -0,0 +1,35 @@
+Cannot use the associated type of
+a trait with uninferred generic parameters.
+
+Erroneous code example:
+
+```compile_fail,E0212
+pub trait Foo<T> {
+    type A;
+
+    fn get(&self, t: T) -> Self::A;
+}
+
+fn foo2<I : for<'x> Foo<&'x isize>>(
+    field: I::A) {} // error!
+```
+
+In this example, we have to instantiate `'x`, and
+we don't know what lifetime to instantiate it with.
+To fix this, spell out the precise lifetimes involved.
+Example:
+
+```
+pub trait Foo<T> {
+    type A;
+
+    fn get(&self, t: T) -> Self::A;
+}
+
+fn foo3<I : for<'x> Foo<&'x isize>>(
+    x: <I as Foo<&isize>>::A) {} // ok!
+
+
+fn foo4<'a, I : for<'x> Foo<&'x isize>>(
+    x: <I as Foo<&'a isize>>::A) {} // ok!
+```
diff --git a/compiler/rustc_mir/src/lib.rs b/compiler/rustc_mir/src/lib.rs
index 2ed115b1297..e6d822086f5 100644
--- a/compiler/rustc_mir/src/lib.rs
+++ b/compiler/rustc_mir/src/lib.rs
@@ -28,6 +28,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(or_patterns)]
 #![feature(once_cell)]
 #![feature(control_flow_enum)]
+#![feature(str_split_once)]
 #![recursion_limit = "256"]
 
 #[macro_use]
diff --git a/compiler/rustc_mir/src/transform/coverage/debug.rs b/compiler/rustc_mir/src/transform/coverage/debug.rs
index e9528557b33..af81d9af0e2 100644
--- a/compiler/rustc_mir/src/transform/coverage/debug.rs
+++ b/compiler/rustc_mir/src/transform/coverage/debug.rs
@@ -148,40 +148,46 @@ impl DebugOptions {
 
         if let Ok(env_debug_options) = std::env::var(RUSTC_COVERAGE_DEBUG_OPTIONS) {
             for setting_str in env_debug_options.replace(" ", "").replace("-", "_").split(',') {
-                let mut setting = setting_str.splitn(2, '=');
-                match setting.next() {
-                    Some(option) if option == "allow_unused_expressions" => {
-                        allow_unused_expressions = bool_option_val(option, setting.next());
+                let (option, value) = match setting_str.split_once('=') {
+                    None => (setting_str, None),
+                    Some((k, v)) => (k, Some(v)),
+                };
+                match option {
+                    "allow_unused_expressions" => {
+                        allow_unused_expressions = bool_option_val(option, value);
                         debug!(
                             "{} env option `allow_unused_expressions` is set to {}",
                             RUSTC_COVERAGE_DEBUG_OPTIONS, allow_unused_expressions
                         );
                     }
-                    Some(option) if option == "counter_format" => {
-                        if let Some(strval) = setting.next() {
-                            counter_format = counter_format_option_val(strval);
-                            debug!(
-                                "{} env option `counter_format` is set to {:?}",
-                                RUSTC_COVERAGE_DEBUG_OPTIONS, counter_format
-                            );
-                        } else {
-                            bug!(
-                                "`{}` option in environment variable {} requires one or more \
-                                plus-separated choices (a non-empty subset of \
-                                `id+block+operation`)",
-                                option,
-                                RUSTC_COVERAGE_DEBUG_OPTIONS
-                            );
-                        }
+                    "counter_format" => {
+                        match value {
+                            None => {
+                                bug!(
+                                    "`{}` option in environment variable {} requires one or more \
+                                    plus-separated choices (a non-empty subset of \
+                                    `id+block+operation`)",
+                                    option,
+                                    RUSTC_COVERAGE_DEBUG_OPTIONS
+                                );
+                            }
+                            Some(val) => {
+                                counter_format = counter_format_option_val(val);
+                                debug!(
+                                    "{} env option `counter_format` is set to {:?}",
+                                    RUSTC_COVERAGE_DEBUG_OPTIONS, counter_format
+                                );
+                            }
+                        };
                     }
-                    Some("") => {}
-                    Some(invalid) => bug!(
-                        "Unsupported setting `{}` in environment variable {}",
-                        invalid,
-                        RUSTC_COVERAGE_DEBUG_OPTIONS
-                    ),
-                    None => {}
-                }
+                    _ => {
+                        bug!(
+                            "Unsupported setting `{}` in environment variable {}",
+                            option,
+                            RUSTC_COVERAGE_DEBUG_OPTIONS
+                        )
+                    }
+                };
             }
         }
 
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 6a0c98805cb..38b46e19cf2 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -1343,8 +1343,10 @@ fn parse_output_types(
     if !debugging_opts.parse_only {
         for list in matches.opt_strs("emit") {
             for output_type in list.split(',') {
-                let mut parts = output_type.splitn(2, '=');
-                let shorthand = parts.next().unwrap();
+                let (shorthand, path) = match output_type.split_once('=') {
+                    None => (output_type, None),
+                    Some((shorthand, path)) => (shorthand, Some(PathBuf::from(path))),
+                };
                 let output_type = OutputType::from_shorthand(shorthand).unwrap_or_else(|| {
                     early_error(
                         error_format,
@@ -1355,7 +1357,6 @@ fn parse_output_types(
                         ),
                     )
                 });
-                let path = parts.next().map(PathBuf::from);
                 output_types.insert(output_type, path);
             }
         }
@@ -1499,11 +1500,10 @@ fn parse_opt_level(
     let max_c = matches
         .opt_strs_pos("C")
         .into_iter()
-        .flat_map(
-            |(i, s)| {
-                if let Some("opt-level") = s.splitn(2, '=').next() { Some(i) } else { None }
-            },
-        )
+        .flat_map(|(i, s)| {
+            // NB: This can match a string without `=`.
+            if let Some("opt-level") = s.splitn(2, '=').next() { Some(i) } else { None }
+        })
         .max();
     if max_o > max_c {
         OptLevel::Default
@@ -1538,11 +1538,10 @@ fn select_debuginfo(
     let max_c = matches
         .opt_strs_pos("C")
         .into_iter()
-        .flat_map(
-            |(i, s)| {
-                if let Some("debuginfo") = s.splitn(2, '=').next() { Some(i) } else { None }
-            },
-        )
+        .flat_map(|(i, s)| {
+            // NB: This can match a string without `=`.
+            if let Some("debuginfo") = s.splitn(2, '=').next() { Some(i) } else { None }
+        })
         .max();
     if max_g > max_c || cg.tracer == TracerMode::Hardware {
         DebugInfo::Full
@@ -1575,23 +1574,26 @@ fn parse_libs(
         .map(|s| {
             // Parse string of the form "[KIND=]lib[:new_name]",
             // where KIND is one of "dylib", "framework", "static".
-            let mut parts = s.splitn(2, '=');
-            let kind = parts.next().unwrap();
-            let (name, kind) = match (parts.next(), kind) {
-                (None, name) => (name, NativeLibKind::Unspecified),
-                (Some(name), "dylib") => (name, NativeLibKind::Dylib),
-                (Some(name), "framework") => (name, NativeLibKind::Framework),
-                (Some(name), "static") => (name, NativeLibKind::StaticBundle),
-                (Some(name), "static-nobundle") => (name, NativeLibKind::StaticNoBundle),
-                (_, s) => {
-                    early_error(
-                        error_format,
-                        &format!(
-                            "unknown library kind `{}`, expected \
-                             one of dylib, framework, or static",
-                            s
-                        ),
-                    );
+            let (name, kind) = match s.split_once('=') {
+                None => (s, NativeLibKind::Unspecified),
+                Some((kind, name)) => {
+                    let kind = match kind {
+                        "dylib" => NativeLibKind::Dylib,
+                        "framework" => NativeLibKind::Framework,
+                        "static" => NativeLibKind::StaticBundle,
+                        "static-nobundle" => NativeLibKind::StaticNoBundle,
+                        s => {
+                            early_error(
+                                error_format,
+                                &format!(
+                                    "unknown library kind `{}`, expected \
+                                     one of dylib, framework, or static",
+                                    s
+                                ),
+                            );
+                        }
+                    };
+                    (name.to_string(), kind)
                 }
             };
             if kind == NativeLibKind::StaticNoBundle
@@ -1603,10 +1605,11 @@ fn parse_libs(
                      accepted on the nightly compiler",
                 );
             }
-            let mut name_parts = name.splitn(2, ':');
-            let name = name_parts.next().unwrap();
-            let new_name = name_parts.next();
-            (name.to_owned(), new_name.map(|n| n.to_owned()), kind)
+            let (name, new_name) = match name.split_once(':') {
+                None => (name, None),
+                Some((name, new_name)) => (name.to_string(), Some(new_name.to_owned())),
+            };
+            (name, new_name, kind)
         })
         .collect()
 }
@@ -1627,20 +1630,13 @@ pub fn parse_externs(
     let is_unstable_enabled = debugging_opts.unstable_options;
     let mut externs: BTreeMap<String, ExternEntry> = BTreeMap::new();
     for arg in matches.opt_strs("extern") {
-        let mut parts = arg.splitn(2, '=');
-        let name = parts
-            .next()
-            .unwrap_or_else(|| early_error(error_format, "--extern value must not be empty"));
-        let path = parts.next().map(|s| s.to_string());
-
-        let mut name_parts = name.splitn(2, ':');
-        let first_part = name_parts.next();
-        let second_part = name_parts.next();
-        let (options, name) = match (first_part, second_part) {
-            (Some(opts), Some(name)) => (Some(opts), name),
-            (Some(name), None) => (None, name),
-            (None, None) => early_error(error_format, "--extern name must not be empty"),
-            _ => unreachable!(),
+        let (name, path) = match arg.split_once('=') {
+            None => (arg, None),
+            Some((name, path)) => (name.to_string(), Some(path.to_string())),
+        };
+        let (options, name) = match name.split_once(':') {
+            None => (None, name),
+            Some((opts, name)) => (Some(opts), name.to_string()),
         };
 
         let entry = externs.entry(name.to_owned());
@@ -1729,17 +1725,12 @@ fn parse_remap_path_prefix(
     matches
         .opt_strs("remap-path-prefix")
         .into_iter()
-        .map(|remap| {
-            let mut parts = remap.rsplitn(2, '='); // reverse iterator
-            let to = parts.next();
-            let from = parts.next();
-            match (from, to) {
-                (Some(from), Some(to)) => (PathBuf::from(from), PathBuf::from(to)),
-                _ => early_error(
-                    error_format,
-                    "--remap-path-prefix must contain '=' between FROM and TO",
-                ),
-            }
+        .map(|remap| match remap.rsplit_once('=') {
+            None => early_error(
+                error_format,
+                "--remap-path-prefix must contain '=' between FROM and TO",
+            ),
+            Some((from, to)) => (PathBuf::from(from), PathBuf::from(to)),
         })
         .collect()
 }
diff --git a/compiler/rustc_session/src/lib.rs b/compiler/rustc_session/src/lib.rs
index d002f597391..36bf8634c6e 100644
--- a/compiler/rustc_session/src/lib.rs
+++ b/compiler/rustc_session/src/lib.rs
@@ -1,6 +1,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(once_cell)]
 #![feature(or_patterns)]
+#![feature(str_split_once)]
 
