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.

Author: bors

src/liballoc/btree/node.rs

@@ -1037,7 +1037,7 @@ impl<'a, K: 'a, V: 'a, NodeType>
         Handle<NodeRef<marker::Mut<'a>, K, V, NodeType>, marker::KV> {
 
     pub fn into_kv_mut(self) -> (&'a mut K, &'a mut V) {
-        let (mut keys, mut vals) = self.node.into_slices_mut();
+        let (keys, vals) = self.node.into_slices_mut();
         unsafe {
             (keys.get_unchecked_mut(self.idx), vals.get_unchecked_mut(self.idx))
         }
@@ -1047,7 +1047,7 @@ impl<'a, K: 'a, V: 'a, NodeType>
 impl<'a, K, V, NodeType> Handle<NodeRef<marker::Mut<'a>, K, V, NodeType>, marker::KV> {
     pub fn kv_mut(&mut self) -> (&mut K, &mut V) {
         unsafe {
-            let (mut keys, mut vals) = self.node.reborrow_mut().into_slices_mut();
+            let (keys, vals) = self.node.reborrow_mut().into_slices_mut();
             (keys.get_unchecked_mut(self.idx), vals.get_unchecked_mut(self.idx))
         }
     }

src/liballoc/vec.rs

@@ -1751,7 +1751,7 @@ impl<'a, T> IntoIterator for &'a mut Vec<T> {
     type Item = &'a mut T;
     type IntoIter = slice::IterMut<'a, T>;
 
-    fn into_iter(mut self) -> slice::IterMut<'a, T> {
+    fn into_iter(self) -> slice::IterMut<'a, T> {
         self.iter_mut()
     }
 }

src/liballoc/vec_deque.rs

@@ -2394,7 +2394,7 @@ impl<'a, T> IntoIterator for &'a mut VecDeque<T> {
     type Item = &'a mut T;
     type IntoIter = IterMut<'a, T>;
 
-    fn into_iter(mut self) -> IterMut<'a, T> {
+    fn into_iter(self) -> IterMut<'a, T> {
         self.iter_mut()
     }
 }
@@ -2558,7 +2558,7 @@ impl<'a, T> Place<T> for PlaceBack<'a, T> {
 impl<'a, T> InPlace<T> for PlaceBack<'a, T> {
     type Owner = &'a mut T;
 
-    unsafe fn finalize(mut self) -> &'a mut T {
+    unsafe fn finalize(self) -> &'a mut T {
         let head = self.vec_deque.head;
         self.vec_deque.head = self.vec_deque.wrap_add(head, 1);
         &mut *(self.vec_deque.ptr().offset(head as isize))
@@ -2605,7 +2605,7 @@ impl<'a, T> Place<T> for PlaceFront<'a, T> {
 impl<'a, T> InPlace<T> for PlaceFront<'a, T> {
     type Owner = &'a mut T;
 
-    unsafe fn finalize(mut self) -> &'a mut T {
+    unsafe fn finalize(self) -> &'a mut T {
         self.vec_deque.tail = self.vec_deque.wrap_sub(self.vec_deque.tail, 1);
         &mut *(self.vec_deque.ptr().offset(self.vec_deque.tail as isize))
     }

src/libcore/ops/function.rs

@@ -187,7 +187,7 @@ mod impls {
         where F : FnMut<A>
     {
         type Output = F::Output;
-        extern "rust-call" fn call_once(mut self, args: A) -> F::Output {
+        extern "rust-call" fn call_once(self, args: A) -> F::Output {
             (*self).call_mut(args)
         }
     }

src/libcore/option.rs

@@ -872,7 +872,7 @@ impl<'a, T> IntoIterator for &'a mut Option<T> {
     type Item = &'a mut T;
     type IntoIter = IterMut<'a, T>;
 
-    fn into_iter(mut self) -> IterMut<'a, T> {
+    fn into_iter(self) -> IterMut<'a, T> {
         self.iter_mut()
     }
 }

src/libcore/result.rs

@@ -909,7 +909,7 @@ impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
     type Item = &'a mut T;
     type IntoIter = IterMut<'a, T>;
 
-    fn into_iter(mut self) -> IterMut<'a, T> {
+    fn into_iter(self) -> IterMut<'a, T> {
         self.iter_mut()
     }
 }

src/libcore/tests/slice.rs

@@ -105,27 +105,27 @@ fn test_chunks_last() {
 
 #[test]
 fn test_chunks_mut_count() {
-    let mut v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
     let c = v.chunks_mut(3);
     assert_eq!(c.count(), 2);
 
-    let mut v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
     let c2 = v2.chunks_mut(2);
     assert_eq!(c2.count(), 3);
 
-    let mut v3: &mut [i32] = &mut [];
+    let v3: &mut [i32] = &mut [];
     let c3 = v3.chunks_mut(2);
     assert_eq!(c3.count(), 0);
 }
 
 #[test]
 fn test_chunks_mut_nth() {
-    let mut v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
     let mut c = v.chunks_mut(2);
     assert_eq!(c.nth(1).unwrap()[1], 3);
     assert_eq!(c.next().unwrap()[0], 4);
 
-    let mut v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
     let mut c2 = v2.chunks_mut(3);
     assert_eq!(c2.nth(1).unwrap()[1], 4);
     assert_eq!(c2.next(), None);
@@ -194,7 +194,7 @@ fn get_range() {
 
 #[test]
 fn get_mut_range() {
-    let mut v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
     assert_eq!(v.get_mut(..), Some(&mut [0, 1, 2, 3, 4, 5][..]));
     assert_eq!(v.get_mut(..2), Some(&mut [0, 1][..]));
     assert_eq!(v.get_mut(2..), Some(&mut [2, 3, 4, 5][..]));

src/librustc/infer/error_reporting/mod.rs

@@ -415,8 +415,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     /// -------- this type is the same as a type argument in the other type, not highlighted
     /// ```
     fn highlight_outer(&self,
-                       mut value: &mut DiagnosticStyledString,
-                       mut other_value: &mut DiagnosticStyledString,
+                       value: &mut DiagnosticStyledString,
+                       other_value: &mut DiagnosticStyledString,
                        name: String,
                        sub: &ty::subst::Substs<'tcx>,
                        pos: usize,

src/librustc/traits/mod.rs

@@ -560,7 +560,7 @@ pub fn fully_normalize<'a, 'gcx, 'tcx, T>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 {
     debug!("fully_normalize(value={:?})", value);
 
-    let mut selcx = &mut SelectionContext::new(infcx);
+    let selcx = &mut SelectionContext::new(infcx);
     // FIXME (@jroesch) ISSUE 26721
     // I'm not sure if this is a bug or not, needs further investigation.
     // It appears that by reusing the fulfillment_cx here we incur more

src/librustc/traits/select.rs

@@ -494,7 +494,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             never_obligation.predicate = never_obligation.predicate.map_bound(|mut trait_pred| {
                 // Swap out () with ! so we can check if the trait is impld for !
                 {
-                    let mut trait_ref = &mut trait_pred.trait_ref;
+                    let trait_ref = &mut trait_pred.trait_ref;
                     let unit_substs = trait_ref.substs;
                     let mut never_substs = Vec::with_capacity(unit_substs.len());
                     never_substs.push(From::from(tcx.types.never));