Add a note to unused variables

Author: GuillaumeGomez

src/librustc/middle/liveness.rs

@@ -1482,12 +1482,16 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                 };
 
                 if is_assigned {
-                    self.ir.tcx.lint_node(lint::builtin::UNUSED_VARIABLES, id, sp,
+                    self.ir.tcx.lint_node_note(lint::builtin::UNUSED_VARIABLES, id, sp,
                         &format!("variable `{}` is assigned to, but never used",
+                                 name),
+                        &format!("to disable this warning, consider using `_{}` instead",
                                  name));
                 } else if name != "self" {
-                    self.ir.tcx.lint_node(lint::builtin::UNUSED_VARIABLES, id, sp,
-                        &format!("unused variable: `{}`", name));
+                    self.ir.tcx.lint_node_note(lint::builtin::UNUSED_VARIABLES, id, sp,
+                        &format!("unused variable: `{}`", name),
+                        &format!("to disable this warning, consider using `_{}` instead",
+                                 name));
                 }
             }
             true

src/librustc/ty/context.rs

@@ -1522,6 +1522,17 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.struct_span_lint_node(lint, id, span.into(), msg).emit()
     }
 
+    pub fn lint_node_note<S: Into<MultiSpan>>(self,
+                                              lint: &'static Lint,
+                                              id: NodeId,
+                                              span: S,
+                                              msg: &str,
+                                              note: &str) {
+        let mut err = self.struct_span_lint_node(lint, id, span.into(), msg);
+        err.note(note);
+        err.emit()
+    }
+
     pub fn lint_level_at_node(self, lint: &'static Lint, mut id: NodeId)
         -> (lint::Level, lint::LintSource)
     {

src/test/ui/span/issue-24690.stderr

@@ -10,6 +10,7 @@ note: lint level defined here
 18 | #![warn(unused)]
    |         ^^^^^^
    = note: #[warn(unused_variables)] implied by #[warn(unused)]
+   = note: to disable this warning, consider using `_theOtherTwo` instead
 
 warning: variable `theTwo` should have a snake case name such as `the_two`
   --> $DIR/issue-24690.rs:22:9