 #[macro_use]
 extern crate bitflags;
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 74f0e7c7c90..22372cb0431 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -179,9 +179,10 @@ macro_rules! options {
     {
         let mut op = $defaultfn();
         for option in matches.opt_strs($prefix) {
-            let mut iter = option.splitn(2, '=');
-            let key = iter.next().unwrap();
-            let value = iter.next();
+            let (key, value) = match option.split_once('=') {
+                None => (option, None),
+                Some((k, v)) => (k.to_string(), Some(v)),
+            };
             let option_to_lookup = key.replace("-", "_");
             let mut found = false;
             for &(candidate, setter, type_desc, _) in $stat {
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 06538ad04e7..a91f6acb896 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -461,6 +461,9 @@ symbols! {
         document_private_items,
         dotdot_in_tuple_patterns,
         dotdoteq_in_patterns,
+        dreg,
+        dreg_low16,
+        dreg_low8,
         drop,
         drop_in_place,
         drop_types_in_const,
@@ -545,6 +548,7 @@ symbols! {
         format_args_capture,
         format_args_nl,
         freeze,
+        freg,
         frem_fast,
         from,
         from_desugaring,
@@ -629,6 +633,7 @@ symbols! {
         iter,
         keyword,
         kind,
+        kreg,
         label,
         label_break_value,
         lang,
@@ -654,6 +659,7 @@ symbols! {
         lint_reasons,
         literal,
         llvm_asm,
+        local,
         local_inner_macros,
         log10f32,
         log10f64,
@@ -856,6 +862,9 @@ symbols! {
         pub_restricted,
         pure,
         pushpop_unsafe,
+        qreg,
+        qreg_low4,
+        qreg_low8,
         quad_precision_float,
         question_mark,
         quote,
@@ -877,6 +886,13 @@ symbols! {
         reexport_test_harness_main,
         reference,
         reflect,
+        reg,
+        reg16,
+        reg32,
+        reg64,
+        reg_abcd,
+        reg_byte,
+        reg_thumb,
         register_attr,
         register_tool,
         relaxed_adts,
@@ -1062,6 +1078,8 @@ symbols! {
         spotlight,
         sqrtf32,
         sqrtf64,
+        sreg,
+        sreg_low16,
         sse4a_target_feature,
         stable,
         staged_api,
@@ -1218,6 +1236,8 @@ symbols! {
         volatile_load,
         volatile_set_memory,
         volatile_store,
+        vreg,
+        vreg_low16,
         warn,
         wasm_import_module,
         wasm_target_feature,
@@ -1229,6 +1249,9 @@ symbols! {
         wrapping_mul,
         wrapping_sub,
         write_bytes,
+        xmm_reg,
+        ymm_reg,
+        zmm_reg,
     }
 }
 
diff --git a/compiler/rustc_target/src/asm/mod.rs b/compiler/rustc_target/src/asm/mod.rs
index aff1ff92d31..3c65c84b0de 100644
--- a/compiler/rustc_target/src/asm/mod.rs
+++ b/compiler/rustc_target/src/asm/mod.rs
@@ -20,16 +20,16 @@ macro_rules! def_reg_class {
         }
 
         impl $arch_regclass {
-            pub fn name(self) -> &'static str {
+            pub fn name(self) -> rustc_span::Symbol {
                 match self {
-                    $(Self::$class => stringify!($class),)*
+                    $(Self::$class => rustc_span::symbol::sym::$class,)*
                 }
             }
 
-            pub fn parse(_arch: super::InlineAsmArch, name: &str) -> Result<Self, &'static str> {
+            pub fn parse(_arch: super::InlineAsmArch, name: rustc_span::Symbol) -> Result<Self, &'static str> {
                 match name {
                     $(
-                        stringify!($class) => Ok(Self::$class),
+                        rustc_span::sym::$class => Ok(Self::$class),
                     )*
                     _ => Err("unknown register class"),
                 }
@@ -327,7 +327,7 @@ pub enum InlineAsmRegClass {
 }
 
 impl InlineAsmRegClass {
-    pub fn name(self) -> &'static str {
+    pub fn name(self) -> Symbol {
         match self {
             Self::X86(r) => r.name(),
             Self::Arm(r) => r.name(),
@@ -422,29 +422,22 @@ impl InlineAsmRegClass {
     }
 
     pub fn parse(arch: InlineAsmArch, name: Symbol) -> Result<Self, &'static str> {
-        // FIXME: use direct symbol comparison for register class names
-        name.with(|name| {
-            Ok(match arch {
-                InlineAsmArch::X86 | InlineAsmArch::X86_64 => {
-                    Self::X86(X86InlineAsmRegClass::parse(arch, name)?)
-                }
-                InlineAsmArch::Arm => Self::Arm(ArmInlineAsmRegClass::parse(arch, name)?),
-                InlineAsmArch::AArch64 => {
-                    Self::AArch64(AArch64InlineAsmRegClass::parse(arch, name)?)
-                }
-                InlineAsmArch::RiscV32 | InlineAsmArch::RiscV64 => {
-                    Self::RiscV(RiscVInlineAsmRegClass::parse(arch, name)?)
-                }
-                InlineAsmArch::Nvptx64 => Self::Nvptx(NvptxInlineAsmRegClass::parse(arch, name)?),
-                InlineAsmArch::Hexagon => {
-                    Self::Hexagon(HexagonInlineAsmRegClass::parse(arch, name)?)
-                }
-                InlineAsmArch::Mips | InlineAsmArch::Mips64 => {
-                    Self::Mips(MipsInlineAsmRegClass::parse(arch, name)?)
-                }
-                InlineAsmArch::SpirV => Self::SpirV(SpirVInlineAsmRegClass::parse(arch, name)?),
-                InlineAsmArch::Wasm32 => Self::Wasm(WasmInlineAsmRegClass::parse(arch, name)?),
-            })
+        Ok(match arch {
+            InlineAsmArch::X86 | InlineAsmArch::X86_64 => {
+                Self::X86(X86InlineAsmRegClass::parse(arch, name)?)
+            }
+            InlineAsmArch::Arm => Self::Arm(ArmInlineAsmRegClass::parse(arch, name)?),
+            InlineAsmArch::AArch64 => Self::AArch64(AArch64InlineAsmRegClass::parse(arch, name)?),
+            InlineAsmArch::RiscV32 | InlineAsmArch::RiscV64 => {
+                Self::RiscV(RiscVInlineAsmRegClass::parse(arch, name)?)
+            }
+            InlineAsmArch::Nvptx64 => Self::Nvptx(NvptxInlineAsmRegClass::parse(arch, name)?),
+            InlineAsmArch::Hexagon => Self::Hexagon(HexagonInlineAsmRegClass::parse(arch, name)?),
+            InlineAsmArch::Mips | InlineAsmArch::Mips64 => {
+                Self::Mips(MipsInlineAsmRegClass::parse(arch, name)?)
+            }
+            InlineAsmArch::SpirV => Self::SpirV(SpirVInlineAsmRegClass::parse(arch, name)?),
+            InlineAsmArch::Wasm32 => Self::Wasm(WasmInlineAsmRegClass::parse(arch, name)?),
         })
     }
 
@@ -484,7 +477,7 @@ impl fmt::Display for InlineAsmRegOrRegClass {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             Self::Reg(r) => write!(f, "\"{}\"", r.name()),
-            Self::RegClass(r) => f.write_str(r.name()),
+            Self::RegClass(r) => write!(f, "{}", r.name()),
         }
     }
 }
diff --git a/compiler/rustc_target/src/lib.rs b/compiler/rustc_target/src/lib.rs
index fb747dfcbd3..1ad57582eba 100644
--- a/compiler/rustc_target/src/lib.rs
+++ b/compiler/rustc_target/src/lib.rs
@@ -15,6 +15,7 @@
 #![feature(never_type)]
 #![feature(associated_type_bounds)]
 #![feature(exhaustive_patterns)]
+#![feature(str_split_once)]
 
 #[macro_use]
 extern crate rustc_macros;
diff --git a/compiler/rustc_target/src/spec/apple_base.rs b/compiler/rustc_target/src/spec/apple_base.rs
index e271a6dec40..88422395216 100644
--- a/compiler/rustc_target/src/spec/apple_base.rs
+++ b/compiler/rustc_target/src/spec/apple_base.rs
@@ -54,10 +54,7 @@ fn macos_deployment_target() -> (u32, u32) {
     let deployment_target = env::var("MACOSX_DEPLOYMENT_TARGET").ok();
     let version = deployment_target
         .as_ref()
-        .and_then(|s| {
-            let mut i = s.splitn(2, '.');
-            i.next().and_then(|a| i.next().map(|b| (a, b)))
-        })
+        .and_then(|s| s.split_once('.'))
         .and_then(|(a, b)| a.parse::<u32>().and_then(|a| b.parse::<u32>().map(|b| (a, b))).ok());
 
     version.unwrap_or((10, 7))
diff --git a/compiler/rustc_typeck/src/collect.rs b/compiler/rustc_typeck/src/collect.rs
index 0ff10abb60a..38da1e5ea03 100644
--- a/compiler/rustc_typeck/src/collect.rs
+++ b/compiler/rustc_typeck/src/collect.rs
@@ -359,8 +359,8 @@ impl AstConv<'tcx> for ItemCtxt<'tcx> {
                 self.tcx().sess,
                 span,
                 E0212,
-                "cannot extract an associated type from a higher-ranked trait bound \
-                 in this context"
+                "cannot use the associated type of a trait \
+                 with uninferred generic parameters"
             );
 
             match self.node() {
diff --git a/library/alloc/src/collections/vec_deque/mod.rs b/library/alloc/src/collections/vec_deque/mod.rs
index 85c809e0d18..57807bc5453 100644
--- a/library/alloc/src/collections/vec_deque/mod.rs
+++ b/library/alloc/src/collections/vec_deque/mod.rs
@@ -1469,6 +1469,8 @@ impl<T> VecDeque<T> {
 
