Make `missing_fragment_specifier` an error in edition 2024

`missing_fragment_specifier` has been a future compatibility warning
since 2017. Uplifting it to an unconditional hard error was attempted in
2020, but eventually reverted due to fallout.

Make it an error only in edition >= 2024, leaving the lint for older
editions. This change will make it easier to support more macro syntax
that relies on usage of `$`.

Fixes <https://github.com/rust-lang/rust/issues/40107>

3 files changed, 35 insertions, 7 deletions

diff --git a/compiler/rustc_expand/src/errors.rs b/compiler/rustc_expand/src/errors.rs
index 0be7403f25b..6f1a0f16c49 100644
--- a/compiler/rustc_expand/src/errors.rs
+++ b/compiler/rustc_expand/src/errors.rs
@@ -417,6 +417,23 @@ pub struct DuplicateMatcherBinding {
 }
 
 #[derive(Diagnostic)]
+#[diag(expand_missing_fragment_specifier)]
+#[note]
+#[help(expand_valid)]
+pub struct MissingFragmentSpecifier {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(
+        expand_suggestion_add_fragspec,
+        style = "verbose",
+        code = ":spec",
+        applicability = "maybe-incorrect"
+    )]
+    pub add_span: Span,
+    pub valid: &'static str,
+}
+
+#[derive(Diagnostic)]
 #[diag(expand_invalid_fragment_specifier)]
 #[help]
 pub struct InvalidFragmentSpecifier {
diff --git a/compiler/rustc_expand/src/mbe/macro_check.rs b/compiler/rustc_expand/src/mbe/macro_check.rs
index 4b730d307fd..161e27fe02c 100644
--- a/compiler/rustc_expand/src/mbe/macro_check.rs
+++ b/compiler/rustc_expand/src/mbe/macro_check.rs
@@ -115,6 +115,7 @@ use rustc_errors::MultiSpan;
 use rustc_lint_defs::BuiltinLintDiag;
 use rustc_session::lint::builtin::{META_VARIABLE_MISUSE, MISSING_FRAGMENT_SPECIFIER};
 use rustc_session::parse::ParseSess;
+use rustc_span::edition::Edition;
 use rustc_span::symbol::kw;
 use rustc_span::{symbol::MacroRulesNormalizedIdent, ErrorGuaranteed, Span};
 
@@ -122,6 +123,8 @@ use smallvec::SmallVec;
 
 use std::iter;
 
+use super::quoted::VALID_FRAGMENT_NAMES_MSG_2021;
+
 /// Stack represented as linked list.
 ///
 /// Those are used for environments because they grow incrementally and are not mutable.
@@ -269,12 +272,20 @@ fn check_binders(
                 // FIXME: Report this as a hard error eventually and remove equivalent errors from
                 // `parse_tt_inner` and `nameize`. Until then the error may be reported twice, once
                 // as a hard error and then once as a buffered lint.
-                psess.buffer_lint(
-                    MISSING_FRAGMENT_SPECIFIER,
-                    span,
-                    node_id,
-                    BuiltinLintDiag::MissingFragmentSpecifier,
-                );
+                if span.edition() >= Edition::Edition2024 {
+                    psess.dcx().emit_err(errors::MissingFragmentSpecifier {
+                        span,
+                        add_span: span.shrink_to_hi(),
+                        valid: VALID_FRAGMENT_NAMES_MSG_2021,
+                    });
+                } else {
+                    psess.buffer_lint(
+                        MISSING_FRAGMENT_SPECIFIER,
+                        span,
+                        node_id,
+                        BuiltinLintDiag::MissingFragmentSpecifier,
+                    );
+                }
             }
             if !macros.is_empty() {
                 psess.dcx().span_bug(span, "unexpected MetaVarDecl in nested lhs");
diff --git a/compiler/rustc_expand/src/mbe/quoted.rs b/compiler/rustc_expand/src/mbe/quoted.rs
index 9c480f17b42..57b6947316d 100644
--- a/compiler/rustc_expand/src/mbe/quoted.rs
+++ b/compiler/rustc_expand/src/mbe/quoted.rs
@@ -16,7 +16,7 @@ use rustc_span::Span;
 const VALID_FRAGMENT_NAMES_MSG: &str = "valid fragment specifiers are \
                                         `ident`, `block`, `stmt`, `expr`, `pat`, `ty`, `lifetime`, \
                                         `literal`, `path`, `meta`, `tt`, `item` and `vis`";
-const VALID_FRAGMENT_NAMES_MSG_2021: &str = "valid fragment specifiers are \
+pub const VALID_FRAGMENT_NAMES_MSG_2021: &str = "valid fragment specifiers are \
                                              `ident`, `block`, `stmt`, `expr`, `expr_2021`, `pat`, \
                                              `ty`, `lifetime`, `literal`, `path`, `meta`, `tt`, \
                                              `item` and `vis`";