diff --git a/src/liballoc/slice.rs b/src/liballoc/slice.rs
index 68f2313843c31..56c53fca62cb1 100644
--- a/src/liballoc/slice.rs
+++ b/src/liballoc/slice.rs
@@ -1400,6 +1400,7 @@ impl<T> [T] {
         let sz_usize = mem::size_of::<(K, usize)>();
 
         let len = self.len();
+        if len < 2 { return }
         if sz_u8  < sz_u16   && len <= ( u8::MAX as usize) { return sort_by_key!( u8, self, f) }
         if sz_u16 < sz_u32   && len <= (u16::MAX as usize) { return sort_by_key!(u16, self, f) }
         if sz_u32 < sz_usize && len <= (u32::MAX as usize) { return sort_by_key!(u32, self, f) }
diff --git a/src/librustc/lib.rs b/src/librustc/lib.rs
index 7da664e6d0255..0b4c3be8b7814 100644
--- a/src/librustc/lib.rs
+++ b/src/librustc/lib.rs
@@ -61,6 +61,7 @@
 #![feature(refcell_replace_swap)]
 #![feature(rustc_diagnostic_macros)]
 #![feature(slice_patterns)]
+#![feature(slice_sort_by_cached_key)]
 #![feature(specialization)]
 #![feature(unboxed_closures)]
 #![feature(trace_macros)]
diff --git a/src/librustc/middle/cstore.rs b/src/librustc/middle/cstore.rs
index add9b621596b6..41334a37dbef6 100644
--- a/src/librustc/middle/cstore.rs
+++ b/src/librustc/middle/cstore.rs
@@ -401,7 +401,7 @@ pub fn used_crates(tcx: TyCtxt, prefer: LinkagePreference)
         .collect::<Vec<_>>();
     let mut ordering = tcx.postorder_cnums(LOCAL_CRATE);
     Lrc::make_mut(&mut ordering).reverse();
-    libs.sort_by_key(|&(a, _)| {
+    libs.sort_by_cached_key(|&(a, _)| {
         ordering.iter().position(|x| *x == a)
     });
     libs
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index 6f88b0aecb6b6..ab710f2e9004d 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -23,6 +23,7 @@
 #![cfg_attr(unix, feature(libc))]
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]
+#![feature(slice_sort_by_cached_key)]
 #![feature(set_stdio)]
 #![feature(rustc_stack_internals)]
 
@@ -83,7 +84,6 @@ use rustc_trans_utils::trans_crate::TransCrate;
 use serialize::json::ToJson;
 
 use std::any::Any;
-use std::cmp::Ordering::Equal;
 use std::cmp::max;
 use std::default::Default;
 use std::env::consts::{DLL_PREFIX, DLL_SUFFIX};
@@ -1177,13 +1177,8 @@ Available lint options:
 
     fn sort_lints(sess: &Session, lints: Vec<(&'static Lint, bool)>) -> Vec<&'static Lint> {
         let mut lints: Vec<_> = lints.into_iter().map(|(x, _)| x).collect();
-        lints.sort_by(|x: &&Lint, y: &&Lint| {
-            match x.default_level(sess).cmp(&y.default_level(sess)) {
-                // The sort doesn't case-fold but it's doubtful we care.
-                Equal => x.name.cmp(y.name),
-                r => r,
-            }
-        });
+        // The sort doesn't case-fold but it's doubtful we care.
+        lints.sort_by_cached_key(|x: &&Lint| (x.default_level(sess), x.name));
         lints
     }
 
diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index 1b208a512e2a4..e40a3057a95cf 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -1414,7 +1414,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
         let mut all_impls: Vec<_> = visitor.impls.into_iter().collect();
 
         // Bring everything into deterministic order for hashing
-        all_impls.sort_unstable_by_key(|&(trait_def_id, _)| {
+        all_impls.sort_by_cached_key(|&(trait_def_id, _)| {
             tcx.def_path_hash(trait_def_id)
         });
 
@@ -1422,7 +1422,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             .into_iter()
             .map(|(trait_def_id, mut impls)| {
                 // Bring everything into deterministic order for hashing
-                impls.sort_unstable_by_key(|&def_index| {
+                impls.sort_by_cached_key(|&def_index| {
                     tcx.hir.definitions().def_path_hash(def_index)
                 });
 
diff --git a/src/librustc_metadata/lib.rs b/src/librustc_metadata/lib.rs
index e89b5a7fc1b4e..76a14cf44f2c2 100644
--- a/src/librustc_metadata/lib.rs
+++ b/src/librustc_metadata/lib.rs
@@ -21,6 +21,7 @@
 #![feature(macro_lifetime_matcher)]
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]
+#![feature(slice_sort_by_cached_key)]
 #![feature(specialization)]
 #![feature(rustc_private)]
 
diff --git a/src/librustc_mir/lib.rs b/src/librustc_mir/lib.rs
index 8762e7550cded..2fe64f492dd8c 100644
--- a/src/librustc_mir/lib.rs
+++ b/src/librustc_mir/lib.rs
@@ -17,6 +17,7 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![deny(warnings)]
 