     #[inline]
     fn is_contiguous(&self) -> bool {
+        // FIXME: Should we consider `head == 0` to mean
+        // that `self` is contiguous?
         self.tail <= self.head
     }
 
@@ -2198,7 +2200,7 @@ impl<T> VecDeque<T> {
         if self.is_contiguous() {
             let tail = self.tail;
             let head = self.head;
-            return unsafe { &mut self.buffer_as_mut_slice()[tail..head] };
+            return unsafe { RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0 };
         }
 
         let buf = self.buf.ptr();
@@ -2224,7 +2226,13 @@ impl<T> VecDeque<T> {
                 self.tail = 0;
                 self.head = len;
             }
-        } else if free >= self.head {
+        } else if free > self.head {
+            // FIXME: We currently do not consider ....ABCDEFGH
+            // to be contiguous because `head` would be `0` in this
+            // case. While we probably want to change this it
+            // isn't trivial as a few places expect `is_contiguous`
+            // to mean that we can just slice using `buf[tail..head]`.
+
             // there is enough free space to copy the head in one go,
             // this means that we first shift the tail forwards, and then
             // copy the head to the correct position.
@@ -2238,7 +2246,7 @@ impl<T> VecDeque<T> {
                 // ...ABCDEFGH.
 
                 self.tail = self.head;
-                self.head = self.tail + len;
+                self.head = self.wrap_add(self.tail, len);
             }
         } else {
             // free is smaller than both head and tail,
@@ -2278,7 +2286,7 @@ impl<T> VecDeque<T> {
 
         let tail = self.tail;
         let head = self.head;
-        unsafe { &mut self.buffer_as_mut_slice()[tail..head] }
+        unsafe { RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0 }
     }
 
     /// Rotates the double-ended queue `mid` places to the left.
@@ -2839,7 +2847,7 @@ impl<T> From<VecDeque<T>> for Vec<T> {
             let len = other.len();
             let cap = other.cap();
 
-            if other.head != 0 {
+            if other.tail != 0 {
                 ptr::copy(buf.add(other.tail), buf, len);
             }
             Vec::from_raw_parts(buf, len, cap)
diff --git a/library/alloc/src/collections/vec_deque/tests.rs b/library/alloc/src/collections/vec_deque/tests.rs
index d74f91c752c..21f52af056b 100644
--- a/library/alloc/src/collections/vec_deque/tests.rs
+++ b/library/alloc/src/collections/vec_deque/tests.rs
@@ -210,6 +210,20 @@ fn make_contiguous_small_free() {
     );
 }
 
+#[test]
+fn make_contiguous_head_to_end() {
+    let mut dq = VecDeque::with_capacity(3);
+    dq.push_front('B');
+    dq.push_front('A');
+    dq.push_back('C');
+    dq.make_contiguous();
+    let expected_tail = 0;
+    let expected_head = 3;
+    assert_eq!(expected_tail, dq.tail);
+    assert_eq!(expected_head, dq.head);
+    assert_eq!((&['A', 'B', 'C'] as &[_], &[] as &[_]), dq.as_slices());
+}
+
 #[test]
 fn test_remove() {
     // This test checks that every single combination of tail position, length, and
diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index ce216e5336e..27b32b69502 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -1413,7 +1413,7 @@ impl String {
         self.len() == 0
     }
 
-    /// Splits the string into two at the given index.
+    /// Splits the string into two at the given byte index.
     ///
     /// Returns a newly allocated `String`. `self` contains bytes `[0, at)`, and
     /// the returned `String` contains bytes `[at, len)`. `at` must be on the
diff --git a/library/core/src/cmp.rs b/library/core/src/cmp.rs
index 47ae1a64190..f752472c3ba 100644
--- a/library/core/src/cmp.rs
+++ b/library/core/src/cmp.rs
@@ -325,6 +325,120 @@ pub enum Ordering {
 }
 
 impl Ordering {
+    /// Returns `true` if the ordering is the `Equal` variant.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ordering_helpers)]
+    /// use std::cmp::Ordering;
+    ///
+    /// assert_eq!(Ordering::Less.is_eq(), false);
+    /// assert_eq!(Ordering::Equal.is_eq(), true);
+    /// assert_eq!(Ordering::Greater.is_eq(), false);
+    /// ```
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "ordering_helpers", issue = "79885")]
+    pub const fn is_eq(self) -> bool {
+        matches!(self, Equal)
+    }
+
+    /// Returns `true` if the ordering is not the `Equal` variant.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ordering_helpers)]
+    /// use std::cmp::Ordering;
+    ///
+    /// assert_eq!(Ordering::Less.is_ne(), true);
+    /// assert_eq!(Ordering::Equal.is_ne(), false);
+    /// assert_eq!(Ordering::Greater.is_ne(), true);
+    /// ```
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "ordering_helpers", issue = "79885")]
+    pub const fn is_ne(self) -> bool {
+        !matches!(self, Equal)
+    }
+
+    /// Returns `true` if the ordering is the `Less` variant.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ordering_helpers)]
+    /// use std::cmp::Ordering;
+    ///
+    /// assert_eq!(Ordering::Less.is_lt(), true);
+    /// assert_eq!(Ordering::Equal.is_lt(), false);
+    /// assert_eq!(Ordering::Greater.is_lt(), false);
+    /// ```
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "ordering_helpers", issue = "79885")]
+    pub const fn is_lt(self) -> bool {
+        matches!(self, Less)
+    }
+
+    /// Returns `true` if the ordering is the `Greater` variant.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ordering_helpers)]
+    /// use std::cmp::Ordering;
+    ///
+    /// assert_eq!(Ordering::Less.is_gt(), false);
+    /// assert_eq!(Ordering::Equal.is_gt(), false);
+    /// assert_eq!(Ordering::Greater.is_gt(), true);
+    /// ```
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "ordering_helpers", issue = "79885")]
+    pub const fn is_gt(self) -> bool {
+        matches!(self, Greater)
+    }
+
+    /// Returns `true` if the ordering is either the `Less` or `Equal` variant.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ordering_helpers)]
+    /// use std::cmp::Ordering;
+    ///
+    /// assert_eq!(Ordering::Less.is_le(), true);
+    /// assert_eq!(Ordering::Equal.is_le(), true);
+    /// assert_eq!(Ordering::Greater.is_le(), false);
+    /// ```
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "ordering_helpers", issue = "79885")]
+    pub const fn is_le(self) -> bool {
+        !matches!(self, Greater)
+    }
+
+    /// Returns `true` if the ordering is either the `Greater` or `Equal` variant.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ordering_helpers)]
+    /// use std::cmp::Ordering;
+    ///
+    /// assert_eq!(Ordering::Less.is_ge(), false);
+    /// assert_eq!(Ordering::Equal.is_ge(), true);
+    /// assert_eq!(Ordering::Greater.is_ge(), true);
+    /// ```
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "ordering_helpers", issue = "79885")]
+    pub const fn is_ge(self) -> bool {
+        !matches!(self, Less)
+    }
+
     /// Reverses the `Ordering`.
     ///
     /// * `Less` becomes `Greater`.
diff --git a/library/core/src/num/int_macros.rs b/library/core/src/num/int_macros.rs
index 33eefe5d265..4fa48427ec6 100644
--- a/library/core/src/num/int_macros.rs
+++ b/library/core/src/num/int_macros.rs
@@ -1211,7 +1211,7 @@ any high-order bits of `rhs` that would cause the shift to exceed the bitwidth o
 
 Note that this is *not* the same as a rotate-left; the RHS of a wrapping shift-left is restricted to
 the range of the type, rather than the bits shifted out of the LHS being returned to the other end.
-The primitive integer types all implement a `[`rotate_left`](#method.rotate_left) function,
+The primitive integer types all implement a [`rotate_left`](#method.rotate_left) function,
 which may be what you want instead.
 
 # Examples
diff --git a/library/std/src/f32.rs b/library/std/src/f32.rs
index 09a9b184e3a..48c77bd13e0 100644
--- a/library/std/src/f32.rs
+++ b/library/std/src/f32.rs
@@ -206,8 +206,10 @@ impl f32 {
     /// Fused multiply-add. Computes `(self * a) + b` with only one rounding
     /// error, yielding a more accurate result than an unfused multiply-add.
     ///
-    /// Using `mul_add` can be more performant than an unfused multiply-add if
-    /// the target architecture has a dedicated `fma` CPU instruction.
+    /// Using `mul_add` *may* be more performant than an unfused multiply-add if
+    /// the target architecture has a dedicated `fma` CPU instruction. However,
+    /// this is not always true, and will be heavily dependant on designing
+    /// algorithms with specific target hardware in mind.
     ///
     /// # Examples
     ///
diff --git a/library/std/src/f64.rs b/library/std/src/f64.rs
index 64bb7cd9fd1..e823ac43fdd 100644
--- a/library/std/src/f64.rs
+++ b/library/std/src/f64.rs
@@ -206,8 +206,10 @@ impl f64 {
     /// Fused multiply-add. Computes `(self * a) + b` with only one rounding
     /// error, yielding a more accurate result than an unfused multiply-add.
     ///
-    /// Using `mul_add` can be more performant than an unfused multiply-add if
-    /// the target architecture has a dedicated `fma` CPU instruction.
+    /// Using `mul_add` *may* be more performant than an unfused multiply-add if
+    /// the target architecture has a dedicated `fma` CPU instruction. However,
+    /// this is not always true, and will be heavily dependant on designing
+    /// algorithms with specific target hardware in mind.
     ///
     /// # Examples
     ///
diff --git a/library/std/src/io/stdio.rs b/library/std/src/io/stdio.rs
index 6ea7704d422..1160011f352 100644
--- a/library/std/src/io/stdio.rs
+++ b/library/std/src/io/stdio.rs
@@ -9,6 +9,7 @@ use crate::cell::{Cell, RefCell};
 use crate::fmt;
 use crate::io::{self, BufReader, Initializer, IoSlice, IoSliceMut, LineWriter};
 use crate::lazy::SyncOnceCell;
+use crate::pin::Pin;
 use crate::sync::atomic::{AtomicBool, Ordering};
 use crate::sync::{Arc, Mutex, MutexGuard};
 use crate::sys::stdio;
@@ -490,7 +491,7 @@ pub struct Stdout {
     // FIXME: this should be LineWriter or BufWriter depending on the state of
     //        stdout (tty or not). Note that if this is not line buffered it
     //        should also flush-on-panic or some form of flush-on-abort.
-    inner: &'static ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>,
+    inner: Pin<&'static ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>>,
 }
 
