Add unnecessary_import_braces lint.

The lint checks any unnecessary braces around one imported item like
`use std::num::{abs};`.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>

src/librustc/lint/builtin.rs

@@ -1105,6 +1105,41 @@ impl LintPass for UnnecessaryParens {
     }
 }
 
+declare_lint!(UNNECESSARY_IMPORT_BRACES, Allow,
+              "unnecessary braces around an imported item")
+
+pub struct UnnecessaryImportBraces;
+
+impl LintPass for UnnecessaryImportBraces {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(UNNECESSARY_IMPORT_BRACES)
+    }
+
+    fn check_view_item(&mut self, cx: &Context, view_item: &ast::ViewItem) {
+        match view_item.node {
+            ast::ViewItemUse(ref view_path) => {
+                match view_path.node {
+                    ast::ViewPathList(_, ref items, _) => {
+                        if items.len() == 1 {
+                            match items[0].node {
+                                ast::PathListIdent {ref name, ..} => {
+                                    let m = format!("braces around {} is unnecessary",
+                                                    token::get_ident(*name).get());
+                                    cx.span_lint(UNNECESSARY_IMPORT_BRACES, view_item.span,
+                                                 m.as_slice());
+                                },
+                                _ => ()
+                            }
+                        }
+                    }
+                    _ => ()
+                }
+            },
+            _ => ()
+        }
+    }
+}
+
 declare_lint!(UNUSED_UNSAFE, Warn,
               "unnecessary use of an `unsafe` block")
 

src/librustc/lint/context.rs

@@ -183,6 +183,7 @@ impl LintStore {
                      NonSnakeCase,
                      NonUppercaseStatics,
                      UnnecessaryParens,
+                     UnnecessaryImportBraces,
                      UnusedUnsafe,
                      UnsafeBlock,
                      UnusedMut,

src/test/compile-fail/lint-unnecessary-import-braces.rs

@@ -0,0 +1,21 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![deny(unnecessary_import_braces)]
+#![allow(dead_code)]
+#![allow(unused_imports)]
+
+use test::{A}; //~ ERROR braces around A is unnecessary
+
+mod test {
+    pub struct A;
+}
+
+fn main() {}