auto merge of #12366 : aepsil0n/rust/feature/unnecessary_parens_around_assigned_values, r=alexcrichton

Fixes #12350.

Parentheses around assignment statements such as

```rust
let mut a = (0);
a = (1);
a += (2);
```

are not necessary and therefore an unnecessary_parens warning is raised when
statements like this occur.

NOTE: In `let` declarations this does not work as intended. Is it possible that they do not count as assignment expressions (`ExprAssign`)? (edit: this is fixed by now)

Furthermore, there are some cases that I fixed in the rest of the code, where parentheses could potentially enhance readability. Compare these lines:

```rust
a = b == c;
a = (b == c);
```

Thus, after having worked on this I'm not entirely sure, whether we should go through with this patch or not. Probably a matter of debate. ;)

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libtest/lib.rs b/src/libtest/lib.rs
index 19c2f80cdbd..eba922ac7b8 100644
--- a/src/libtest/lib.rs
+++ b/src/libtest/lib.rs
@@ -1047,7 +1047,7 @@ impl MetricMap {
             let r = match selfmap.find(k) {
                 None => MetricRemoved,
                 Some(v) => {
-                    let delta = (v.value - vold.value);
+                    let delta = v.value - vold.value;
                     let noise = match noise_pct {
                         None => f64::max(vold.noise.abs(), v.noise.abs()),
                         Some(pct) => vold.value * pct / 100.0