typeck: simplify error type using () field

Using new support for spanless subdiagnostics from `()` fields in the
diagnostic derive, simplify the "explicit generic args with impl trait"
diagnostic's struct.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_error_messages/locales/en-US/typeck.ftl

@@ -99,6 +99,4 @@ typeck-explicit-generic-args-with-impl-trait =
     cannot provide explicit generic arguments when `impl Trait` is used in argument position
     .label = explicit generic argument not allowed
     .note = see issue #83701 <https://github.com/rust-lang/rust/issues/83701> for more information
-
-typeck-explicit-generic-args-with-impl-trait-feature =
-    add `#![feature(explicit_generic_args_with_impl_trait)]` to the crate attributes to enable
+    .help = add `#![feature(explicit_generic_args_with_impl_trait)]` to the crate attributes to enable

compiler/rustc_typeck/src/astconv/generics.rs

@@ -3,10 +3,7 @@
     AstConv, CreateSubstsForGenericArgsCtxt, ExplicitLateBound, GenericArgCountMismatch,
     GenericArgCountResult, GenericArgPosition,
 };
-use crate::errors::{
-    AssocTypeBindingNotAllowed, ExplicitGenericArgsWithImplTrait,
-    ExplicitGenericArgsWithImplTraitFeature,
-};
+use crate::errors::{AssocTypeBindingNotAllowed, ExplicitGenericArgsWithImplTrait};
 use crate::structured_errors::{GenericArgsInfo, StructuredDiagnostic, WrongNumberOfGenericArgs};
 use rustc_ast::ast::ParamKindOrd;
 use rustc_errors::{struct_span_err, Applicability, Diagnostic, MultiSpan};
@@ -639,11 +636,10 @@ pub(crate) fn check_impl_trait(
                 })
                 .collect::<Vec<_>>();
 
-            let mut err = tcx.sess.create_err(ExplicitGenericArgsWithImplTrait { spans });
-            if tcx.sess.is_nightly_build() {
-                err.subdiagnostic(ExplicitGenericArgsWithImplTraitFeature);
-            }
-            err.emit();
+            tcx.sess.emit_err(ExplicitGenericArgsWithImplTrait {
+                spans,
+                is_nightly_build: tcx.sess.is_nightly_build().then_some(()),
+            });
         }
 
         impl_trait

compiler/rustc_typeck/src/errors.rs

@@ -247,8 +247,6 @@ pub struct ExplicitGenericArgsWithImplTrait {
     #[primary_span]
     #[label]
     pub spans: Vec<Span>,
+    #[help]
+    pub is_nightly_build: Option<()>,
 }
-
-#[derive(SessionSubdiagnostic)]
-#[help(slug = "typeck-explicit-generic-args-with-impl-trait-feature")]
-pub struct ExplicitGenericArgsWithImplTraitFeature;