Iterate over the smaller list

If there are two lists of different sizes,
iterating over the smaller list and then
looking up in the larger list is cheaper
than vice versa, because lookups scale
sublinearly.

2 files changed, 12 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index cc378f1ea47..ae2ab2b0252 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -265,6 +265,10 @@ impl<'tcx> AssociatedItems<'tcx> {
         self.items.iter().map(|(_, v)| *v)
     }
 
+    pub fn len(&self) -> usize {
+        self.items.len()
+    }
+
     /// Returns an iterator over all associated items with the given name, ignoring hygiene.
     pub fn filter_by_name_unhygienic(
         &self,
diff --git a/compiler/rustc_typeck/src/coherence/inherent_impls_overlap.rs b/compiler/rustc_typeck/src/coherence/inherent_impls_overlap.rs
index be77d049cae..673cddbdb5f 100644
--- a/compiler/rustc_typeck/src/coherence/inherent_impls_overlap.rs
+++ b/compiler/rustc_typeck/src/coherence/inherent_impls_overlap.rs
@@ -22,6 +22,14 @@ impl InherentOverlapChecker<'tcx> {
         let impl_items1 = self.tcx.associated_items(impl1);
         let impl_items2 = self.tcx.associated_items(impl2);
 
+        let mut impl_items1 = &impl_items1;
+        let mut impl_items2 = &impl_items2;
+
+        // Performance optimization: iterate over the smaller list
+        if impl_items1.len() > impl_items2.len() {
+            std::mem::swap(&mut impl_items1, &mut impl_items2);
+        }
+
         for item1 in impl_items1.in_definition_order() {
             let collision = impl_items2.filter_by_name_unhygienic(item1.ident.name).any(|item2| {
                 // Symbols and namespace match, compare hygienically.