 /// A locked reference to the `Stdout` handle.
@@ -550,25 +551,29 @@ pub struct StdoutLock<'a> {
 pub fn stdout() -> Stdout {
     static INSTANCE: SyncOnceCell<ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>> =
         SyncOnceCell::new();
+
+    fn cleanup() {
+        if let Some(instance) = INSTANCE.get() {
+            // Flush the data and disable buffering during shutdown
+            // by replacing the line writer by one with zero
+            // buffering capacity.
+            // We use try_lock() instead of lock(), because someone
+            // might have leaked a StdoutLock, which would
+            // otherwise cause a deadlock here.
+            if let Some(lock) = Pin::static_ref(instance).try_lock() {
+                *lock.borrow_mut() = LineWriter::with_capacity(0, stdout_raw());
+            }
+        }
+    }
+
     Stdout {
-        inner: INSTANCE.get_or_init(|| unsafe {
-            let _ = sys_common::at_exit(|| {
-                if let Some(instance) = INSTANCE.get() {
-                    // Flush the data and disable buffering during shutdown
-                    // by replacing the line writer by one with zero
-                    // buffering capacity.
-                    // We use try_lock() instead of lock(), because someone
-                    // might have leaked a StdoutLock, which would
-                    // otherwise cause a deadlock here.
-                    if let Some(lock) = instance.try_lock() {
-                        *lock.borrow_mut() = LineWriter::with_capacity(0, stdout_raw());
-                    }
-                }
-            });
-            let r = ReentrantMutex::new(RefCell::new(LineWriter::new(stdout_raw())));
-            r.init();
-            r
-        }),
+        inner: Pin::static_ref(&INSTANCE).get_or_init_pin(
+            || unsafe {
+                let _ = sys_common::at_exit(cleanup);
+                ReentrantMutex::new(RefCell::new(LineWriter::new(stdout_raw())))
+            },
+            |mutex| unsafe { mutex.init() },
+        ),
     }
 }
 
@@ -700,7 +705,7 @@ impl fmt::Debug for StdoutLock<'_> {
 /// an error.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Stderr {
-    inner: &'static ReentrantMutex<RefCell<StderrRaw>>,
+    inner: Pin<&'static ReentrantMutex<RefCell<StderrRaw>>>,
 }
 
 /// A locked reference to the `Stderr` handle.
@@ -756,21 +761,16 @@ pub struct StderrLock<'a> {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stderr() -> Stderr {
-    // Note that unlike `stdout()` we don't use `Lazy` here which registers a
-    // destructor. Stderr is not buffered nor does the `stderr_raw` type consume
-    // any owned resources, so there's no need to run any destructors at some
-    // point in the future.
-    //
-    // This has the added benefit of allowing `stderr` to be usable during
-    // process shutdown as well!
+    // Note that unlike `stdout()` we don't use `at_exit` here to register a
+    // destructor. Stderr is not buffered , so there's no need to run a
+    // destructor for flushing the buffer
     static INSTANCE: SyncOnceCell<ReentrantMutex<RefCell<StderrRaw>>> = SyncOnceCell::new();
 
     Stderr {
-        inner: INSTANCE.get_or_init(|| unsafe {
-            let r = ReentrantMutex::new(RefCell::new(stderr_raw()));
-            r.init();
-            r
-        }),
+        inner: Pin::static_ref(&INSTANCE).get_or_init_pin(
+            || unsafe { ReentrantMutex::new(RefCell::new(stderr_raw())) },
+            |mutex| unsafe { mutex.init() },
+        ),
     }
 }
 
diff --git a/library/std/src/lazy.rs b/library/std/src/lazy.rs
index e0095e64faf..68f57958bb2 100644
--- a/library/std/src/lazy.rs
+++ b/library/std/src/lazy.rs
@@ -10,6 +10,7 @@ use crate::{
     mem::MaybeUninit,
     ops::{Deref, Drop},
     panic::{RefUnwindSafe, UnwindSafe},
+    pin::Pin,
     sync::Once,
 };
 
@@ -297,6 +298,60 @@ impl<T> SyncOnceCell<T> {
         Ok(unsafe { self.get_unchecked() })
     }
 
+    /// Internal-only API that gets the contents of the cell, initializing it
+    /// in two steps with `f` and `g` if the cell was empty.
+    ///
+    /// `f` is called to construct the value, which is then moved into the cell
+    /// and given as a (pinned) mutable reference to `g` to finish
+    /// initialization.
+    ///
+    /// This allows `g` to inspect an manipulate the value after it has been
+    /// moved into its final place in the cell, but before the cell is
+    /// considered initialized.
+    ///
+    /// # Panics
+    ///
+    /// If `f` or `g` panics, the panic is propagated to the caller, and the
+    /// cell remains uninitialized.
+    ///
+    /// With the current implementation, if `g` panics, the value from `f` will
+    /// not be dropped. This should probably be fixed if this is ever used for
+    /// a type where this matters.
+    ///
+    /// It is an error to reentrantly initialize the cell from `f`. The exact
+    /// outcome is unspecified. Current implementation deadlocks, but this may
+    /// be changed to a panic in the future.
+    pub(crate) fn get_or_init_pin<F, G>(self: Pin<&Self>, f: F, g: G) -> Pin<&T>
+    where
+        F: FnOnce() -> T,
+        G: FnOnce(Pin<&mut T>),
+    {
+        if let Some(value) = self.get_ref().get() {
+            // SAFETY: The inner value was already initialized, and will not be
+            // moved anymore.
+            return unsafe { Pin::new_unchecked(value) };
+        }
+
+        let slot = &self.value;
+
+        // Ignore poisoning from other threads
+        // If another thread panics, then we'll be able to run our closure
+        self.once.call_once_force(|_| {
+            let value = f();
+            // SAFETY: We use the Once (self.once) to guarantee unique access
+            // to the UnsafeCell (slot).
+            let value: &mut T = unsafe { (&mut *slot.get()).write(value) };
+            // SAFETY: The value has been written to its final place in
+            // self.value. We do not to move it anymore, which we promise here
+            // with a Pin<&mut T>.
+            g(unsafe { Pin::new_unchecked(value) });
+        });
+
+        // SAFETY: The inner value has been initialized, and will not be moved
+        // anymore.
+        unsafe { Pin::new_unchecked(self.get_ref().get_unchecked()) }
+    }
+
     /// Consumes the `SyncOnceCell`, returning the wrapped value. Returns
     /// `None` if the cell was empty.
     ///
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 6c240cb4c3e..fc48997f1bb 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -266,6 +266,7 @@
 #![feature(format_args_nl)]
 #![feature(gen_future)]
 #![feature(generator_trait)]
+#![feature(get_mut_unchecked)]
 #![feature(global_asm)]
 #![feature(hashmap_internals)]
 #![feature(int_error_internals)]
@@ -293,6 +294,7 @@
 #![feature(panic_info_message)]
 #![feature(panic_internals)]
 #![feature(panic_unwind)]
+#![feature(pin_static_ref)]
 #![feature(prelude_import)]
 #![feature(ptr_internals)]
 #![feature(raw)]
@@ -312,6 +314,7 @@
 #![feature(stdsimd)]
 #![feature(stmt_expr_attributes)]
 #![feature(str_internals)]
+#![feature(str_split_once)]
 #![feature(test)]
 #![feature(thread_local)]
 #![feature(thread_local_internals)]
diff --git a/library/std/src/sys/unix/kernel_copy/tests.rs b/library/std/src/sys/unix/kernel_copy/tests.rs
index 3937a1ffa38..77369cdd35f 100644
--- a/library/std/src/sys/unix/kernel_copy/tests.rs
+++ b/library/std/src/sys/unix/kernel_copy/tests.rs
@@ -1,18 +1,18 @@
-use crate::env::temp_dir;
 use crate::fs::OpenOptions;
 use crate::io;
 use crate::io::Result;
 use crate::io::SeekFrom;
 use crate::io::{BufRead, Read, Seek, Write};
 use crate::os::unix::io::AsRawFd;
+use crate::sys_common::io::test::tmpdir;
 
 #[test]
 fn copy_specialization() -> Result<()> {
     use crate::io::{BufReader, BufWriter};
 
-    let path = crate::env::temp_dir();
-    let source_path = path.join("copy-spec.source");
-    let sink_path = path.join("copy-spec.sink");
+    let tmp_path = tmpdir();
+    let source_path = tmp_path.join("copy-spec.source");
+    let sink_path = tmp_path.join("copy-spec.sink");
 
     let result: Result<()> = try {
         let mut source = crate::fs::OpenOptions::new()
@@ -68,7 +68,8 @@ fn copy_specialization() -> Result<()> {
 #[bench]
 fn bench_file_to_file_copy(b: &mut test::Bencher) {
     const BYTES: usize = 128 * 1024;
-    let src_path = temp_dir().join("file-copy-bench-src");
+    let temp_path = tmpdir();
+    let src_path = temp_path.join("file-copy-bench-src");
     let mut src = crate::fs::OpenOptions::new()
         .create(true)
         .truncate(true)
@@ -78,7 +79,7 @@ fn bench_file_to_file_copy(b: &mut test::Bencher) {
         .unwrap();
     src.write(&vec![0u8; BYTES]).unwrap();
 
