Rollup merge of #44590 - oli-obk:allow_unused_mut_on_vars, r=eddyb

Get `allow(unused_mut)` to work on `let` bindings

fixes #40491

src/test/compile-fail/lint-unused-mut-variables.rs

@@ -110,3 +110,11 @@ fn foo(mut a: isize) {
     let mut a = 3;
     let mut b = vec![2];
 }
+
+// make sure the lint attribute can be turned off on let statements
+#[deny(unused_mut)]
+fn bar() {
+    #[allow(unused_mut)]
+    let mut a = 3;
+    let mut b = vec![2]; //~ ERROR: variable does not need to be mutable
+}