Fast path for coercions of TY == TY

Author: compiler-errors

compiler/rustc_hir_typeck/src/coercion.rs

@@ -206,6 +206,30 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             return self.coerce_from_inference_variable(a, b);
         }
 
+        // No coercion needed; types are identical. The only exception is when we have
+        // a mutable reference, which needs a pair of reborrow adjustments inserted.
+        if a == b {
+            if let ty::Ref(_, pointee, ty::Mutability::Mut) = *a.kind() {
+                return Ok(InferOk {
+                    value: (
+                        vec![
+                            Adjustment { kind: Adjust::Deref(None), target: pointee },
+                            Adjustment {
+                                kind: Adjust::Borrow(AutoBorrow::Ref(AutoBorrowMutability::Mut {
+                                    allow_two_phase_borrow: AllowTwoPhase::No,
+                                })),
+                                target: a,
+                            },
+                        ],
+                        a,
+                    ),
+                    obligations: Default::default(),
+                });
+            } else {
+                return Ok(InferOk { value: (vec![], a), obligations: Default::default() });
+            }
+        }
+
         // Consider coercing the subtype to a DST
         //
         // NOTE: this is wrapped in a `commit_if_ok` because it creates

tests/ui/nll/issue-52663-trait-object.stderr

@@ -6,7 +6,6 @@ LL |         let tmp0 = 3;
 LL |         let tmp1 = &tmp0;
    |                    ^^^^^ borrowed value does not live long enough
 LL |         Box::new(tmp1) as Box<dyn Foo + '_>
-   |         ----------------------------------- borrow later captured here by trait object
 LL |     };
    |     - `tmp0` dropped here while still borrowed
 

tests/ui/span/regions-close-over-type-parameter-2.stderr

@@ -6,7 +6,6 @@ LL |         let tmp0 = 3;
 LL |         let tmp1 = &tmp0;
    |                    ^^^^^ borrowed value does not live long enough
 LL |         repeater3(tmp1)
-   |         --------------- borrow later captured here by trait object
 LL |     };
    |     - `tmp0` dropped here while still borrowed