-    let sink_path = temp_dir().join("file-copy-bench-sink");
+    let sink_path = temp_path.join("file-copy-bench-sink");
     let mut sink = crate::fs::OpenOptions::new()
         .create(true)
         .truncate(true)
@@ -97,7 +98,8 @@ fn bench_file_to_file_copy(b: &mut test::Bencher) {
 #[bench]
 fn bench_file_to_socket_copy(b: &mut test::Bencher) {
     const BYTES: usize = 128 * 1024;
-    let src_path = temp_dir().join("pipe-copy-bench-src");
+    let temp_path = tmpdir();
+    let src_path = temp_path.join("pipe-copy-bench-src");
     let mut src = OpenOptions::new()
         .create(true)
         .truncate(true)
@@ -128,7 +130,8 @@ fn bench_file_to_socket_copy(b: &mut test::Bencher) {
 #[bench]
 fn bench_file_to_uds_copy(b: &mut test::Bencher) {
     const BYTES: usize = 128 * 1024;
-    let src_path = temp_dir().join("uds-copy-bench-src");
+    let temp_path = tmpdir();
+    let src_path = temp_path.join("uds-copy-bench-src");
     let mut src = OpenOptions::new()
         .create(true)
         .truncate(true)
diff --git a/library/std/src/sys/windows/thread_local_key.rs b/library/std/src/sys/windows/thread_local_key.rs
index fb2bb0613ee..065365e5572 100644
--- a/library/std/src/sys/windows/thread_local_key.rs
+++ b/library/std/src/sys/windows/thread_local_key.rs
@@ -1,4 +1,4 @@
-use crate::mem;
+use crate::mem::ManuallyDrop;
 use crate::ptr;
 use crate::sync::atomic::AtomicPtr;
 use crate::sync::atomic::Ordering::SeqCst;
@@ -111,16 +111,13 @@ struct Node {
 }
 
 unsafe fn register_dtor(key: Key, dtor: Dtor) {
-    let mut node = Box::new(Node { key, dtor, next: ptr::null_mut() });
+    let mut node = ManuallyDrop::new(Box::new(Node { key, dtor, next: ptr::null_mut() }));
 
     let mut head = DTORS.load(SeqCst);
     loop {
         node.next = head;
-        match DTORS.compare_exchange(head, &mut *node, SeqCst, SeqCst) {
-            Ok(_) => {
-                mem::forget(node);
-                return;
-            }
+        match DTORS.compare_exchange(head, &mut **node, SeqCst, SeqCst) {
+            Ok(_) => return, // nothing to drop, we successfully added the node to the list
             Err(cur) => head = cur,
         }
     }
diff --git a/library/std/src/sys_common/net.rs b/library/std/src/sys_common/net.rs
index 48ba4ddfc0b..38ba0d2fbdb 100644
--- a/library/std/src/sys_common/net.rs
+++ b/library/std/src/sys_common/net.rs
@@ -177,11 +177,8 @@ impl TryFrom<&str> for LookupHost {
         }
 
         // split the string by ':' and convert the second part to u16
-        let mut parts_iter = s.rsplitn(2, ':');
-        let port_str = try_opt!(parts_iter.next(), "invalid socket address");
-        let host = try_opt!(parts_iter.next(), "invalid socket address");
+        let (host, port_str) = try_opt!(s.rsplit_once(':'), "invalid socket address");
         let port: u16 = try_opt!(port_str.parse().ok(), "invalid port value");
-
         (host, port).try_into()
     }
 }
diff --git a/library/std/src/sys_common/remutex.rs b/library/std/src/sys_common/remutex.rs
index 162eab2388d..475bfca9b6d 100644
--- a/library/std/src/sys_common/remutex.rs
+++ b/library/std/src/sys_common/remutex.rs
@@ -1,10 +1,10 @@
 #[cfg(all(test, not(target_os = "emscripten")))]
 mod tests;
 
-use crate::fmt;
-use crate::marker;
+use crate::marker::PhantomPinned;
 use crate::ops::Deref;
 use crate::panic::{RefUnwindSafe, UnwindSafe};
+use crate::pin::Pin;
 use crate::sys::mutex as sys;
 
 /// A re-entrant mutual exclusion
@@ -15,6 +15,7 @@ use crate::sys::mutex as sys;
 pub struct ReentrantMutex<T> {
     inner: sys::ReentrantMutex,
     data: T,
+    _pinned: PhantomPinned,
 }
 
 unsafe impl<T: Send> Send for ReentrantMutex<T> {}
@@ -37,10 +38,10 @@ impl<T> RefUnwindSafe for ReentrantMutex<T> {}
 /// guarded data.
 #[must_use = "if unused the ReentrantMutex will immediately unlock"]
 pub struct ReentrantMutexGuard<'a, T: 'a> {
-    lock: &'a ReentrantMutex<T>,
+    lock: Pin<&'a ReentrantMutex<T>>,
 }
 
-impl<T> !marker::Send for ReentrantMutexGuard<'_, T> {}
+impl<T> !Send for ReentrantMutexGuard<'_, T> {}
 
 impl<T> ReentrantMutex<T> {
     /// Creates a new reentrant mutex in an unlocked state.
@@ -51,7 +52,11 @@ impl<T> ReentrantMutex<T> {
     /// once this mutex is in its final resting place, and only then are the
     /// lock/unlock methods safe.
     pub const unsafe fn new(t: T) -> ReentrantMutex<T> {
-        ReentrantMutex { inner: sys::ReentrantMutex::uninitialized(), data: t }
+        ReentrantMutex {
+            inner: sys::ReentrantMutex::uninitialized(),
+            data: t,
+            _pinned: PhantomPinned,
+        }
     }
 
     /// Initializes this mutex so it's ready for use.
@@ -60,8 +65,8 @@ impl<T> ReentrantMutex<T> {
     ///
     /// Unsafe to call more than once, and must be called after this will no
     /// longer move in memory.
-    pub unsafe fn init(&self) {
-        self.inner.init();
+    pub unsafe fn init(self: Pin<&mut Self>) {
+        self.get_unchecked_mut().inner.init()
     }
 
     /// Acquires a mutex, blocking the current thread until it is able to do so.
@@ -76,9 +81,9 @@ impl<T> ReentrantMutex<T> {
     /// If another user of this mutex panicked while holding the mutex, then
     /// this call will return failure if the mutex would otherwise be
     /// acquired.
-    pub fn lock(&self) -> ReentrantMutexGuard<'_, T> {
+    pub fn lock(self: Pin<&Self>) -> ReentrantMutexGuard<'_, T> {
         unsafe { self.inner.lock() }
-        ReentrantMutexGuard::new(&self)
+        ReentrantMutexGuard { lock: self }
     }
 
     /// Attempts to acquire this lock.
@@ -93,8 +98,12 @@ impl<T> ReentrantMutex<T> {
     /// If another user of this mutex panicked while holding the mutex, then
     /// this call will return failure if the mutex would otherwise be
     /// acquired.
-    pub fn try_lock(&self) -> Option<ReentrantMutexGuard<'_, T>> {
-        if unsafe { self.inner.try_lock() } { Some(ReentrantMutexGuard::new(&self)) } else { None }
+    pub fn try_lock(self: Pin<&Self>) -> Option<ReentrantMutexGuard<'_, T>> {
+        if unsafe { self.inner.try_lock() } {
+            Some(ReentrantMutexGuard { lock: self })
+        } else {
+            None
+        }
     }
 }
 
@@ -107,30 +116,6 @@ impl<T> Drop for ReentrantMutex<T> {
     }
 }
 
-impl<T: fmt::Debug + 'static> fmt::Debug for ReentrantMutex<T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self.try_lock() {
-            Some(guard) => f.debug_struct("ReentrantMutex").field("data", &*guard).finish(),
-            None => {
-                struct LockedPlaceholder;
-                impl fmt::Debug for LockedPlaceholder {
-                    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                        f.write_str("<locked>")
-                    }
-                }
-
-                f.debug_struct("ReentrantMutex").field("data", &LockedPlaceholder).finish()
-            }
-        }
-    }
-}
-
-impl<'mutex, T> ReentrantMutexGuard<'mutex, T> {
-    fn new(lock: &'mutex ReentrantMutex<T>) -> ReentrantMutexGuard<'mutex, T> {
-        ReentrantMutexGuard { lock }
-    }
-}
-
 impl<T> Deref for ReentrantMutexGuard<'_, T> {
     type Target = T;
 
diff --git a/library/std/src/sys_common/remutex/tests.rs b/library/std/src/sys_common/remutex/tests.rs
index 9c686e579d7..88453ded2f9 100644
--- a/library/std/src/sys_common/remutex/tests.rs
+++ b/library/std/src/sys_common/remutex/tests.rs
@@ -1,4 +1,6 @@
+use crate::boxed::Box;
 use crate::cell::RefCell;
+use crate::pin::Pin;
 use crate::sync::Arc;
 use crate::sys_common::remutex::{ReentrantMutex, ReentrantMutexGuard};
 use crate::thread;
@@ -6,10 +8,11 @@ use crate::thread;
 #[test]
 fn smoke() {
     let m = unsafe {
-        let m = ReentrantMutex::new(());
-        m.init();
+        let mut m = Box::pin(ReentrantMutex::new(()));
+        m.as_mut().init();
         m
     };
+    let m = m.as_ref();
     {
         let a = m.lock();
         {
@@ -27,18 +30,19 @@ fn smoke() {
 #[test]
 fn is_mutex() {
     let m = unsafe {
-        let m = Arc::new(ReentrantMutex::new(RefCell::new(0)));
-        m.init();
-        m
+        // FIXME: Simplify this if Arc gets a Arc::get_pin_mut.
+        let mut m = Arc::new(ReentrantMutex::new(RefCell::new(0)));
+        Pin::new_unchecked(Arc::get_mut_unchecked(&mut m)).init();
+        Pin::new_unchecked(m)
     };
     let m2 = m.clone();
-    let lock = m.lock();
+    let lock = m.as_ref().lock();
     let child = thread::spawn(move || {
-        let lock = m2.lock();
+        let lock = m2.as_ref().lock();
         assert_eq!(*lock.borrow(), 4950);
     });
     for i in 0..100 {
-        let lock = m.lock();
+        let lock = m.as_ref().lock();
         *lock.borrow_mut() += i;
     }
     drop(lock);
@@ -48,20 +52,21 @@ fn is_mutex() {
 #[test]
 fn trylock_works() {
     let m = unsafe {
-        let m = Arc::new(ReentrantMutex::new(()));
-        m.init();
-        m
+        // FIXME: Simplify this if Arc gets a Arc::get_pin_mut.
+        let mut m = Arc::new(ReentrantMutex::new(()));
+        Pin::new_unchecked(Arc::get_mut_unchecked(&mut m)).init();
+        Pin::new_unchecked(m)
     };
     let m2 = m.clone();
-    let _lock = m.try_lock();
-    let _lock2 = m.try_lock();
+    let _lock = m.as_ref().try_lock();
+    let _lock2 = m.as_ref().try_lock();
     thread::spawn(move || {
-        let lock = m2.try_lock();
+        let lock = m2.as_ref().try_lock();
         assert!(lock.is_none());
     })
     .join()
     .unwrap();
-    let _lock3 = m.try_lock();
+    let _lock3 = m.as_ref().try_lock();
 }
 
 pub struct Answer<'a>(pub ReentrantMutexGuard<'a, RefCell<u32>>);
diff --git a/library/test/src/lib.rs b/library/test/src/lib.rs
index 5c12a54eef1..656d9669e81 100644
--- a/library/test/src/lib.rs
+++ b/library/test/src/lib.rs
@@ -30,6 +30,7 @@
 #![feature(termination_trait_lib)]
 #![feature(test)]
 #![feature(total_cmp)]
+#![feature(str_split_once)]
 
