Add initial impl of check_pat() for UnusedParens

This uses a copied version of `check_unused_parens_expr` that is
specific to `ast::Pat`. `check_unused_parens_` could possibly be made
more generic to work with any `ast::*` that has `node` and `span`
fields.

This also only checks for the case of parens around the wildcard
pattern. It covers the case highlighted in the issue, but could check
for a lot more.

src/librustc_lint/unused.rs

@@ -264,7 +264,7 @@ declare_lint! {
 pub struct UnusedParens;
 
 impl UnusedParens {
-    fn check_unused_parens_core(&self,
+    fn check_unused_parens_expr(&self,
                                 cx: &EarlyContext,
                                 value: &ast::Expr,
                                 msg: &str,
@@ -313,6 +313,56 @@ impl UnusedParens {
             }
         }
     }
+
+    fn check_unused_parens_pat(&self,
+                                cx: &EarlyContext,
+                                value: &ast::Pat,
+                                msg: &str,
+                                struct_lit_needs_parens: bool) {
+        if let ast::PatKind::Paren(_) = value.node {
+            // Does there need to be a check similar to `parser::contains_exterior_struct_lit`
+            // here?
+            if !struct_lit_needs_parens {
+                let span_msg = format!("unnecessary parentheses around {}", msg);
+                let mut err = cx.struct_span_lint(UNUSED_PARENS,
+                                                  value.span,
+                                                  &span_msg);
+                // Remove exactly one pair of parentheses (rather than naïvely
+                // stripping all paren characters)
+                let mut ate_left_paren = false;
+                let mut ate_right_paren = false;
+                let parens_removed = pprust::pat_to_string(value)
+                    .trim_matches(|c| {
+                        match c {
+                            '(' => {
+                                if ate_left_paren {
+                                    false
+                                } else {
+                                    ate_left_paren = true;
+                                    true
+                                }
+                            },
+                            ')' => {
+                                if ate_right_paren {
+                                    false
+                                } else {
+                                    ate_right_paren = true;
+                                    true
+                                }
+                            },
+                            _ => false,
+                        }
+                    }).to_owned();
+                err.span_suggestion_short_with_applicability(
+                    value.span,
+                    "remove these parentheses",
+                    parens_removed,
+                    Applicability::MachineApplicable
+                );
+                err.emit();
+            }
+        }
+    }
 }
 
 impl LintPass for UnusedParens {
@@ -354,18 +404,32 @@ impl EarlyLintPass for UnusedParens {
                 }
                 let msg = format!("{} argument", call_kind);
                 for arg in args_to_check {
-                    self.check_unused_parens_core(cx, arg, &msg, false);
+                    self.check_unused_parens_expr(cx, arg, &msg, false);
                 }
                 return;
             }
         };
-        self.check_unused_parens_core(cx, &value, msg, struct_lit_needs_parens);
+        self.check_unused_parens_expr(cx, &value, msg, struct_lit_needs_parens);
+    }
+
+    fn check_pat(&mut self, cx: &EarlyContext, p: &ast::Pat) {
+        use ast::PatKind::*;
+        let (value, msg) = match p.node {
+            Paren(ref pat) => {
+                match pat.node {
+                    Wild => (p, "wildcard pattern"),
+                    _ => return,
+                }
+            }
+            _ => return,
+        };
+        self.check_unused_parens_pat(cx, &value, msg, false);
     }
 
     fn check_stmt(&mut self, cx: &EarlyContext, s: &ast::Stmt) {
         if let ast::StmtKind::Local(ref local) = s.node {
             if let Some(ref value) = local.init {
-                self.check_unused_parens_core(cx, &value, "assigned value", false);
+                self.check_unused_parens_expr(cx, &value, "assigned value", false);
             }
         }
     }