Add a hint that the expressions produce a value

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/unused-borrows.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
 

src/test/ui/lint/unused/issue-85913.rs

@@ -0,0 +1,13 @@
+#![deny(unused_must_use)]
+
+pub fn fun() -> i32 {
+    function() && return 1;
+    //~^ ERROR: unused logical operation that must be used
+    return 0;
+}
+
+fn function() -> bool {
+    true
+}
+
+fn main() {}

src/test/ui/lint/unused/issue-85913.stderr

@@ -0,0 +1,18 @@
+error: unused logical operation that must be used
+  --> $DIR/issue-85913.rs:4:5
+   |
+LL |     function() && return 1;
+   |     ^^^^^^^^^^^^^^^^^^^^^^ the logical operation produces a value
+   |
+note: the lint level is defined here
+  --> $DIR/issue-85913.rs:1:9
+   |
+LL | #![deny(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = function() && return 1;
+   |     ^^^^^^^
+
+error: aborting due to previous error
+