correct unused-parens lint suggestion to strip exact pair

src/librustc_lint/unused.rs

@@ -334,8 +334,32 @@ impl UnusedParens {
                 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::expr_to_string(value)
-                    .trim_matches(|c| c == '(' || c == ')').to_owned();
+                    .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(value.span,
                                           "remove these parentheses",
                                           parens_removed);

src/test/ui/lint/unused_parens_json_suggestion.rs

@@ -0,0 +1,25 @@
+// Copyright 2017 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.
+
+// compile-flags: --error-format json
+
+// ignore-windows (see Issue #44968)
+
+// The output for humans should just highlight the whole span without showing
+// the suggested replacement, but we also want to test that suggested
+// replacement only removes one set of parentheses, rather than naïvely
+// stripping away any starting or ending parenthesis characters—hence this
+// test of the JSON error format.
+
+fn main() {
+    // We want to suggest the properly-balanced expression `1 / (2 + 3)`, not
+    // the malformed `1 / (2 + 3`
+    let _a = (1 / (2 + 3));
+}

src/test/ui/lint/unused_parens_json_suggestion.stderr

@@ -0,0 +1 @@
+{"message":"unnecessary parentheses around assigned value","code":null,"level":"warning","spans":[{"file_name":"$DIR/unused_parens_json_suggestion.rs","byte_start":1014,"byte_end":1027,"line_start":24,"line_end":24,"column_start":14,"column_end":27,"is_primary":true,"text":[{"text":"    let _a = (1 / (2 + 3));","highlight_start":14,"highlight_end":27}],"label":null,"suggested_replacement":null,"expansion":null}],"children":[{"message":"#[warn(unused_parens)] on by default","code":null,"level":"note","spans":[],"children":[],"rendered":null},{"message":"remove these parentheses","code":null,"level":"help","spans":[{"file_name":"$DIR/unused_parens_json_suggestion.rs","byte_start":1014,"byte_end":1027,"line_start":24,"line_end":24,"column_start":14,"column_end":27,"is_primary":true,"text":[{"text":"    let _a = (1 / (2 + 3));","highlight_start":14,"highlight_end":27}],"label":null,"suggested_replacement":"1 / (2 + 3)","expansion":null}],"children":[],"rendered":"    let _a = 1 / (2 + 3);"}],"rendered":null}