Fix diagnostics for coroutines with () as input.

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

@@ -2635,49 +2635,47 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
  // This shouldn't be common unless manually implementing one of the
  // traits manually, but don't make it more confusing when it does
  // happen.
- Ok(
- if Some(expected_trait_ref.def_id) != self.tcx.lang_items().coroutine_trait()
- && not_tupled
- {
- self.report_and_explain_type_error(
- TypeTrace::trait_refs(
- &obligation.cause,
- true,
- expected_trait_ref,
- found_trait_ref,
- ),
- ty::error::TypeError::Mismatch,
- )
- } else if found.len() == expected.len() {
- self.report_closure_arg_mismatch(
- span,
- found_span,
- found_trait_ref,
- expected_trait_ref,
- obligation.cause.code(),
- found_node,
- obligation.param_env,
- )
- } else {
- let (closure_span, closure_arg_span, found) = found_did
- .and_then(|did| {
- let node = self.tcx.hir().get_if_local(did)?;
- let (found_span, closure_arg_span, found) =
- self.get_fn_like_arguments(node)?;
- Some((Some(found_span), closure_arg_span, found))
- })
- .unwrap_or((found_span, None, found));
-
- self.report_arg_count_mismatch(
+ if Some(expected_trait_ref.def_id) != self.tcx.lang_items().coroutine_trait() && not_tupled
+ {
+ return Ok(self.report_and_explain_type_error(
+ TypeTrace::trait_refs(&obligation.cause, true, expected_trait_ref, found_trait_ref),
+ ty::error::TypeError::Mismatch,
+ ));
+ }
+ if found.len() != expected.len() {
+ let (closure_span, closure_arg_span, found) = found_did
+ .and_then(|did| {
+ let node = self.tcx.hir().get_if_local(did)?;
+ let (found_span, closure_arg_span, found) = self.get_fn_like_arguments(node)?;
+ Some((Some(found_span), closure_arg_span, found))
+ })
+ .unwrap_or((found_span, None, found));
+
+ // If the coroutine take a single () as its argument,
+ // the trait argument would found the coroutine take 0 arguments,
+ // but get_fn_like_arguments would give 1 argument.
+ // This would result in "Expected to take 1 argument, but it takes 1 argument".
+ // Check again to avoid this.
+ if found.len() != expected.len() {
+ return Ok(self.report_arg_count_mismatch(
  span,
  closure_span,
  expected,
  found,
  found_trait_ty.is_closure(),
  closure_arg_span,
- )
- },
- )
+ ));
+ }
+ }
+ Ok(self.report_closure_arg_mismatch(
+ span,
+ found_span,
+ found_trait_ref,
+ expected_trait_ref,
+ obligation.cause.code(),
+ found_node,
+ obligation.param_env,
+ ))
  }
 
  /// Given some node representing a fn-like thing in the HIR map,

tests/ui/coroutine/arg-count-mismatch-on-unit-input.rs

@@ -0,0 +1,11 @@
+#![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
+
+use std::ops::Coroutine;
+
+fn foo() -> impl Coroutine<u8> {
+ //~^ ERROR type mismatch in coroutine arguments
+ #[coroutine]
+ |_: ()| {}
+}
+
+fn main() { }

tests/ui/coroutine/arg-count-mismatch-on-unit-input.stderr

@@ -0,0 +1,15 @@
+error[E0631]: type mismatch in coroutine arguments
+ --> $DIR/arg-count-mismatch-on-unit-input.rs:5:13
+ |
+LL | fn foo() -> impl Coroutine<u8> {
+ | ^^^^^^^^^^^^^^^^^^ expected due to this
+...
+LL | |_: ()| {}
+ | ------- found signature defined here
+ |
+ = note: expected coroutine signature `fn(u8) -> _`
+ found coroutine signature `fn(()) -> _`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0631`.