Auto merge of #48605 - KiChjang:unused-mut-warning, r=nikomatsakis

Allow MIR borrowck to catch unused mutable locals

Fixes #47279.

r? @nikomatsakis

src/test/run-pass/borrowck/borrowck-unused-mut-locals.rs

@@ -0,0 +1,56 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(nll)]
+#![deny(unused_mut)]
+
+#[derive(Debug)]
+struct A {}
+
+fn init_a() -> A {
+    A {}
+}
+
+#[derive(Debug)]
+struct B<'a> {
+    ed: &'a mut A,
+}
+
+fn init_b<'a>(ed: &'a mut A) -> B<'a> {
+    B { ed }
+}
+
+#[derive(Debug)]
+struct C<'a> {
+    pd: &'a mut B<'a>,
+}
+
+fn init_c<'a>(pd: &'a mut B<'a>) -> C<'a> {
+    C { pd }
+}
+
+#[derive(Debug)]
+struct D<'a> {
+    sd: &'a mut C<'a>,
+}
+
+fn init_d<'a>(sd: &'a mut C<'a>) -> D<'a> {
+    D { sd }
+}
+
+fn main() {
+    let mut a = init_a();
+    let mut b = init_b(&mut a);
+    let mut c = init_c(&mut b);
+
+    let d = init_d(&mut c);
+
+    println!("{:?}", d)
+}