Let's #[expect] some lints: Stabilize lint_reasons (RFC 2383)

compiler/rustc_builtin_macros/src/lib.rs

@@ -5,14 +5,14 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
+#![cfg_attr(bootstrap, feature(lint_reasons))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(decl_macro)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
-#![feature(lint_reasons)]
 #![feature(proc_macro_internals)]
 #![feature(proc_macro_quote)]
 #![feature(rustdoc_internals)]

compiler/rustc_data_structures/src/lib.rs

@@ -10,6 +10,7 @@
 #![allow(internal_features)]
 #![allow(rustc::default_hash_types)]
 #![allow(rustc::potential_query_instability)]
+#![cfg_attr(bootstrap, feature(lint_reasons))]
 #![cfg_attr(not(parallel_compiler), feature(cell_leak))]
 #![deny(unsafe_op_in_unsafe_fn)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
@@ -24,7 +25,6 @@
 #![feature(extend_one)]
 #![feature(hash_raw_entry)]
 #![feature(hasher_prefixfree_extras)]
-#![feature(lint_reasons)]
 #![feature(macro_metavar_expr)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]

compiler/rustc_errors/src/json.rs

@@ -135,7 +135,12 @@ impl Emitter for JsonEmitter {
         let data: Vec<FutureBreakageItem<'_>> = diags
             .into_iter()
             .map(|mut diag| {
-                if diag.level == crate::Level::Allow {
+                // The `FutureBreakageItem` is collected and serialized.
+                // However, the `allow` and `expect` lint levels can't usually
+                // be serialized. The lint level is overwritten to allow the
+                // serialization again and force a lint emission.
+                // (This is an educated guess. I didn't originally add this)
+                if matches!(diag.level, crate::Level::Allow | crate::Level::Expect(..)) {
                     diag.level = crate::Level::Warning;
                 }
                 FutureBreakageItem {

compiler/rustc_feature/src/accepted.rs

@@ -232,6 +232,8 @@ declare_features! (
     (accepted, label_break_value, "1.65.0", Some(48594)),
     /// Allows `let...else` statements.
     (accepted, let_else, "1.65.0", Some(87335)),
+    /// Allows using `reason` in lint attributes and the `#[expect(lint)]` lint check.
+    (accepted, lint_reasons, "CURRENT_RUSTC_VERSION", Some(54503)),
     /// Allows `break {expr}` with a value inside `loop`s.
     (accepted, loop_break_value, "1.19.0", Some(37339)),
     /// Allows use of `?` as the Kleene "at most one" operator in macros.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -369,9 +369,9 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         allow, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#),
         DuplicatesOk, EncodeCrossCrate::No,
     ),
-    gated!(
-        expect, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#), DuplicatesOk,
-        EncodeCrossCrate::No, lint_reasons, experimental!(expect)
+    ungated!(
+        expect, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#),
+        DuplicatesOk, EncodeCrossCrate::No,
     ),
     ungated!(
         forbid, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#),

compiler/rustc_feature/src/unstable.rs

@@ -512,8 +512,6 @@ declare_features! (
     /// Allows using `#[link(kind = "link-arg", name = "...")]`
     /// to pass custom arguments to the linker.
     (unstable, link_arg_attribute, "1.76.0", Some(99427)),
-    /// Allows using `reason` in lint attributes and the `#[expect(lint)]` lint check.
-    (unstable, lint_reasons, "1.31.0", Some(54503)),
     /// Give access to additional metadata about declarative macro meta-variables.
     (unstable, macro_metavar_expr, "1.61.0", Some(83527)),
     /// Provides a way to concatenate identifiers using metavariable expressions.

compiler/rustc_lint/src/expect.rs

@@ -3,18 +3,13 @@ use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::lint::builtin::UNFULFILLED_LINT_EXPECTATIONS;
 use rustc_session::lint::LintExpectationId;
-use rustc_span::symbol::sym;
 use rustc_span::Symbol;
 
 pub(crate) fn provide(providers: &mut Providers) {
     *providers = Providers { check_expectations, ..*providers };
 }
 
 fn check_expectations(tcx: TyCtxt<'_>, tool_filter: Option<Symbol>) {
-    if !tcx.features().active(sym::lint_reasons) {
-        return;
-    }
-
     let lint_expectations = tcx.lint_expectations(());
     let fulfilled_expectations = tcx.dcx().steal_fulfilled_expectation_ids();
 

