Rollup merge of rust-lang#52647 - csmoe:closure_arg_ignore, r=estebank

Suggest to take and ignore args while closure args count mismatching

Closes rust-lang#52473

Author: Mark-Simulacrum

src/librustc/traits/error_reporting.rs

@@ -1050,6 +1050,30 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         if let Some(found_span) = found_span {
             err.span_label(found_span, format!("takes {}", found_str));
 
+            // Suggest to take and ignore the arguments with expected_args_length `_`s if
+            // found arguments is empty (assume the user just wants to ignore args in this case).
+            // For example, if `expected_args_length` is 2, suggest `|_, _|`.
+            if found_args.is_empty() && is_closure {
+                let mut underscores = "_".repeat(expected_args.len())
+                                      .split("")
+                                      .filter(|s| !s.is_empty())
+                                      .collect::<Vec<_>>()
+                                      .join(", ");
+                err.span_suggestion_with_applicability(
+                    found_span,
+                    &format!(
+                        "consider changing the closure to take and ignore the expected argument{}",
+                        if expected_args.len() < 2 {
+                            ""
+                        } else {
+                            "s"
+                        }
+                    ),
+                    format!("|{}|", underscores),
+                    Applicability::MachineApplicable,
+                );
+            }
+
             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
                 if fields.len() == expected_args.len() {
                     let sugg = fields.iter()

src/test/ui/mismatched_types/closure-arg-count.stderr

@@ -5,6 +5,10 @@ LL |     [1, 2, 3].sort_by(|| panic!());
    |               ^^^^^^^ -- takes 0 arguments
    |               |
    |               expected closure that takes 2 arguments
+help: consider changing the closure to take and ignore the expected arguments
+   |
+LL |     [1, 2, 3].sort_by(|_, _| panic!());
+   |                       ^^^^^^
 
 error[E0593]: closure is expected to take 2 arguments, but it takes 1 argument
   --> $DIR/closure-arg-count.rs:17:15
@@ -51,6 +55,10 @@ note: required by `f`
    |
 LL | fn f<F: Fn<usize>>(_: F) {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^
+help: consider changing the closure to take and ignore the expected argument
+   |
+LL |     f(|_| panic!());
+   |       ^^^
 
 error[E0593]: closure is expected to take a single 2-tuple as argument, but it takes 2 distinct arguments
   --> $DIR/closure-arg-count.rs:26:53