Fix closure arg extraction in extract_callable_info

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -1091,7 +1091,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         Some((
                             DefIdOrName::DefId(def_id),
                             fn_sig.output(),
-                            fn_sig.inputs().map_bound(|inputs| &inputs[1..]),
+                            fn_sig.inputs().map_bound(|inputs| inputs[0].tuple_fields().as_slice()),
                         ))
                     }
                     ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
@@ -1101,7 +1101,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             .iter()
                             .find_map(|pred| {
                                 if let ty::ClauseKind::Projection(proj) = pred.kind().skip_binder()
-                        && self.tcx.is_lang_item(proj.projection_term.def_id,LangItem::FnOnceOutput)
+                        && self.tcx.is_lang_item(proj.projection_term.def_id, LangItem::FnOnceOutput)
                         // args tuple will always be args[1]
                         && let ty::Tuple(args) = proj.projection_term.args.type_at(1).kind()
                                 {

tests/ui/closures/correct-args-on-call-suggestion.rs

@@ -0,0 +1,7 @@
+// Ensure we give the right args when we suggest calling a closure.
+
+fn main() {
+    let x = |a: i32, b: i32| a + b;
+    let y: i32 = x;
+    //~^ ERROR mismatched types
+}

tests/ui/closures/correct-args-on-call-suggestion.stderr

@@ -0,0 +1,20 @@
+error[E0308]: mismatched types
+  --> $DIR/correct-args-on-call-suggestion.rs:5:18
+   |
+LL |     let x = |a: i32, b: i32| a + b;
+   |             ---------------- the found closure
+LL |     let y: i32 = x;
+   |            ---   ^ expected `i32`, found closure
+   |            |
+   |            expected due to this
+   |
+   = note: expected type `i32`
+           found closure `{closure@$DIR/correct-args-on-call-suggestion.rs:4:13: 4:29}`
+help: use parentheses to call this closure
+   |
+LL |     let y: i32 = x(/* i32 */, /* i32 */);
+   |                   ++++++++++++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.