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