Don't actually make bound ty/const for RTN

compiler/rustc_hir_analysis/src/astconv/bounds.rs

@@ -329,7 +329,7 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
         }
 
         let projection_ty = if let ty::AssocKind::Fn = assoc_kind {
-            let mut emitted_bad_param_err = false;
+            let mut emitted_bad_param_err = None;
             // If we have an method return type bound, then we need to substitute
             // the method's early bound params with suitable late-bound params.
             let mut num_bound_vars = candidate.bound_vars().len();
@@ -346,46 +346,30 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
                         )
                         .into(),
                         ty::GenericParamDefKind::Type { .. } => {
-                            if !emitted_bad_param_err {
+                            let guar = *emitted_bad_param_err.get_or_insert_with(|| {
                                 tcx.dcx().emit_err(
                                     crate::errors::ReturnTypeNotationIllegalParam::Type {
                                         span: path_span,
                                         param_span: tcx.def_span(param.def_id),
                                     },
-                                );
-                                emitted_bad_param_err = true;
-                            }
-                            Ty::new_bound(
-                                tcx,
-                                ty::INNERMOST,
-                                ty::BoundTy {
-                                    var: ty::BoundVar::from_usize(num_bound_vars),
-                                    kind: ty::BoundTyKind::Param(param.def_id, param.name),
-                                },
-                            )
-                            .into()
+                                )
+                            });
+                            Ty::new_error(tcx, guar).into()
                         }
                         ty::GenericParamDefKind::Const { .. } => {
-                            if !emitted_bad_param_err {
+                            let guar = *emitted_bad_param_err.get_or_insert_with(|| {
                                 tcx.dcx().emit_err(
                                     crate::errors::ReturnTypeNotationIllegalParam::Const {
                                         span: path_span,
                                         param_span: tcx.def_span(param.def_id),
                                     },
-                                );
-                                emitted_bad_param_err = true;
-                            }
+                                )
+                            });
                             let ty = tcx
                                 .type_of(param.def_id)
                                 .no_bound_vars()
                                 .expect("ct params cannot have early bound vars");
-                            ty::Const::new_bound(
-                                tcx,
-                                ty::INNERMOST,
-                                ty::BoundVar::from_usize(num_bound_vars),
-                                ty,
-                            )
-                            .into()
+                            ty::Const::new_error(tcx, guar, ty).into()
                         }
                     };
                     num_bound_vars += 1;

tests/ui/associated-type-bounds/return-type-notation/issue-120208-higher-ranked-const.rs

@@ -0,0 +1,17 @@
+// edition: 2021
+
+#![feature(return_type_notation)]
+//~^ WARN the feature `return_type_notation` is incomplete
+
+trait HealthCheck {
+    async fn check<const N: usize>() -> bool;
+}
+
+async fn do_health_check_par<HC>(hc: HC)
+where
+    HC: HealthCheck<check(): Send> + Send + 'static,
+    //~^ ERROR return type notation is not allowed for functions that have const parameters
+{
+}
+
+fn main() {}

tests/ui/associated-type-bounds/return-type-notation/issue-120208-higher-ranked-const.stderr

@@ -0,0 +1,20 @@
+warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-120208-higher-ranked-const.rs:3:12
+   |
+LL | #![feature(return_type_notation)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: return type notation is not allowed for functions that have const parameters
+  --> $DIR/issue-120208-higher-ranked-const.rs:12:21
+   |
+LL |     async fn check<const N: usize>() -> bool;
+   |                    -------------- const parameter declared here
+...
+LL |     HC: HealthCheck<check(): Send> + Send + 'static,
+   |                     ^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error; 1 warning emitted
+