 #![feature(slice_patterns)]
+#![feature(slice_sort_by_cached_key)]
 #![feature(from_ref)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
diff --git a/src/librustc_mir/monomorphize/partitioning.rs b/src/librustc_mir/monomorphize/partitioning.rs
index da4cb4ec78904..f29f86af4aba7 100644
--- a/src/librustc_mir/monomorphize/partitioning.rs
+++ b/src/librustc_mir/monomorphize/partitioning.rs
@@ -112,11 +112,11 @@ use rustc::ty::{self, TyCtxt, InstanceDef};
 use rustc::ty::item_path::characteristic_def_id_of_type;
 use rustc::util::nodemap::{FxHashMap, FxHashSet};
 use std::collections::hash_map::Entry;
+use std::cmp;
 use syntax::ast::NodeId;
 use syntax::symbol::{Symbol, InternedString};
 use rustc::mir::mono::MonoItem;
 use monomorphize::item::{MonoItemExt, InstantiationMode};
-use core::usize;
 
 pub use rustc::mir::mono::CodegenUnit;
 
@@ -189,11 +189,9 @@ pub trait CodegenUnitExt<'tcx> {
             }, item.symbol_name(tcx))
         }
 
-        let items: Vec<_> = self.items().iter().map(|(&i, &l)| (i, l)).collect();
-        let mut items : Vec<_> = items.iter()
-            .map(|il| (il, item_sort_key(tcx, il.0))).collect();
-        items.sort_by(|&(_, ref key1), &(_, ref key2)| key1.cmp(key2));
-        items.into_iter().map(|(&item_linkage, _)| item_linkage).collect()
+        let mut items: Vec<_> = self.items().iter().map(|(&i, &l)| (i, l)).collect();
+        items.sort_by_cached_key(|&(i, _)| item_sort_key(tcx, i));
+        items
     }
 }
 
@@ -509,7 +507,7 @@ fn merge_codegen_units<'tcx>(initial_partitioning: &mut PreInliningPartitioning<
     // Merge the two smallest codegen units until the target size is reached.
     while codegen_units.len() > target_cgu_count {
         // Sort small cgus to the back
-        codegen_units.sort_by_key(|cgu| usize::MAX - cgu.size_estimate());
+        codegen_units.sort_by_cached_key(|cgu| cmp::Reverse(cgu.size_estimate()));
         let mut smallest = codegen_units.pop().unwrap();
         let second_smallest = codegen_units.last_mut().unwrap();
 
diff --git a/src/librustc_resolve/lib.rs b/src/librustc_resolve/lib.rs
index 2bf17cd1317d4..9ca7d1a372b11 100644
--- a/src/librustc_resolve/lib.rs
+++ b/src/librustc_resolve/lib.rs
@@ -14,6 +14,7 @@
 #![deny(warnings)]
 
 #![feature(rustc_diagnostic_macros)]
+#![feature(slice_sort_by_cached_key)]
 
 #[macro_use]
 extern crate log;
@@ -1150,13 +1151,9 @@ impl<'a> ModuleData<'a> {
 
     fn for_each_child_stable<F: FnMut(Ident, Namespace, &'a NameBinding<'a>)>(&self, mut f: F) {
         let resolutions = self.resolutions.borrow();
-        let mut resolutions = resolutions.iter().map(|(&(ident, ns), &resolution)| {
-                                                    // Pre-compute keys for sorting
-                                                    (ident.name.as_str(), ns, ident, resolution)
-                                                })
-                                                .collect::<Vec<_>>();
-        resolutions.sort_unstable_by_key(|&(str, ns, ..)| (str, ns));
-        for &(_, ns, ident, resolution) in resolutions.iter() {
+        let mut resolutions = resolutions.iter().collect::<Vec<_>>();
+        resolutions.sort_by_cached_key(|&(&(ident, ns), _)| (ident.name.as_str(), ns));
+        for &(&(ident, ns), &resolution) in resolutions.iter() {
             resolution.borrow().binding.map(|binding| f(ident, ns, binding));
         }
     }
@@ -3341,7 +3338,9 @@ impl<'a> Resolver<'a> {
                         let is_mod = |def| match def { Def::Mod(..) => true, _ => false };
                         let mut candidates =
                             self.lookup_import_candidates(name, TypeNS, is_mod);
-                        candidates.sort_by_key(|c| (c.path.segments.len(), c.path.to_string()));
+                        candidates.sort_by_cached_key(|c| {
+                            (c.path.segments.len(), c.path.to_string())
+                        });
                         if let Some(candidate) = candidates.get(0) {
                             format!("Did you mean `{}`?", candidate.path)
                         } else {
@@ -3579,7 +3578,7 @@ impl<'a> Resolver<'a> {
 