 // Public reexports
 pub use self::bench::{black_box, Bencher};
diff --git a/library/test/src/time.rs b/library/test/src/time.rs
index 130792fa5d7..e0b6eadffa1 100644
--- a/library/test/src/time.rs
+++ b/library/test/src/time.rs
@@ -105,30 +105,24 @@ impl TimeThreshold {
     /// value.
     pub fn from_env_var(env_var_name: &str) -> Option<Self> {
         let durations_str = env::var(env_var_name).ok()?;
+        let (warn_str, critical_str) = durations_str.split_once(',').unwrap_or_else(|| {
+            panic!(
+                "Duration variable {} expected to have 2 numbers separated by comma, but got {}",
+                env_var_name, durations_str
+            )
+        });
 
-        // Split string into 2 substrings by comma and try to parse numbers.
-        let mut durations = durations_str.splitn(2, ',').map(|v| {
+        let parse_u64 = |v| {
             u64::from_str(v).unwrap_or_else(|_| {
                 panic!(
                     "Duration value in variable {} is expected to be a number, but got {}",
                     env_var_name, v
                 )
             })
-        });
-
-        // Callback to be called if the environment variable has unexpected structure.
-        let panic_on_incorrect_value = || {
-            panic!(
-                "Duration variable {} expected to have 2 numbers separated by comma, but got {}",
-                env_var_name, durations_str
-            );
         };
 
-        let (warn, critical) = (
-            durations.next().unwrap_or_else(panic_on_incorrect_value),
-            durations.next().unwrap_or_else(panic_on_incorrect_value),
-        );
-
+        let warn = parse_u64(warn_str);
+        let critical = parse_u64(critical_str);
         if warn > critical {
             panic!("Test execution warn time should be less or equal to the critical time");
         }
diff --git a/src/ci/docker/host-x86_64/x86_64-gnu-nopt/Dockerfile b/src/ci/docker/host-x86_64/x86_64-gnu-nopt/Dockerfile
index f4071961f8e..77510d7ac62 100644
--- a/src/ci/docker/host-x86_64/x86_64-gnu-nopt/Dockerfile
+++ b/src/ci/docker/host-x86_64/x86_64-gnu-nopt/Dockerfile
@@ -1,6 +1,7 @@
-FROM ubuntu:16.04
+FROM ubuntu:20.04
 
-RUN apt-get update && apt-get install -y --no-install-recommends \
+# Avoid interactive prompts while installing `tzdata` dependency with `DEBIAN_FRONTEND`.
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
   g++ \
   make \
   ninja-build \
diff --git a/src/doc/unstable-book/src/language-features/const-fn.md b/src/doc/unstable-book/src/language-features/const-fn.md
index 50dbbaf5674..bcf7f78b8fe 100644
--- a/src/doc/unstable-book/src/language-features/const-fn.md
+++ b/src/doc/unstable-book/src/language-features/const-fn.md
@@ -6,24 +6,5 @@ The tracking issue for this feature is: [#57563]
 
 ------------------------
 
-The `const_fn` feature allows marking free functions and inherent methods as
-`const`, enabling them to be called in constants contexts, with constant
-arguments.
-
-## Examples
-
-```rust
-#![feature(const_fn)]
-
-const fn double(x: i32) -> i32 {
-    x * 2
-}
-
-const FIVE: i32 = 5;
-const TEN: i32 = double(FIVE);
-
-fn main() {
-    assert_eq!(5, FIVE);
-    assert_eq!(10, TEN);
-}
-```
+The `const_fn` feature enables additional functionality not stabilized in the
+[minimal subset of `const_fn`](https://github.com/rust-lang/rust/issues/53555)
diff --git a/src/librustdoc/config.rs b/src/librustdoc/config.rs
index e60970af0d3..2d58614b139 100644
--- a/src/librustdoc/config.rs
+++ b/src/librustdoc/config.rs
@@ -397,12 +397,9 @@ impl Options {
             matches
                 .opt_strs("default-setting")
                 .iter()
-                .map(|s| {
-                    let mut kv = s.splitn(2, '=');
-                    // never panics because `splitn` always returns at least one element
-                    let k = kv.next().unwrap().to_string();
-                    let v = kv.next().unwrap_or("true").to_string();
-                    (k, v)
+                .map(|s| match s.split_once('=') {
+                    None => (s.clone(), "true".to_string()),
+                    Some((k, v)) => (k.to_string(), v.to_string()),
                 })
                 .collect(),
         ];
@@ -707,11 +704,9 @@ fn parse_extern_html_roots(
 ) -> Result<BTreeMap<String, String>, &'static str> {
     let mut externs = BTreeMap::new();
     for arg in &matches.opt_strs("extern-html-root-url") {
-        let mut parts = arg.splitn(2, '=');
-        let name = parts.next().ok_or("--extern-html-root-url must not be empty")?;
-        let url = parts.next().ok_or("--extern-html-root-url must be of the form name=url")?;
+        let (name, url) =
+            arg.split_once('=').ok_or("--extern-html-root-url must be of the form name=url")?;
         externs.insert(name.to_string(), url.to_string());
     }
-
     Ok(externs)
 }
diff --git a/src/librustdoc/html/layout.rs b/src/librustdoc/html/layout.rs
index e8039942f4f..f2c74c46d7d 100644
--- a/src/librustdoc/html/layout.rs
+++ b/src/librustdoc/html/layout.rs
@@ -98,7 +98,7 @@ crate fn render<T: Print, S: Print>(
                            placeholder=\"Click or press ‘S’ to search, ‘?’ for more options…\" \
                            type=\"search\">\
                 </div>\
-                <span class=\"help-button\">?</span>
+                <button class=\"help-button\">?</button>
                 <a id=\"settings-menu\" href=\"{root_path}settings.html\">\
                     <img src=\"{static_root_path}wheel{suffix}.svg\" \
                          width=\"18\" \
diff --git a/src/librustdoc/html/render/mod.rs b/src/librustdoc/html/render/mod.rs
index 901f00b21da..efee4c0be06 100644
--- a/src/librustdoc/html/render/mod.rs
+++ b/src/librustdoc/html/render/mod.rs
@@ -167,10 +167,8 @@ impl Context {
         // `style-suffix.min.css`.  Path::extension would just return `css`
         // which would result in `style.min-suffix.css` which isn't what we
         // want.
-        let mut iter = filename.splitn(2, '.');
-        let base = iter.next().unwrap();
-        let ext = iter.next().unwrap();
-        let filename = format!("{}{}.{}", base, self.shared.resource_suffix, ext,);
+        let (base, ext) = filename.split_once('.').unwrap();
+        let filename = format!("{}{}.{}", base, self.shared.resource_suffix, ext);
         self.dst.join(&filename)
     }
 }
diff --git a/src/librustdoc/html/static/main.js b/src/librustdoc/html/static/main.js
index a0ccb078c4a..47847ccb5f6 100644
--- a/src/librustdoc/html/static/main.js
+++ b/src/librustdoc/html/static/main.js
@@ -1633,10 +1633,10 @@ function defocusSearchBar() {
 
         function makeTabHeader(tabNb, text, nbElems) {
             if (currentTab === tabNb) {
-                return "<div class=\"selected\">" + text +
-                       " <div class=\"count\">(" + nbElems + ")</div></div>";
+                return "<button class=\"selected\">" + text +
+                       " <div class=\"count\">(" + nbElems + ")</div></button>";
             }
-            return "<div>" + text + " <div class=\"count\">(" + nbElems + ")</div></div>";
+            return "<button>" + text + " <div class=\"count\">(" + nbElems + ")</div></button>";
         }
 
         function showResults(results) {
diff --git a/src/librustdoc/html/static/rustdoc.css b/src/librustdoc/html/static/rustdoc.css
index 8eef65a231d..61905b8eca8 100644
--- a/src/librustdoc/html/static/rustdoc.css
+++ b/src/librustdoc/html/static/rustdoc.css
@@ -1176,21 +1176,22 @@ pre.rust {
 	height: 35px;
 }
 
-#titles > div {
+#titles > button {
 	float: left;
 	width: 33.3%;
 	text-align: center;
 	font-size: 18px;
 	cursor: pointer;
+	border: 0;
 	border-top: 2px solid;
 }
 
-#titles > div:not(:last-child) {
+#titles > button:not(:last-child) {
 	margin-right: 1px;
 	width: calc(33.3% - 1px);
 }
 
-#titles > div > div.count {
+#titles > button > div.count {
 	display: inline-block;
 	font-size: 16px;
 }
@@ -1459,7 +1460,7 @@ h4 > .notable-traits {
 		top: 24px;
 	}
 
-	#titles > div > div.count {
+	#titles > button > div.count {
 		float: left;
 		width: 100%;
 	}
@@ -1565,7 +1566,7 @@ h4 > .notable-traits {
 }
 
 @media (max-width: 416px) {
-	#titles, #titles > div {
+	#titles, #titles > button {
 		height: 73px;
 	}
 
diff --git a/src/librustdoc/html/static/themes/ayu.css b/src/librustdoc/html/static/themes/ayu.css
index c1f796f09e8..76bbe4f6201 100644
--- a/src/librustdoc/html/static/themes/ayu.css
+++ b/src/librustdoc/html/static/themes/ayu.css
@@ -403,22 +403,22 @@ pre.ignore:hover, .information:hover + pre.ignore {
 	border-color: #5c6773;
 }
 
-#titles > div.selected {
+#titles > button.selected {
 	background-color: #141920 !important;
 	border-bottom: 1px solid #ffb44c !important;
 	border-top: none;
 }
 
-#titles > div:not(.selected) {
+#titles > button:not(.selected) {
 	background-color: transparent !important;
 	border: none;
 }
 
-#titles > div:hover {
+#titles > button:hover {
 	border-bottom: 1px solid rgba(242, 151, 24, 0.3);
 }
 
-#titles > div > div.count {
+#titles > button > div.count {
 	color: #888;
 }
 
@@ -434,7 +434,7 @@ above the `@media (max-width: 700px)` rules due to a bug in the css checker */
 .block a.current.derive,.content span.macro,.content a.macro,.block a.current.macro {}
 .content .highlighted.trait {}
 .content span.struct,.content a.struct,.block a.current.struct {}
-#titles>div:hover,#titles>div.selected {}
+#titles>button:hover,#titles>button.selected {}
 .content .highlighted.traitalias {}
 .content span.type,.content a.type,.block a.current.type {}
 .content span.union,.content a.union,.block a.current.union {}
diff --git a/src/librustdoc/html/static/themes/dark.css b/src/librustdoc/html/static/themes/dark.css
index 946ca0a40c9..86ce99284eb 100644
--- a/src/librustdoc/html/static/themes/dark.css
+++ b/src/librustdoc/html/static/themes/dark.css
@@ -352,16 +352,17 @@ pre.ignore:hover, .information:hover + pre.ignore {
 	border-color: #777;
 }
 
