Short circuit full corherence check when dealing with types with different reference mutability

Author: rylev

compiler/rustc_middle/src/ty/sty.rs

@@ -1837,6 +1837,15 @@ impl<'tcx> TyS<'tcx> {
         )
     }
 
+    /// Get the mutability of the reference or `None` when not a reference
+    #[inline]
+    pub fn ref_mutability(&self) -> Option<hir::Mutability> {
+        match self.kind() {
+            Ref(_, _, mutability) => Some(*mutability),
+            _ => None,
+        }
+    }
+
     #[inline]
     pub fn is_unsafe_ptr(&self) -> bool {
         matches!(self.kind(), RawPtr(_))

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -75,18 +75,19 @@ where
     let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
     let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
 
-    // Check if any of the input types definitely mismatch.
+    // Check if any of the input types definitely do not unify.
     if impl1_ref
         .iter()
         .flat_map(|tref| tref.substs.types())
         .zip(impl2_ref.iter().flat_map(|tref| tref.substs.types()))
         .chain(iter::once((impl1_self, impl2_self)))
         .any(|(ty1, ty2)| {
-            let ty1 = fast_reject::simplify_type(tcx, ty1, false);
-            let ty2 = fast_reject::simplify_type(tcx, ty2, false);
-            if let (Some(ty1), Some(ty2)) = (ty1, ty2) {
+            let t1 = fast_reject::simplify_type(tcx, ty1, false);
+            let t2 = fast_reject::simplify_type(tcx, ty2, false);
+            if let (Some(t1), Some(t2)) = (t1, t2) {
                 // Simplified successfully
-                ty1 != ty2
+                // Types cannot unify if they differ in their reference mutability or simplify to different types
+                ty1.ref_mutability() != ty2.ref_mutability() || t1 != t2
             } else {
                 // Types might unify
                 false