fix: handle ref mut bindings in contains_explicit_ref_binding

the standalone `contains_explicit_ref_binding` function only checked for
`BindingAnnotation::Ref`, missing `BindingAnnotation::RefMut`. this caused
`let ref mut x = expr` to incorrectly take the coercion path instead of
preserving the exact type of the rhs expression. the method version used
for match arms already handles both `Ref` and `RefMut` correctly.

Author: Albab-Hasan

crates/hir-ty/src/infer/pat.rs

@@ -673,7 +673,7 @@ impl<'db> InferenceContext<'_, 'db> {
 pub(super) fn contains_explicit_ref_binding(body: &Body, pat_id: PatId) -> bool {
     let mut res = false;
     body.walk_pats(pat_id, &mut |pat| {
-        res |= matches!(body[pat], Pat::Bind { id, .. } if body[id].mode == BindingAnnotation::Ref);
+        res |= matches!(body[pat], Pat::Bind { id, .. } if matches!(body[id].mode, BindingAnnotation::Ref | BindingAnnotation::RefMut));
     });
     res
 }

crates/hir-ty/src/tests/never_type.rs

@@ -760,6 +760,32 @@ fn coerce_ref_binding() -> ! {
     )
 }
 
+#[test]
+fn diverging_place_match_ref_mut() {
+    check_infer_with_mismatches(
+        r#"
+//- minicore: sized
+fn coerce_ref_mut_binding() -> ! {
+    unsafe {
+        let x: *mut ! = 0 as _;
+        let ref mut _x: () = *x;
+    }
+}
+"#,
+        expect![[r#"
+            33..120 '{     ...   } }': !
+            39..118 'unsafe...     }': !
+            60..61 'x': *mut !
+            72..73 '0': i32
+            72..78 '0 as _': *mut !
+            92..102 'ref mut _x': &'? mut ()
+            109..111 '*x': !
+            110..111 'x': *mut !
+            109..111: expected (), got !
+        "#]],
+    )
+}
+
 #[test]
 fn never_place_isnt_diverging() {
     check_infer_with_mismatches(