Auto merge of #43582 - ivanbakel:unused_mut_ref, r=arielb1

Fixed mutable vars being marked used when they weren't

#### NB : bootstrapping is slow on my machine, even with `keep-stage` - fixes for occurances in the current codebase are <s>in the pipeline</s> done. This PR is being put up for review of the fix of the issue.

Fixes #43526, Fixes #30280, Fixes #25049

### Issue
Whenever the compiler detected a mutable deref being used mutably, it marked an associated value as being used mutably as well. In the case of derefencing local variables which were mutable references, this incorrectly marked the reference itself being used mutably, instead of its contents - with the consequence of making the following code emit no warnings
```
fn do_thing<T>(mut arg : &mut T) {
    ... // don't touch arg - just deref it to access the T
}
```

### Fix
Make dereferences not be counted as a mutable use, but only when they're on borrows on local variables.
#### Why not on things other than local variables?
  * Whenever you capture a variable in a closure, it gets turned into a hidden reference - when you use it in the closure, it gets dereferenced. If the closure uses the variable mutably, that is actually a mutable use of the thing being dereffed to, so it has to be counted.
  * If you deref a mutable `Box` to access the contents mutably, you are using the `Box` mutably - so it has to be counted.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libsyntax_ext/format.rs b/src/libsyntax_ext/format.rs
index 9734bb867f1..16b06424c92 100644
--- a/src/libsyntax_ext/format.rs
+++ b/src/libsyntax_ext/format.rs
@@ -529,7 +529,7 @@ impl<'a, 'b> Context<'a, 'b> {
 
     /// Actually builds the expression which the format_args! block will be
     /// expanded to
-    fn into_expr(mut self) -> P<ast::Expr> {
+    fn into_expr(self) -> P<ast::Expr> {
         let mut locals = Vec::new();
         let mut counts = Vec::new();
         let mut pats = Vec::new();