Auto merge of #87607 - JohnTitor:help-to-unused-must-use-op, r=estebank

Add a hint that the expressions produce a value Fixes #85913 The second commit is semi-_unrelated_ but it allows us to run the related tests just on `src/test/ui/lint`.

Author: bors

compiler/rustc_lint/src/unused.rs

@@ -161,7 +161,15 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
 
  if let Some(must_use_op) = must_use_op {
  cx.struct_span_lint(UNUSED_MUST_USE, expr.span, |lint| {
- lint.build(&format!("unused {} that must be used", must_use_op)).emit()
+ let mut lint = lint.build(&format!("unused {} that must be used", must_use_op));
+ lint.span_label(expr.span, &format!("the {} produces a value", must_use_op));
+ lint.span_suggestion_verbose(
+ expr.span.shrink_to_lo(),
+ "use `let _ = ...` to ignore the resulting value",
+ "let _ = ".to_string(),
+ Applicability::MachineApplicable,
+ );
+ lint.emit();
  });
  op_warned = true;
  }

src/test/ui/lint/fn_must_use.stderr

@@ -47,13 +47,23 @@ warning: unused comparison that must be used
  --> $DIR/fn_must_use.rs:74:5
  |
 LL | 2 == 3;
- | ^^^^^^
+ | ^^^^^^ the comparison produces a value
+ |
+help: use `let _ = ...` to ignore the resulting value
+ |
+LL | let _ = 2 == 3;
+ | ^^^^^^^
 
 warning: unused comparison that must be used
  --> $DIR/fn_must_use.rs:75:5
  |
 LL | m == n;
- | ^^^^^^
+ | ^^^^^^ the comparison produces a value
+ |
+help: use `let _ = ...` to ignore the resulting value
+ |
+LL | let _ = m == n;
+ | ^^^^^^^
 
 warning: 8 warnings emitted
 

src/test/ui/lint/must-use-ops.stderr

@@ -2,43 +2,72 @@ error: unused borrow that must be used
  --> $DIR/unused-borrows.rs:6:5
  |
 LL | &42;
- | ^^^
+ | ^^^ the borrow produces a value
  |
 note: the lint level is defined here
  --> $DIR/unused-borrows.rs:1:9
  |
 LL | #![deny(unused_must_use)]
  | ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+ |
+LL | let _ = &42;
+ | ^^^^^^^
 
 error: unused borrow that must be used
  --> $DIR/unused-borrows.rs:9:5
  |
 LL | &mut foo(42);
- | ^^^^^^^^^^^^
+ | ^^^^^^^^^^^^ the borrow produces a value
+ |
+help: use `let _ = ...` to ignore the resulting value
+ |
+LL | let _ = &mut foo(42);
+ | ^^^^^^^
 
 error: unused borrow that must be used
  --> $DIR/unused-borrows.rs:12:5
  |
 LL | &&42;
- | ^^^^
+ | ^^^^ the borrow produces a value
+ |
+help: use `let _ = ...` to ignore the resulting value
+ |
+LL | let _ = &&42;
+ | ^^^^^^^
 
 error: unused borrow that must be used
  --> $DIR/unused-borrows.rs:15:5
  |
 LL | &&mut 42;
- | ^^^^^^^^
+ | ^^^^^^^^ the borrow produces a value
+ |
+help: use `let _ = ...` to ignore the resulting value
+ |
+LL | let _ = &&mut 42;
+ | ^^^^^^^
 
 error: unused borrow that must be used
  --> $DIR/unused-borrows.rs:18:5
  |
 LL | &mut &42;
- | ^^^^^^^^
+ | ^^^^^^^^ the borrow produces a value
+ |
+help: use `let _ = ...` to ignore the resulting value
+ |
+LL | let _ = &mut &42;
+ | ^^^^^^^
 
 error: unused borrow that must be used
  --> $DIR/unused-borrows.rs:23:5
  |
 LL | && foo(42);
- | ^^^^^^^^^^
+ | ^^^^^^^^^^ the borrow produces a value
+ |
+help: use `let _ = ...` to ignore the resulting value
+ |
+LL | let _ = && foo(42);
+ | ^^^^^^^
 
 error: aborting due to 6 previous errors