compiler/rustc_lint/src/levels.rs

@@ -37,7 +37,6 @@ use rustc_session::lint::{
     },
     Level, Lint, LintExpectationId, LintId,
 };
-use rustc_session::parse::feature_err;
 use rustc_session::Session;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{Span, DUMMY_SP};
@@ -788,15 +787,6 @@ impl<'s, P: LintLevelsProvider> LintLevelsBuilder<'s, P> {
                     ast::MetaItemKind::NameValue(ref name_value) => {
                         if item.path == sym::reason {
                             if let ast::LitKind::Str(rationale, _) = name_value.kind {
-                                if !self.features.lint_reasons {
-                                    feature_err(
-                                        &self.sess,
-                                        sym::lint_reasons,
-                                        item.span,
-                                        "lint reasons are experimental",
-                                    )
-                                    .emit();
-                                }
                                 reason = Some(rationale);
                             } else {
                                 sess.dcx().emit_err(MalformedAttribute {

compiler/rustc_lint_defs/src/builtin.rs

@@ -608,13 +608,13 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// The `unfulfilled_lint_expectations` lint detects lint trigger expectations
-    /// that have not been fulfilled.
+    /// The `unfulfilled_lint_expectations` lint detects when a lint expectation is
+    /// unfulfilled.
     ///
     /// ### Example
     ///
     /// ```rust
-    /// #![feature(lint_reasons)]
+    /// #![cfg_attr(bootstrap, feature(lint_reasons))]
     ///
     /// #[expect(unused_variables)]
     /// let x = 10;
@@ -625,24 +625,14 @@ declare_lint! {
     ///
     /// ### Explanation
     ///
-    /// It was expected that the marked code would emit a lint. This expectation
-    /// has not been fulfilled.
+    /// The `#[expect]` attribute can be used to create a lint expectation. The
+    /// expectation is fulfilled, if a `#[warn]` attribute at the same location
+    /// would result in a lint emission. If the expectation is unfulfilled,
+    /// because no lint was emitted, this lint will be emitted on the attribute.
     ///
-    /// The `expect` attribute can be removed if this is intended behavior otherwise
-    /// it should be investigated why the expected lint is no longer issued.
-    ///
-    /// In rare cases, the expectation might be emitted at a different location than
-    /// shown in the shown code snippet. In most cases, the `#[expect]` attribute
-    /// works when added to the outer scope. A few lints can only be expected
-    /// on a crate level.
-    ///
-    /// Part of RFC 2383. The progress is being tracked in [#54503]
-    ///
-    /// [#54503]: https://github.com/rust-lang/rust/issues/54503
     pub UNFULFILLED_LINT_EXPECTATIONS,
     Warn,
-    "unfulfilled lint expectation",
-    @feature_gate = rustc_span::sym::lint_reasons;
+    "unfulfilled lint expectation"
 }
 
 declare_lint! {

src/doc/rustc/src/lints/levels.md

@@ -1,12 +1,13 @@
 # Lint Levels
 
-In `rustc`, lints are divided into five *levels*:
+In `rustc`, lints are divided into six *levels*:
 
 1. allow
-2. warn
-3. force-warn
-4. deny
-5. forbid
+2. expect
+3. warn
+4. force-warn
+5. deny
+6. forbid
 
 Each lint has a default level (explained in the lint listing later in this
 chapter), and the compiler has a default warning level. First, let's explain
@@ -33,6 +34,40 @@ But this code violates the `missing_docs` lint.
 These lints exist mostly to be manually turned on via configuration, as we'll
 talk about later in this section.
 
+## expect
+
+Sometimes, it can be helpful to suppress lints, but at the same time ensure that
+the code in question still emits them. The 'expect' level does exactly this. If
+the lint in question is not emitted, the `unfulfilled_lint_expectation` lint
+triggers on the `expect` attribute, notifying you that the expectation is no
+longer fulfilled.
+
+```rust
+fn main() {
+    #[expect(unused_variables)]
+    let unused = "Everyone ignores me";
+
+    #[expect(unused_variables)] // `unused_variables` lint is not emitted
+    let used = "I'm useful";    // the expectation is therefore unfulfilled
+    println!("The `used` value is equal to: {:?}", used);
+}
+```
+
+This will produce the following warning:
+
+```txt
+warning: this lint expectation is unfulfilled
+ --> src/main.rs:7:14
+  |
+7 |     #[expect(unused_variables)]
+  |              ^^^^^^^^^^^^^^^^
+  |
+  = note: `#[warn(unfulfilled_lint_expectations)]` on by default
+```
+
+This level can only be defined via the `#[expect]` attribute, there is no equivalent
+flag. Lints with the special 'force-warn' level will still be emitted as usual.
+
 ## warn
 
