Implement the double_parens lint.

Author: theemathas

clippy_lints/src/double_parens.rs

@@ -1,5 +1,5 @@
 use syntax::ast::*;
-use rustc::lint::{EarlyContext, LintArray, LintPass, EarlyLintPass};
+use rustc::lint::{EarlyContext, LintContext, LintArray, LintPass, EarlyLintPass};
 
 /// **What it does:** Checks for unnecessary double parentheses.
 ///
@@ -30,6 +30,33 @@ impl LintPass for DoubleParens {
 
 impl EarlyLintPass for DoubleParens {
     fn check_expr(&mut self, cx: &EarlyContext, expr: &Expr) {
-        // insert check here.
+        match expr.node {
+            ExprKind::Paren(ref in_paren) => {
+                match in_paren.node {
+                    ExprKind::Paren(_) |
+                    ExprKind::Tup(_) => {
+                        cx.span_lint(DOUBLE_PARENS, expr.span, "Consider removing unnecessary double parentheses");
+                    },
+                    _ => {},
+                }
+            },
+            ExprKind::Call(_, ref params) => {
+                if params.len() == 1 {
+                    let param = &params[0];
+                    if let ExprKind::Paren(_) = param.node {
+                        cx.span_lint(DOUBLE_PARENS, param.span, "Consider removing unnecessary double parentheses");
+                    }
+                }
+            },
+            ExprKind::MethodCall(_, _, ref params) => {
+                if params.len() == 2 {
+                    let param = &params[1];
+                    if let ExprKind::Paren(_) = param.node {
+                        cx.span_lint(DOUBLE_PARENS, param.span, "Consider removing unnecessary double parentheses");
+                    }
+                }
+            },
+            _ => {},
+        }
     }
 }