use feature_err to report unstable expr_2021

Author: eholk

compiler/rustc_expand/messages.ftl

@@ -42,9 +42,6 @@ expand_explain_doc_comment_inner =
 expand_explain_doc_comment_outer =
     outer doc comments expand to `#[doc = "..."]`, which is what this macro attempted to match
 
-expand_expr_2021_is_experimental =
-    expr_2021 is experimental
-
 expand_expr_repeat_no_syntax_vars =
     attempted to repeat an expression containing no syntax variables matched as repeating at this depth
 

compiler/rustc_expand/src/errors.rs

@@ -456,10 +456,3 @@ pub struct ExpectedParenOrBrace<'a> {
     pub span: Span,
     pub token: Cow<'a, str>,
 }
-
-#[derive(Diagnostic)]
-#[diag(expand_expr_2021_is_experimental)]
-pub struct Expr2021IsExperimental {
-    #[primary_span]
-    pub span: Span,
-}

compiler/rustc_expand/src/mbe/quoted.rs

@@ -95,8 +95,13 @@ pub(super) fn parse(
                                     if kind == token::NonterminalKind::Expr2021
                                         && !features.expr_fragment_specifier_2024
                                     {
-                                        sess.dcx()
-                                            .emit_err(errors::Expr2021IsExperimental { span });
+                                        rustc_session::parse::feature_err(
+                                            sess,
+                                            sym::expr_fragment_specifier_2024,
+                                            span,
+                                            "fragment specifier `expr_2021` is unstable",
+                                        )
+                                        .emit();
                                     }
                                     result.push(TokenTree::MetaVarDecl(span, ident, Some(kind)));
                                     continue;

tests/ui/macros/feature-gate-expr_fragment_specifier_2024.rs

@@ -1,7 +1,7 @@
 //@  compile-flags: --edition=2024 -Z unstable-options
 
 macro_rules! m {
-    ($e:expr_2021) => { //~ ERROR: expr_2021 is experimental
+    ($e:expr_2021) => { //~ ERROR: fragment specifier `expr_2021` is unstable
         $e
     };
 }

tests/ui/macros/feature-gate-expr_fragment_specifier_2024.stderr

@@ -1,8 +1,13 @@
-error: expr_2021 is experimental
+error[E0658]: fragment specifier `expr_2021` is unstable
   --> $DIR/feature-gate-expr_fragment_specifier_2024.rs:4:6
    |
 LL |     ($e:expr_2021) => {
    |      ^^^^^^^^^^^^
+   |
+   = note: see issue #123742 <https://github.com/rust-lang/rust/issues/123742> for more information
+   = help: add `#![feature(expr_fragment_specifier_2024)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0658`.