 The 'warn' lint level will produce a warning if you violate the lint. For example,
@@ -240,6 +275,21 @@ And use multiple attributes together:
 pub fn foo() {}
 ```
 
+All lint attributes support an additional `reason` parameter, to give context why
+a certain attribute was added. This reason will be displayed as part of the lint
+message, if the lint is emitted at the defined level.
+
+```rust
+use std::path::PathBuf;
+pub fn get_path() -> PathBuf {
+    #[allow(unused_mut, reason = "this is only modified on some platforms")]
+    let mut file_name = PathBuf::from("git");
+    #[cfg(target_os = "windows")]
+    file_name.set_extension("exe");
+    file_name
+}
+```
+
 ### Capping lints
 
 `rustc` supports a flag, `--cap-lints LEVEL` that sets the "lint cap level."

src/tools/clippy/CHANGELOG.md

@@ -1943,7 +1943,7 @@ Released 2022-05-19
   [#8218](https://github.com/rust-lang/rust-clippy/pull/8218)
 * [`needless_match`]
   [#8471](https://github.com/rust-lang/rust-clippy/pull/8471)
-* [`allow_attributes_without_reason`] (Requires `#![feature(lint_reasons)]`)
+* [`allow_attributes_without_reason`]
   [#8504](https://github.com/rust-lang/rust-clippy/pull/8504)
 * [`print_in_format_impl`]
   [#8253](https://github.com/rust-lang/rust-clippy/pull/8253)

src/tools/clippy/book/src/lint_configuration.md

@@ -669,6 +669,8 @@ The minimum rust version that the project supports. Defaults to the `rust-versio
 
 ---
 **Affected lints:**
+* [`allow_attributes`](https://rust-lang.github.io/rust-clippy/master/index.html#allow_attributes)
+* [`allow_attributes_without_reason`](https://rust-lang.github.io/rust-clippy/master/index.html#allow_attributes_without_reason)
 * [`almost_complete_range`](https://rust-lang.github.io/rust-clippy/master/index.html#almost_complete_range)
 * [`approx_constant`](https://rust-lang.github.io/rust-clippy/master/index.html#approx_constant)
 * [`assigning_clones`](https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones)

src/tools/clippy/clippy_config/src/conf.rs

@@ -265,7 +265,7 @@ define_Conf! {
     ///
     /// Suppress lints whenever the suggested change would cause breakage for other crates.
     (avoid_breaking_exported_api: bool = true),
-    /// Lint: MANUAL_SPLIT_ONCE, MANUAL_STR_REPEAT, CLONED_INSTEAD_OF_COPIED, REDUNDANT_FIELD_NAMES, OPTION_MAP_UNWRAP_OR, REDUNDANT_STATIC_LIFETIMES, FILTER_MAP_NEXT, CHECKED_CONVERSIONS, MANUAL_RANGE_CONTAINS, USE_SELF, MEM_REPLACE_WITH_DEFAULT, MANUAL_NON_EXHAUSTIVE, OPTION_AS_REF_DEREF, MAP_UNWRAP_OR, MATCH_LIKE_MATCHES_MACRO, MANUAL_STRIP, MISSING_CONST_FOR_FN, UNNESTED_OR_PATTERNS, FROM_OVER_INTO, PTR_AS_PTR, IF_THEN_SOME_ELSE_NONE, APPROX_CONSTANT, DEPRECATED_CFG_ATTR, INDEX_REFUTABLE_SLICE, MAP_CLONE, BORROW_AS_PTR, MANUAL_BITS, ERR_EXPECT, CAST_ABS_TO_UNSIGNED, UNINLINED_FORMAT_ARGS, MANUAL_CLAMP, MANUAL_LET_ELSE, UNCHECKED_DURATION_SUBTRACTION, COLLAPSIBLE_STR_REPLACE, SEEK_FROM_CURRENT, SEEK_REWIND, UNNECESSARY_LAZY_EVALUATIONS, TRANSMUTE_PTR_TO_REF, ALMOST_COMPLETE_RANGE, NEEDLESS_BORROW, DERIVABLE_IMPLS, MANUAL_IS_ASCII_CHECK, MANUAL_REM_EUCLID, MANUAL_RETAIN, TYPE_REPETITION_IN_BOUNDS, TUPLE_ARRAY_CONVERSIONS, MANUAL_TRY_FOLD, MANUAL_HASH_ONE, ITER_KV_MAP, MANUAL_C_STR_LITERALS, ASSIGNING_CLONES, LEGACY_NUMERIC_CONSTANTS.