-#titles > div:not(.selected) {
+#titles > button:not(.selected) {
 	background-color: #252525;
 	border-top-color: #252525;
 }
 
-#titles > div:hover, #titles > div.selected {
+#titles > button:hover, #titles > button.selected {
 	border-top-color: #0089ff;
+	background-color: #353535;
 }
 
-#titles > div > div.count {
+#titles > button > div.count {
 	color: #888;
 }
 
diff --git a/src/librustdoc/html/static/themes/light.css b/src/librustdoc/html/static/themes/light.css
index e0b9a04921a..52cfdf6f7a3 100644
--- a/src/librustdoc/html/static/themes/light.css
+++ b/src/librustdoc/html/static/themes/light.css
@@ -343,16 +343,17 @@ pre.ignore:hover, .information:hover + pre.ignore {
 	border-color: #999;
 }
 
-#titles > div:not(.selected) {
+#titles > button:not(.selected) {
 	background-color: #e6e6e6;
 	border-top-color: #e6e6e6;
 }
 
-#titles > div:hover, #titles > div.selected {
+#titles > button:hover, #titles > button.selected {
 	border-top-color: #0089ff;
+	background-color: #353535;
 }
 
-#titles > div > div.count {
+#titles > button > div.count {
 	color: #888;
 }
 
diff --git a/src/librustdoc/lib.rs b/src/librustdoc/lib.rs
index 26bf4b569ff..f851d1a2372 100644
--- a/src/librustdoc/lib.rs
+++ b/src/librustdoc/lib.rs
@@ -16,6 +16,7 @@
 #![feature(once_cell)]
 #![feature(type_ascription)]
 #![feature(split_inclusive)]
+#![feature(str_split_once)]
 #![recursion_limit = "256"]
 
 #[macro_use]
diff --git a/src/librustdoc/passes/collect_intra_doc_links.rs b/src/librustdoc/passes/collect_intra_doc_links.rs
index 551c086a8d4..5ce64c4cd83 100644
--- a/src/librustdoc/passes/collect_intra_doc_links.rs
+++ b/src/librustdoc/passes/collect_intra_doc_links.rs
@@ -435,8 +435,10 @@ impl<'a, 'tcx> LinkCollector<'a, 'tcx> {
 
         // Try looking for methods and associated items.
         let mut split = path_str.rsplitn(2, "::");
-        // this can be an `unwrap()` because we ensure the link is never empty
-        let (item_str, item_name) = split.next().map(|i| (i, Symbol::intern(i))).unwrap();
+        // NB: `split`'s first element is always defined, even if the delimiter was not present.
+        let item_str = split.next().unwrap();
+        assert!(!item_str.is_empty());
+        let item_name = Symbol::intern(item_str);
         let path_root = split
             .next()
             .map(|f| f.to_owned())
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.fixed b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.fixed
index 760d2b433c8..bca69a97677 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.fixed
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.fixed
@@ -11,7 +11,7 @@ pub trait Foo<T> {
 
 fn foo2<I : for<'x> Foo<&'x isize>>(
     x: <I as Foo<&isize>>::A)
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 {
     // This case is illegal because we have to instantiate `'x`, and
     // we don't know what region to instantiate it with.
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.rs b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.rs
index 6eb584ea645..1e23dd8890b 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.rs
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.rs
@@ -11,7 +11,7 @@ pub trait Foo<T> {
 
 fn foo2<I : for<'x> Foo<&'x isize>>(
     x: I::A)
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 {
     // This case is illegal because we have to instantiate `'x`, and
     // we don't know what region to instantiate it with.
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.stderr b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.stderr
index f2137f68665..989624bdd93 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.stderr
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.stderr
@@ -1,4 +1,4 @@
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-fn.rs:13:8
    |
 LL |     x: I::A)
@@ -6,3 +6,4 @@ LL |     x: I::A)
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0212`.
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.rs b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.rs
index 58f186d7775..ed30d86cb5b 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.rs
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.rs
@@ -9,14 +9,14 @@ pub trait Foo<T> {
 
 struct SomeStruct<I: for<'x> Foo<&'x isize>> {
     field: I::A
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 }
 
 enum SomeEnum<'b, I: for<'a> Foo<&'a isize>> {
     TupleVariant(I::A),
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
     StructVariant { field: I::A },
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
     OkVariant(&'b usize),
 }
 
@@ -33,7 +33,7 @@ struct YetAnotherStruct<'a, I: for<'x> Foo<&'x isize>> {
 struct Why<'a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k, 'n, 'o, 'p, 'q, 'r, 's, 't, 'u, 'v, 'w, 'x,
     'y, 'z, 'aa, I: for<'l, 'm> Foo<&'l &'m isize>> {
     field: I::A,
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 }
 
 pub fn main() {}
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.stderr b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.stderr
index e3fd2860ebc..cadc3e9eab1 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.stderr
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.stderr
@@ -1,4 +1,4 @@
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-struct.rs:11:12
    |
 LL |     field: I::A
@@ -10,7 +10,7 @@ LL | struct SomeStruct<'a, I: for<'x> Foo<&'x isize>> {
 LL |     field: <I as Foo<&'a isize>>::A
    |
 
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-struct.rs:16:18
    |
 LL |     TupleVariant(I::A),
@@ -22,7 +22,7 @@ LL | enum SomeEnum<'c, 'b, I: for<'a> Foo<&'a isize>> {
 LL |     TupleVariant(<I as Foo<&'c isize>>::A),
    |
 
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-struct.rs:18:28
    |
 LL |     StructVariant { field: I::A },
@@ -36,7 +36,7 @@ LL |
 LL |     StructVariant { field: <I as Foo<&'c isize>>::A },
    |
 
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-struct.rs:35:12
    |
 LL |     field: I::A,
@@ -51,3 +51,4 @@ LL |     field: <I as Foo<&'bb &'bb isize>>::A,
 
 error: aborting due to 4 previous errors
 
+For more information about this error, try `rustc --explain E0212`.
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.fixed b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.fixed
index acf32bccbec..66d8613f184 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.fixed
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.fixed
@@ -11,7 +11,7 @@ pub trait Foo<T> {
 
 trait SomeTrait<I : for<'x> Foo<&'x isize>> {
     fn some_method(&self, arg: <I as Foo<&isize>>::A);
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 }
 
 trait AnotherTrait<I : for<'x> Foo<&'x isize>> {
@@ -30,7 +30,7 @@ struct Peach<X>(std::marker::PhantomData<X>);
 
 impl<X: for<'a> Banana<'a>> Peach<X> {
     fn mango(&self) -> <X as Banana<'_>>::Assoc {
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
         Default::default()
     }
 }
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.rs b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.rs
index a249f89685e..0a1b29de19e 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.rs
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.rs
@@ -11,7 +11,7 @@ pub trait Foo<T> {
 
 trait SomeTrait<I : for<'x> Foo<&'x isize>> {
     fn some_method(&self, arg: I::A);
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 }
 
 trait AnotherTrait<I : for<'x> Foo<&'x isize>> {
@@ -30,7 +30,7 @@ struct Peach<X>(std::marker::PhantomData<X>);
 
 impl<X: for<'a> Banana<'a>> Peach<X> {
     fn mango(&self) -> X::Assoc {
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
         Default::default()
     }
 }
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.stderr b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.stderr
index a37fec24493..d457f9f8468 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.stderr
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.stderr
@@ -1,10 +1,10 @@
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-trait-method.rs:13:32
    |
 LL |     fn some_method(&self, arg: I::A);
    |                                ^^^^ help: use a fully qualified path with inferred lifetimes: `<I as Foo<&isize>>::A`
 
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-trait-method.rs:32:24
    |
 LL |     fn mango(&self) -> X::Assoc {
@@ -12,3 +12,4 @@ LL |     fn mango(&self) -> X::Assoc {
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0212`.
diff --git a/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.rs b/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.rs
index 34aee7f6935..f2d97b7bac3 100644
--- a/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.rs
+++ b/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.rs
@@ -3,10 +3,10 @@
 fn main() {}
 
 trait X {
-    default const A: u8; //~ ERROR `default` is only allowed on items in `impl` definitions
-    default const B: u8 = 0;  //~ ERROR `default` is only allowed on items in `impl` definitions
-    default type D; //~ ERROR `default` is only allowed on items in `impl` definitions
-    default type C: Ord; //~ ERROR `default` is only allowed on items in `impl` definitions
-    default fn f1(); //~ ERROR `default` is only allowed on items in `impl` definitions
-    default fn f2() {} //~ ERROR `default` is only allowed on items in `impl` definitions
+    default const A: u8; //~ ERROR `default` is only allowed on items in trait impls
+    default const B: u8 = 0;  //~ ERROR `default` is only allowed on items in trait impls
+    default type D; //~ ERROR `default` is only allowed on items in trait impls
+    default type C: Ord; //~ ERROR `default` is only allowed on items in trait impls
+    default fn f1(); //~ ERROR `default` is only allowed on items in trait impls
+    default fn f2() {} //~ ERROR `default` is only allowed on items in trait impls
 }
diff --git a/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.stderr b/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.stderr
index cdc9ee8d9e7..76fa860334d 100644
--- a/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.stderr
+++ b/src/test/ui/parser/trait-item-with-defaultness-fail-semantic.stderr
@@ -1,4 +1,4 @@
-error: `default` is only allowed on items in `impl` definitions
+error: `default` is only allowed on items in trait impls
   --> $DIR/trait-item-with-defaultness-fail-semantic.rs:6:5
    |
 LL |     default const A: u8;
@@ -6,7 +6,7 @@ LL |     default const A: u8;
    |     |
    |     `default` because of this
 
-error: `default` is only allowed on items in `impl` definitions
+error: `default` is only allowed on items in trait impls
   --> $DIR/trait-item-with-defaultness-fail-semantic.rs:7:5
    |
 LL |     default const B: u8 = 0;
@@ -14,7 +14,7 @@ LL |     default const B: u8 = 0;
    |     |
    |     `default` because of this
 
-error: `default` is only allowed on items in `impl` definitions
+error: `default` is only allowed on items in trait impls
   --> $DIR/trait-item-with-defaultness-fail-semantic.rs:8:5
    |
 LL |     default type D;
@@ -22,7 +22,7 @@ LL |     default type D;
    |     |
    |     `default` because of this
 
-error: `default` is only allowed on items in `impl` definitions
+error: `default` is only allowed on items in trait impls
   --> $DIR/trait-item-with-defaultness-fail-semantic.rs:9:5
    |
 LL |     default type C: Ord;
@@ -30,7 +30,7 @@ LL |     default type C: Ord;
    |     |
    |     `default` because of this
 
-error: `default` is only allowed on items in `impl` definitions
+error: `default` is only allowed on items in trait impls
   --> $DIR/trait-item-with-defaultness-fail-semantic.rs:10:5
    |
 LL |     default fn f1();
@@ -38,7 +38,7 @@ LL |     default fn f1();
    |     |
    |     `default` because of this
 
-error: `default` is only allowed on items in `impl` definitions
+error: `default` is only allowed on items in trait impls
   --> $DIR/trait-item-with-defaultness-fail-semantic.rs:11:5
    |
 LL |     default fn f2() {}
diff --git a/src/tools/linkchecker/main.rs b/src/tools/linkchecker/main.rs
index f213944e0ab..dcfe1bb803f 100644
--- a/src/tools/linkchecker/main.rs
+++ b/src/tools/linkchecker/main.rs
@@ -14,6 +14,8 @@
 //! A few exceptions are allowed as there's known bugs in rustdoc, but this
 //! should catch the majority of "broken link" cases.
 
