Fix if_let_mutex not checking Mutexes behind refs

Fixes rust-lang#9193

clippy_lints/src/if_let_mutex.rs

@@ -129,7 +129,7 @@ fn is_mutex_lock_call<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Opt
     if_chain! {
         if let ExprKind::MethodCall(path, [self_arg, ..], _) = &expr.kind;
         if path.ident.as_str() == "lock";
-        let ty = cx.typeck_results().expr_ty(self_arg);
+        let ty = cx.typeck_results().expr_ty(self_arg).peel_refs();
         if is_type_diagnostic_item(cx, ty, sym::Mutex);
         then {
             Some(self_arg)

tests/ui/if_let_mutex.rs

@@ -39,4 +39,12 @@ fn if_let_different_mutex() {
     };
 }
 
+fn mutex_ref(mutex: &Mutex<i32>) {
+    if let Ok(i) = mutex.lock() {
+        do_stuff(i);
+    } else {
+        let _x = mutex.lock();
+    };
+}
+
 fn main() {}

tests/ui/if_let_mutex.stderr

@@ -25,5 +25,17 @@ LL | |     };
    |
    = help: move the lock call outside of the `if let ...` expression
 
-error: aborting due to 2 previous errors
+error: calling `Mutex::lock` inside the scope of another `Mutex::lock` causes a deadlock
+  --> $DIR/if_let_mutex.rs:43:5
+   |
+LL | /     if let Ok(i) = mutex.lock() {
+LL | |         do_stuff(i);
+LL | |     } else {
+LL | |         let _x = mutex.lock();
+LL | |     };
+   | |_____^
+   |
+   = help: move the lock call outside of the `if let ...` expression
+
+error: aborting due to 3 previous errors