code suggestions for unused-mut, while-true lints; UI test

src/librustc_lint/builtin.rs

@@ -76,9 +76,13 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for WhileTrue {
             if let hir::ExprLit(ref lit) = cond.node {
                 if let ast::LitKind::Bool(true) = lit.node {
                     if lit.span.ctxt() == SyntaxContext::empty() {
-                        cx.span_lint(WHILE_TRUE,
+                        let msg = "denote infinite loops with `loop { ... }`";
-                                    e.span,
+                        let mut err = cx.struct_span_lint(WHILE_TRUE, e.span, msg);
-                                    "denote infinite loops with loop { ... }");
+                        let condition_span = cx.tcx.sess.codemap().def_span(e.span);
+                        err.span_suggestion_short(condition_span,
+                                                  "use `loop`",
+                                                  "loop".to_owned());
+                        err.emit();
                     }
                 }
             }

src/librustc_lint/unused.rs

@@ -71,9 +71,13 @@ impl UnusedMut {
         let used_mutables = cx.tcx.used_mut_nodes.borrow();
         for (_, v) in &mutables {
             if !v.iter().any(|e| used_mutables.contains(e)) {
-                cx.span_lint(UNUSED_MUT,
+                let binding_span = cx.tcx.hir.span(v[0]);
-                             cx.tcx.hir.span(v[0]),
+                let mut_span = cx.tcx.sess.codemap().span_until_char(binding_span, ' ');
-                             "variable does not need to be mutable");
+                let mut err = cx.struct_span_lint(UNUSED_MUT,
+                                                  binding_span,
+                                                  "variable does not need to be mutable");
+                err.span_suggestion_short(mut_span, "remove this `mut`", "".to_owned());
+                err.emit();
             }
         }
     }

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/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