Support composing two autoderef adjustments

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -337,9 +337,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         *entry.get_mut() = adj;
                     }
 
+                    (
+                        &mut [.., Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(..)), .. }],
+                        &[Adjustment { kind: Adjust::Deref(_), .. }, ..],
+                    ) => {
+                        // A reborrow has no effect before a dereference, so we can safely merge adjustments.
+                        // A more general version of the above case.
+                        let entry_mut = entry.get_mut();
+                        entry_mut.pop();
+                        entry_mut.extend_from_slice(&adj[1..]);
+                    }
+
                     _ => {
-                        // FIXME: currently we never try to compose autoderefs
-                        // and ReifyFnPointer/UnsafeFnPointer, but we could.
+                        // FIXME: currently we never try to compose ReifyFnPointer/UnsafeFnPointer, but we could.
                         self.dcx().span_delayed_bug(
                             expr.span,
                             format!(

tests/ui/coercion/compose_autoderefs.rs

@@ -0,0 +1,32 @@
+//@ edition:2021
+//@ check-pass
+
+use core::ops::Deref;
+
+struct A;
+struct B;
+struct C;
+
+impl Deref for C {
+    type Target = B;
+    fn deref(&self) -> &Self::Target {
+        &B
+    }
+}
+
+impl Deref for B {
+    type Target = A;
+    fn deref(&self) -> &Self::Target {
+        &A
+    }
+}
+
+fn f(v: u8) {
+    let _ = match v {
+        0 => &C,
+        1 => &B,
+        _ => &A,
+    };
+}
+
+fn main() {}