Rollup merge of #54787 - varkor:unused-mut-in-desugaring, r=nikomatsakis

Only warn about unused `mut` in user-written code

Fixes https://github.com/rust-lang/rust/issues/54586.

r? @pnkfelix
cc @blitzerr

src/test/ui/mut/no-mut-lint-for-desugared-mut.rs

@@ -0,0 +1,8 @@
+// run-pass
+
+#![deny(unused_mut)]
+#![allow(unreachable_code)]
+
+fn main() {
+    for _ in { return (); 0..3 } {} // ok
+}