Add UI test for issue 73592

Author: nbdd0121

Diff for: src/test/ui/typeck/issue-73592-borrow_mut-through-deref.rs

@@ -0,0 +1,58 @@
+// check-pass
+//
+// rust-lang/rust#73592: borrow_mut through Deref should work.
+//
+// Before #72280, when we see something like `&mut *rcvr.method()`, we
+// incorrectly requires `rcvr` to be type-checked as a mut place. While this
+// requirement is usually correct for smart pointers, it is overly restrictive
+// for types like `Mutex` or `RefCell` which can produce a guard that
+// implements `DerefMut` from `&self`.
+//
+// Making it more confusing, because we use Deref as the fallback when DerefMut
+// is implemented, we won't see an issue when the smart pointer does not
+// implement `DerefMut`. It only causes an issue when `rcvr` is obtained via a
+// type that implements both `Deref` or `DerefMut`.
+//
+// This bug is only discovered in #73592 after it is already fixed as a side-effect
+// of a refactoring made in #72280.
+
+#![warn(unused_mut)]
+
+use std::pin::Pin;
+use std::cell::RefCell;
+
+struct S(RefCell<()>);
+
+fn test_pin(s: Pin<&S>) {
+    // This works before #72280.
+    let _ = &mut *s.0.borrow_mut();
+}
+
+fn test_pin_mut(s: Pin<&mut S>) {
+    // This should compile but didn't before #72280.
+    let _ = &mut *s.0.borrow_mut();
+}
+
+fn test_vec(s: &Vec<RefCell<()>>) {
+    // This should compile but didn't before #72280.
+    let _ = &mut *s[0].borrow_mut();
+}
+
+fn test_mut_pin(mut s: Pin<&S>) {
+    //~^ WARN variable does not need to be mutable
+    let _ = &mut *s.0.borrow_mut();
+}
+
+fn test_mut_pin_mut(mut s: Pin<&mut S>) {
+    //~^ WARN variable does not need to be mutable
+    let _ = &mut *s.0.borrow_mut();
+}
+
+fn main() {
+    let mut s = S(RefCell::new(()));
+    test_pin(Pin::new(&s));
+    test_pin_mut(Pin::new(&mut s));
+    test_mut_pin(Pin::new(&s));
+    test_mut_pin_mut(Pin::new(&mut s));
+    test_vec(&vec![s.0]);
+}

Diff for: src/test/ui/typeck/issue-73592-borrow_mut-through-deref.stderr

@@ -0,0 +1,24 @@
+warning: variable does not need to be mutable
+  --> $DIR/issue-73592-borrow_mut-through-deref.rs:41:17
+   |
+LL | fn test_mut_pin(mut s: Pin<&S>) {
+   |                 ----^
+   |                 |
+   |                 help: remove this `mut`
+   |
+note: the lint level is defined here
+  --> $DIR/issue-73592-borrow_mut-through-deref.rs:19:9
+   |
+LL | #![warn(unused_mut)]
+   |         ^^^^^^^^^^
+
+warning: variable does not need to be mutable
+  --> $DIR/issue-73592-borrow_mut-through-deref.rs:46:21
+   |
+LL | fn test_mut_pin_mut(mut s: Pin<&mut S>) {
+   |                     ----^
+   |                     |
+   |                     help: remove this `mut`
+
+warning: 2 warnings emitted
+