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, 2 insertions, 2 deletions

diff --git a/src/libstd/os.rs b/src/libstd/os.rs
index 458e31fd86f..fdd81179325 100644
--- a/src/libstd/os.rs
+++ b/src/libstd/os.rs
@@ -119,7 +119,7 @@ pub mod win32 {
                 } else if k == n &&
                           libc::GetLastError() ==
                           libc::ERROR_INSUFFICIENT_BUFFER as DWORD {
-                    n *= (2 as DWORD);
+                    n *= 2 as DWORD;
                 } else if k >= n {
                     n = k;
                 } else {
@@ -225,7 +225,7 @@ pub fn env_as_bytes() -> ~[(~[u8],~[u8])] {
             for p in input.iter() {
                 let vs: ~[&[u8]] = p.splitn(1, |b| *b == '=' as u8).collect();
                 let key = vs[0].to_owned();
-                let val = (if vs.len() < 2 { ~[] } else { vs[1].to_owned() });
+                let val = if vs.len() < 2 { ~[] } else { vs[1].to_owned() };
                 pairs.push((key, val));
             }
             pairs