+#![feature(str_split_once)]
+
 use std::collections::hash_map::Entry;
 use std::collections::{HashMap, HashSet};
 use std::env;
@@ -232,11 +234,12 @@ fn check(cache: &mut Cache, root: &Path, file: &Path, errors: &mut bool) -> Opti
         {
             return;
         }
-        let mut parts = url.splitn(2, '#');
-        let url = parts.next().unwrap();
-        let fragment = parts.next();
-        let mut parts = url.splitn(2, '?');
-        let url = parts.next().unwrap();
+        let (url, fragment) = match url.split_once('#') {
+            None => (url, None),
+            Some((url, fragment)) => (url, Some(fragment)),
+        };
+        // NB: the `splitn` always succeeds, even if the delimiter is not present.
+        let url = url.splitn(2, '?').next().unwrap();
 
         // Once we've plucked out the URL, parse it using our base url and
         // then try to extract a file path.
diff --git a/src/tools/tidy/src/cargo.rs b/src/tools/tidy/src/cargo.rs
index 7bdd78a91e7..e06616a59f3 100644
--- a/src/tools/tidy/src/cargo.rs
+++ b/src/tools/tidy/src/cargo.rs
@@ -59,11 +59,10 @@ fn verify(tomlfile: &Path, libfile: &Path, bad: &mut bool) {
             break;
         }
 
-        let mut parts = line.splitn(2, '=');
-        let krate = parts.next().unwrap().trim();
-        if parts.next().is_none() {
-            continue;
-        }
+        let krate = match line.split_once('=') {
+            None => continue,
+            Some((krate, _)) => krate.trim(),
+        };
 
         // Don't worry about depending on core/std while not writing `extern crate
         // core/std` -- that's intentional.
diff --git a/src/tools/tidy/src/error_codes_check.rs b/src/tools/tidy/src/error_codes_check.rs
index 82a5234ac5b..a7199fdfce6 100644
--- a/src/tools/tidy/src/error_codes_check.rs
+++ b/src/tools/tidy/src/error_codes_check.rs
@@ -85,47 +85,61 @@ fn extract_error_codes(
     for line in f.lines() {
         let s = line.trim();
         if !reached_no_explanation && s.starts_with('E') && s.contains("include_str!(\"") {
-            if let Some(err_code) = s.splitn(2, ':').next() {
-                let err_code = err_code.to_owned();
-                if !error_codes.contains_key(&err_code) {
-                    error_codes.insert(err_code.clone(), false);
-                }
-                // Now we extract the tests from the markdown file!
-                let md = some_or_continue!(s.splitn(2, "include_str!(\"").nth(1));
-                let md_file_name = some_or_continue!(md.splitn(2, "\")").next());
-                let path = some_or_continue!(path.parent())
-                    .join(md_file_name)
-                    .canonicalize()
-                    .expect("failed to canonicalize error explanation file path");
-                match read_to_string(&path) {
-                    Ok(content) => {
-                        if !IGNORE_EXPLANATION_CHECK.contains(&err_code.as_str())
-                            && !check_if_error_code_is_test_in_explanation(&content, &err_code)
-                        {
-                            errors.push(format!(
-                                "`{}` doesn't use its own error code in compile_fail example",
-                                path.display(),
-                            ));
-                        }
-                        if check_error_code_explanation(&content, error_codes, err_code) {
-                            errors.push(format!(
-                                "`{}` uses invalid tag `compile-fail` instead of `compile_fail`",
-                                path.display(),
-                            ));
-                        }
+            let err_code = s
+                .split_once(':')
+                .expect(
+                    format!(
+                        "Expected a line with the format `E0xxx: include_str!(\"..\")`, but got {} without a `:` delimiter",
+                        s,
+                    ).as_str()
+                )
+                .0
+                .to_owned();
+            if !error_codes.contains_key(&err_code) {
+                error_codes.insert(err_code.clone(), false);
+            }
+            // Now we extract the tests from the markdown file!
+            let md_file_name = match s.split_once("include_str!(\"") {
+                None => continue,
+                Some((_, md)) => match md.split_once("\")") {
+                    None => continue,
+                    Some((file_name, _)) => file_name,
+                },
+            };
+            let path = some_or_continue!(path.parent())
+                .join(md_file_name)
+                .canonicalize()
+                .expect("failed to canonicalize error explanation file path");
+            match read_to_string(&path) {
+                Ok(content) => {
+                    if !IGNORE_EXPLANATION_CHECK.contains(&err_code.as_str())
+                        && !check_if_error_code_is_test_in_explanation(&content, &err_code)
+                    {
+                        errors.push(format!(
+                            "`{}` doesn't use its own error code in compile_fail example",
+                            path.display(),
+                        ));
                     }
-                    Err(e) => {
-                        eprintln!("Couldn't read `{}`: {}", path.display(), e);
+                    if check_error_code_explanation(&content, error_codes, err_code) {
+                        errors.push(format!(
+                            "`{}` uses invalid tag `compile-fail` instead of `compile_fail`",
+                            path.display(),
+                        ));
                     }
                 }
+                Err(e) => {
+                    eprintln!("Couldn't read `{}`: {}", path.display(), e);
+                }
             }
         } else if reached_no_explanation && s.starts_with('E') {
-            if let Some(err_code) = s.splitn(2, ',').next() {
-                let err_code = err_code.to_owned();
-                if !error_codes.contains_key(&err_code) {
-                    // this check should *never* fail!
-                    error_codes.insert(err_code, false);
-                }
+            let err_code = match s.split_once(',') {
+                None => s,
+                Some((err_code, _)) => err_code,
+            }
+            .to_string();
+            if !error_codes.contains_key(&err_code) {
+                // this check should *never* fail!
+                error_codes.insert(err_code, false);
             }
         } else if s == ";" {
             reached_no_explanation = true;
@@ -137,12 +151,15 @@ fn extract_error_codes_from_tests(f: &str, error_codes: &mut HashMap<String, boo
     for line in f.lines() {
         let s = line.trim();
         if s.starts_with("error[E") || s.starts_with("warning[E") {
-            if let Some(err_code) = s.splitn(2, ']').next() {
-                if let Some(err_code) = err_code.splitn(2, '[').nth(1) {
-                    let nb = error_codes.entry(err_code.to_owned()).or_insert(false);
-                    *nb = true;
-                }
-            }
+            let err_code = match s.split_once(']') {
+                None => continue,
+                Some((err_code, _)) => match err_code.split_once('[') {
+                    None => continue,
+                    Some((_, err_code)) => err_code,
+                },
+            };
+            let nb = error_codes.entry(err_code.to_owned()).or_insert(false);
+            *nb = true;
         }
     }
 }
diff --git a/src/tools/tidy/src/extdeps.rs b/src/tools/tidy/src/extdeps.rs
index 6b0ecde7de4..55440fdcf3f 100644
--- a/src/tools/tidy/src/extdeps.rs
+++ b/src/tools/tidy/src/extdeps.rs
@@ -28,7 +28,7 @@ pub fn check(root: &Path, bad: &mut bool) {
         }
 
         // Extract source value.
-        let source = line.splitn(2, '=').nth(1).unwrap().trim();
+        let source = line.split_once('=').unwrap().1.trim();
 
         // Allow all soft-dev repos.
         // We also allow our personal forks for scenarios where we are breaking a CI cycle and need
diff --git a/src/tools/tidy/src/features.rs b/src/tools/tidy/src/features.rs
index d8029ea04f0..3c2880d0d5e 100644
--- a/src/tools/tidy/src/features.rs
+++ b/src/tools/tidy/src/features.rs
@@ -112,6 +112,7 @@ pub fn check(
                 let gate_test_str = "gate-test-";
 
                 let feature_name = match line.find(gate_test_str) {
+                    // NB: the `splitn` always succeeds, even if the delimiter is not present.
                     Some(i) => line[i + gate_test_str.len()..].splitn(2, ' ').next().unwrap(),
                     None => continue,
                 };
diff --git a/src/tools/tidy/src/lib.rs b/src/tools/tidy/src/lib.rs
index cc4c43f0468..e11d293210b 100644
--- a/src/tools/tidy/src/lib.rs
+++ b/src/tools/tidy/src/lib.rs
@@ -3,6 +3,8 @@
 //! This library contains the tidy lints and exposes it
 //! to be used by tools.
 
+#![feature(str_split_once)]
+
 use std::fs::File;
 use std::io::Read;
 use walkdir::{DirEntry, WalkDir};
diff --git a/src/tools/tidy/src/ui_tests.rs b/src/tools/tidy/src/ui_tests.rs
index 47b328dae47..03f4efea983 100644
--- a/src/tools/tidy/src/ui_tests.rs
+++ b/src/tools/tidy/src/ui_tests.rs
@@ -19,14 +19,11 @@ pub fn check(path: &Path, bad: &mut bool) {
                     //
                     // For now, just make sure that there is a corresponding
                     // `$testname.rs` file.
-                    let testname = file_path
-                        .file_name()
-                        .unwrap()
-                        .to_str()
-                        .unwrap()
-                        .splitn(2, '.')
-                        .next()
-                        .unwrap();
+                    //
+                    // NB: We do not use file_stem() as some file names have multiple `.`s and we
+                    // must strip all of them.
+                    let testname =
+                        file_path.file_name().unwrap().to_str().unwrap().split_once('.').unwrap().0;
                     if !file_path.with_file_name(testname).with_extension("rs").exists() {
                         println!("Stray file with UI testing output: {:?}", file_path);
                         *bad = true;