Auto merge of #44942 - zackmdavis:lint_suggestions, r=estebank

code suggestions for unused-mut, while-true, deprecated-attribute, and unused-parens lints

![lint_suggestions](https://user-images.githubusercontent.com/1076988/31044068-b2074de8-a57c-11e7-9319-6668508b6d1f.png)

r? @estebank

src/test/compile-fail/feature-gate-no-debug.rs

@@ -10,5 +10,5 @@
 
 #![allow(deprecated)]
 
-#[no_debug] //~ ERROR the `#[no_debug]` attribute is
+#[no_debug] //~ ERROR the `#[no_debug]` attribute was
 fn main() {}

src/test/compile-fail/issue-1962.rs

@@ -11,7 +11,7 @@
 // compile-flags: -D while-true
 fn main() {
   let mut i = 0;
-  while true  { //~ ERROR denote infinite loops with loop
+  while true  { //~ ERROR denote infinite loops with `loop
     i += 1;
     if i == 5 { break; }
   }

src/test/ui/lint/suggestions.rs

@@ -0,0 +1,20 @@
+// 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.
+
+#![warn(unused_mut)] // UI tests pass `-A unused`—see Issue #43896
+#![feature(no_debug)]
+
+#[no_debug] // should suggest removal of deprecated attribute
+fn main() {
+    while true { // should suggest `loop`
+        let mut a = (1); // should suggest no `mut`, no parens
+        println!("{}", a);
+    }
+}

src/test/ui/lint/suggestions.stderr

@@ -0,0 +1,45 @@
+warning: unnecessary parentheses around assigned value
+  --> $DIR/suggestions.rs:17:21
+   |
+17 |         let mut a = (1); // should suggest no `mut`, no parens
+   |                     ^^^ help: remove these parentheses
+   |
+   = note: #[warn(unused_parens)] on by default
+
+warning: use of deprecated attribute `no_debug`: the `#[no_debug]` attribute was an experimental feature that has been deprecated due to lack of demand. See https://github.com/rust-lang/rust/issues/29721
+  --> $DIR/suggestions.rs:14:1
+   |
+14 | #[no_debug] // should suggest removal of deprecated attribute
+   | ^^^^^^^^^^^ help: remove this attribute
+   |
+   = note: #[warn(deprecated)] on by default
+
+warning: denote infinite loops with `loop { ... }`
+  --> $DIR/suggestions.rs:16:5
+   |
+16 |       while true { // should suggest `loop`
+   |       ^---------
+   |       |
+   |  _____help: use `loop`
+   | |
+17 | |         let mut a = (1); // should suggest no `mut`, no parens
+18 | |         println!("{}", a);
+19 | |     }
+   | |_____^
+   |
+   = note: #[warn(while_true)] on by default
+
+warning: variable does not need to be mutable
+  --> $DIR/suggestions.rs:17:13
+   |
+17 |         let mut a = (1); // should suggest no `mut`, no parens
+   |             ---^^
+   |             |
+   |             help: remove this `mut`
+   |
+note: lint level defined here
+  --> $DIR/suggestions.rs:11:9
+   |
+11 | #![warn(unused_mut)] // UI tests pass `-A unused`—see Issue #43896
+   |         ^^^^^^^^^^
+

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}

src/test/ui/path-lookahead.stderr

@@ -2,7 +2,7 @@ warning: unnecessary parentheses around `return` value
   --> $DIR/path-lookahead.rs:18:10
    |
 18 |   return (<T as ToString>::to_string(&arg)); //~WARN unnecessary parentheses around `return` value
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove these parentheses
    |
    = note: #[warn(unused_parens)] on by default