         let name = path[path.len() - 1].name;
         // Make sure error reporting is deterministic.
-        names.sort_by_key(|name| name.as_str());
+        names.sort_by_cached_key(|name| name.as_str());
         match find_best_match_for_name(names.iter(), &name.as_str(), None) {
             Some(found) if found != name => Some(found),
             _ => None,
diff --git a/src/librustc_trans/base.rs b/src/librustc_trans/base.rs
index 0329264a3125f..09aba830d050d 100644
--- a/src/librustc_trans/base.rs
+++ b/src/librustc_trans/base.rs
@@ -82,7 +82,8 @@ use std::ffi::CString;
 use std::str;
 use std::sync::Arc;
 use std::time::{Instant, Duration};
-use std::{i32, usize};
+use std::i32;
+use std::cmp;
 use std::sync::mpsc;
 use syntax_pos::Span;
 use syntax_pos::symbol::InternedString;
@@ -830,7 +831,7 @@ pub fn trans_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     // a bit more efficiently.
     let codegen_units = {
         let mut codegen_units = codegen_units;
-        codegen_units.sort_by_key(|cgu| usize::MAX - cgu.size_estimate());
+        codegen_units.sort_by_cached_key(|cgu| cmp::Reverse(cgu.size_estimate()));
         codegen_units
     };
 
diff --git a/src/librustc_trans/lib.rs b/src/librustc_trans/lib.rs
index 344f959c1414c..73c676021ec36 100644
--- a/src/librustc_trans/lib.rs
+++ b/src/librustc_trans/lib.rs
@@ -27,6 +27,7 @@
 #![feature(libc)]
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]
+#![feature(slice_sort_by_cached_key)]
 #![feature(optin_builtin_traits)]
 #![feature(inclusive_range_fields)]
 #![feature(underscore_lifetimes)]
diff --git a/src/librustc_typeck/check/method/probe.rs b/src/librustc_typeck/check/method/probe.rs
index fa2022e8cc994..de5709566225c 100644
--- a/src/librustc_typeck/check/method/probe.rs
+++ b/src/librustc_typeck/check/method/probe.rs
@@ -799,7 +799,7 @@ impl<'a, 'gcx, 'tcx> ProbeContext<'a, 'gcx, 'tcx> {
             .collect();
 
         // sort them by the name so we have a stable result
-        names.sort_by_key(|n| n.as_str());
+        names.sort_by_cached_key(|n| n.as_str());
         names
     }
 
diff --git a/src/librustc_typeck/lib.rs b/src/librustc_typeck/lib.rs
index 6f71db998bd41..2ead5e9d91306 100644
--- a/src/librustc_typeck/lib.rs
+++ b/src/librustc_typeck/lib.rs
@@ -82,6 +82,7 @@ This API is completely unstable and subject to change.
 #![feature(refcell_replace_swap)]
 #![feature(rustc_diagnostic_macros)]
 #![feature(slice_patterns)]
+#![feature(slice_sort_by_cached_key)]
 #![feature(dyn_trait)]
 
 #[macro_use] extern crate log;
diff --git a/src/librustdoc/clean/auto_trait.rs b/src/librustdoc/clean/auto_trait.rs
index a87e1df5efc2c..322527839e02e 100644
--- a/src/librustdoc/clean/auto_trait.rs
+++ b/src/librustdoc/clean/auto_trait.rs
@@ -1435,9 +1435,7 @@ impl<'a, 'tcx, 'rcx> AutoTraitFinder<'a, 'tcx, 'rcx> {
     // involved (impls rarely have more than a few bounds) means that it
     // shouldn't matter in practice.
     fn unstable_debug_sort<T: Debug>(&self, vec: &mut Vec<T>) {
-        vec.sort_unstable_by(|first, second| {
-            format!("{:?}", first).cmp(&format!("{:?}", second))
-        });
+        vec.sort_by_cached_key(|x| format!("{:?}", x))
     }
 
     fn is_fn_ty(&self, tcx: &TyCtxt, ty: &Type) -> bool {
diff --git a/src/librustdoc/lib.rs b/src/librustdoc/lib.rs
index 42e87f88fd40d..e34cf6222cf83 100644
--- a/src/librustdoc/lib.rs
+++ b/src/librustdoc/lib.rs
@@ -20,6 +20,7 @@
 #![feature(box_syntax)]
 #![feature(fs_read_write)]
 #![feature(set_stdio)]
+#![feature(slice_sort_by_cached_key)]
 #![feature(test)]
 #![feature(unicode)]
 #![feature(vec_remove_item)]
diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index c456dc45d2182..7774095e20421 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -22,6 +22,7 @@
 
 #![feature(unicode)]
 #![feature(rustc_diagnostic_macros)]
+#![feature(slice_sort_by_cached_key)]
 #![feature(non_exhaustive)]
 #![feature(const_atomic_usize_new)]
 #![feature(rustc_attrs)]
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index e6da5bcaa3ae6..6ed7088d0521d 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -689,7 +689,7 @@ impl<'a> Parser<'a> {
                 .chain(inedible.iter().map(|x| TokenType::Token(x.clone())))
                 .chain(self.expected_tokens.iter().cloned())
                 .collect::<Vec<_>>();
-            expected.sort_by(|a, b| a.to_string().cmp(&b.to_string()));
+            expected.sort_by_cached_key(|x| x.to_string());
             expected.dedup();
             let expect = tokens_to_string(&expected[..]);
             let actual = self.this_token_to_string();