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.

src/libstd/collections/hash/map.rs

@@ -1618,7 +1618,7 @@ impl<'a, K, V, S> IntoIterator for &'a mut HashMap<K, V, S>
     type Item = (&'a K, &'a mut V);
     type IntoIter = IterMut<'a, K, V>;
 
-    fn into_iter(mut self) -> IterMut<'a, K, V> {
+    fn into_iter(self) -> IterMut<'a, K, V> {
         self.iter_mut()
     }
 }

src/libstd/collections/hash/table.rs

@@ -563,7 +563,7 @@ impl<'t, K, V> FullBucket<K, V, &'t mut RawTable<K, V>> {
     ///
     /// This works similarly to `put`, building an `EmptyBucket` out of the
     /// taken bucket.
-    pub fn take(mut self) -> (EmptyBucket<K, V, &'t mut RawTable<K, V>>, K, V) {
+    pub fn take(self) -> (EmptyBucket<K, V, &'t mut RawTable<K, V>>, K, V) {
         self.table.size -= 1;
 
         unsafe {

src/libstd/error.rs

@@ -514,7 +514,7 @@ mod tests {
     #[test]
     fn downcasting() {
         let mut a = A;
-        let mut a = &mut a as &mut (Error + 'static);
+        let a = &mut a as &mut (Error + 'static);
         assert_eq!(a.downcast_ref::<A>(), Some(&A));
         assert_eq!(a.downcast_ref::<B>(), None);
         assert_eq!(a.downcast_mut::<A>(), Some(&mut A));

src/libstd/io/cursor.rs

@@ -456,7 +456,7 @@ mod tests {
     #[test]
     fn test_slice_reader() {
         let in_buf = vec![0, 1, 2, 3, 4, 5, 6, 7];
-        let mut reader = &mut &in_buf[..];
+        let reader = &mut &in_buf[..];
         let mut buf = [];
         assert_eq!(reader.read(&mut buf).unwrap(), 0);
         let mut buf = [0];

src/libstd/sync/once.rs

@@ -267,7 +267,7 @@ impl Once {
     #[cold]
     fn call_inner(&'static self,
                   ignore_poisoning: bool,
-                  mut init: &mut FnMut(bool)) {
+                  init: &mut FnMut(bool)) {
         let mut state = self.state.load(Ordering::SeqCst);
 
         'outer: loop {

src/libstd/sys/unix/rand.rs

@@ -12,13 +12,13 @@ pub use self::imp::OsRng;
 
 use mem;
 
-fn next_u32(mut fill_buf: &mut FnMut(&mut [u8])) -> u32 {
+fn next_u32(fill_buf: &mut FnMut(&mut [u8])) -> u32 {
     let mut buf: [u8; 4] = [0; 4];
     fill_buf(&mut buf);
     unsafe { mem::transmute::<[u8; 4], u32>(buf) }
 }
 
-fn next_u64(mut fill_buf: &mut FnMut(&mut [u8])) -> u64 {
+fn next_u64(fill_buf: &mut FnMut(&mut [u8])) -> u64 {
     let mut buf: [u8; 8] = [0; 8];
     fill_buf(&mut buf);
     unsafe { mem::transmute::<[u8; 8], u64>(buf) }

src/libstd/sys/windows/fs.rs

@@ -768,8 +768,8 @@ fn symlink_junction_inner(target: &Path, junction: &Path) -> io::Result<()> {
 
     unsafe {
         let mut data = [0u8; c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE];
-        let mut db = data.as_mut_ptr()
-                        as *mut c::REPARSE_MOUNTPOINT_DATA_BUFFER;
+        let db = data.as_mut_ptr()
+                    as *mut c::REPARSE_MOUNTPOINT_DATA_BUFFER;
         let buf = &mut (*db).ReparseTarget as *mut _;
         let mut i = 0;
         // FIXME: this conversion is very hacky