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. ;)

3 files changed, 3 insertions, 3 deletions

diff --git a/src/libsyntax/abi.rs b/src/libsyntax/abi.rs
index 6b0f2c6e516..82e4d08d077 100644
--- a/src/libsyntax/abi.rs
+++ b/src/libsyntax/abi.rs
@@ -202,7 +202,7 @@ impl AbiSet {
     }
 
     pub fn add(&mut self, abi: Abi) {
-        self.bits |= (1 << abi.index());
+        self.bits |= 1 << abi.index();
     }
 
     pub fn each(&self, op: |abi: Abi| -> bool) -> bool {
diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index e8edc1a0dfc..c436dc018e7 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1224,6 +1224,6 @@ mod test {
             },
         };
         // doesn't matter which encoder we use....
-        let _f = (&e as &serialize::Encodable<json::Encoder>);
+        let _f = &e as &serialize::Encodable<json::Encoder>;
     }
 }
diff --git a/src/libsyntax/diagnostic.rs b/src/libsyntax/diagnostic.rs
index 9455df063f1..8cf0f128d22 100644
--- a/src/libsyntax/diagnostic.rs
+++ b/src/libsyntax/diagnostic.rs
@@ -329,7 +329,7 @@ fn highlight_lines(cm: &codemap::CodeMap,
         for _ in range(0, skip) { s.push_char(' '); }
         let orig = fm.get_line(lines.lines[0] as int);
         for pos in range(0u, left-skip) {
-            let curChar = (orig[pos] as char);
+            let curChar = orig[pos] as char;
             // Whenever a tab occurs on the previous line, we insert one on
             // the error-point-squiggly-line as well (instead of a space).
             // That way the squiggly line will usually appear in the correct