+    /// Lint: MANUAL_SPLIT_ONCE, MANUAL_STR_REPEAT, CLONED_INSTEAD_OF_COPIED, REDUNDANT_FIELD_NAMES, OPTION_MAP_UNWRAP_OR, REDUNDANT_STATIC_LIFETIMES, FILTER_MAP_NEXT, CHECKED_CONVERSIONS, MANUAL_RANGE_CONTAINS, USE_SELF, MEM_REPLACE_WITH_DEFAULT, MANUAL_NON_EXHAUSTIVE, OPTION_AS_REF_DEREF, MAP_UNWRAP_OR, MATCH_LIKE_MATCHES_MACRO, MANUAL_STRIP, MISSING_CONST_FOR_FN, UNNESTED_OR_PATTERNS, FROM_OVER_INTO, PTR_AS_PTR, IF_THEN_SOME_ELSE_NONE, APPROX_CONSTANT, DEPRECATED_CFG_ATTR, INDEX_REFUTABLE_SLICE, MAP_CLONE, BORROW_AS_PTR, MANUAL_BITS, ERR_EXPECT, CAST_ABS_TO_UNSIGNED, UNINLINED_FORMAT_ARGS, MANUAL_CLAMP, MANUAL_LET_ELSE, UNCHECKED_DURATION_SUBTRACTION, COLLAPSIBLE_STR_REPLACE, SEEK_FROM_CURRENT, SEEK_REWIND, UNNECESSARY_LAZY_EVALUATIONS, TRANSMUTE_PTR_TO_REF, ALMOST_COMPLETE_RANGE, NEEDLESS_BORROW, DERIVABLE_IMPLS, MANUAL_IS_ASCII_CHECK, MANUAL_REM_EUCLID, MANUAL_RETAIN, TYPE_REPETITION_IN_BOUNDS, TUPLE_ARRAY_CONVERSIONS, MANUAL_TRY_FOLD, MANUAL_HASH_ONE, ITER_KV_MAP, MANUAL_C_STR_LITERALS, ASSIGNING_CLONES, LEGACY_NUMERIC_CONSTANTS, ALLOW_ATTRIBUTES, ALLOW_ATTRIBUTES_WITHOUT_REASON.
     ///
     /// The minimum rust version that the project supports. Defaults to the `rust-version` field in `Cargo.toml`
     #[default_text = ""]

src/tools/clippy/clippy_config/src/msrvs.rs

@@ -17,6 +17,7 @@ macro_rules! msrv_aliases {
 
 // names may refer to stabilized feature flags or library items
 msrv_aliases! {
+    1,81,0  { LINT_REASONS_STABILIZATION }
     1,77,0 { C_STR_LITERALS }
     1,76,0 { PTR_FROM_REF }
     1,71,0 { TUPLE_ARRAY_CONVERSIONS, BUILD_HASHER_HASH_ONE }

src/tools/clippy/clippy_lints/src/attrs/allow_attributes.rs

@@ -0,0 +1,26 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::is_from_proc_macro;
+use rustc_ast::{AttrStyle, Attribute};
+use rustc_errors::Applicability;
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::lint::in_external_macro;
+use super::ALLOW_ATTRIBUTES;
+
+// Separate each crate's features.
+pub fn check<'cx>(cx: &LateContext<'cx>, attr: &'cx Attribute) {
+    if !in_external_macro(cx.sess(), attr.span)
+        && let AttrStyle::Outer = attr.style
+        && let Some(ident) = attr.ident()
+        && !is_from_proc_macro(cx, &attr)
+    {
+        span_lint_and_sugg(
+            cx,
+            ALLOW_ATTRIBUTES,
+            ident.span,
+            "#[allow] attribute found",
+            "replace it with",
+            "expect".into(),
+            Applicability::MachineApplicable,
+        );
+    }
+}

src/tools/clippy/clippy_lints/src/attrs/allow_attributes_without_reason.rs

@@ -8,11 +8,6 @@ use rustc_span::sym;
 use rustc_span::symbol::Symbol;
 
 pub(super) fn check<'cx>(cx: &LateContext<'cx>, name: Symbol, items: &[NestedMetaItem], attr: &'cx Attribute) {
-    // Check for the feature
-    if !cx.tcx.features().lint_reasons {
-        return;
-    }
-
     // Check if the reason is present
     if let Some(item) = items.last().and_then(NestedMetaItem::meta_item)
         && let MetaItemKind::NameValue(_) = &item.kind