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>

7 files changed, 114 insertions, 25 deletions

diff --git a/compiler/rustc_expand/messages.ftl b/compiler/rustc_expand/messages.ftl
index cc0b110d2bc..18d95a398fd 100644
--- a/compiler/rustc_expand/messages.ftl
+++ b/compiler/rustc_expand/messages.ftl
@@ -105,6 +105,11 @@ expand_meta_var_dif_seq_matchers = {$msg}
 expand_meta_var_expr_unrecognized_var =
     variable `{$key}` is not recognized in meta-variable expression
 
+expand_missing_fragment_specifier = missing fragment specifier
+    .note = fragment specifiers must be specified in the 2024 edition
+    .suggestion_add_fragspec = try adding a specifier here
+    .valid = {$valid}
+
 expand_module_circular =
     circular modules: {$modules}
 
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`";
diff --git a/tests/ui/macros/macro-missing-fragment.stderr b/tests/ui/macros/macro-missing-fragment.e2015.stderr
index abe4d4cd68a..a068dc5c054 100644
--- a/tests/ui/macros/macro-missing-fragment.stderr
+++ b/tests/ui/macros/macro-missing-fragment.e2015.stderr
@@ -1,11 +1,11 @@
 error: missing fragment specifier
-  --> $DIR/macro-missing-fragment.rs:4:20
+  --> $DIR/macro-missing-fragment.rs:9:20
    |
 LL |     ( $( any_token $field_rust_type )* ) => {};
    |                    ^^^^^^^^^^^^^^^^
 
 warning: missing fragment specifier
-  --> $DIR/macro-missing-fragment.rs:4:20
+  --> $DIR/macro-missing-fragment.rs:9:20
    |
 LL |     ( $( any_token $field_rust_type )* ) => {};
    |                    ^^^^^^^^^^^^^^^^
@@ -13,13 +13,13 @@ LL |     ( $( any_token $field_rust_type )* ) => {};
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>
 note: the lint level is defined here
-  --> $DIR/macro-missing-fragment.rs:1:9
+  --> $DIR/macro-missing-fragment.rs:6:9
    |
 LL | #![warn(missing_fragment_specifier)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 warning: missing fragment specifier
-  --> $DIR/macro-missing-fragment.rs:12:7
+  --> $DIR/macro-missing-fragment.rs:19:7
    |
 LL |     ( $name ) => {};
    |       ^^^^^
@@ -28,7 +28,7 @@ LL |     ( $name ) => {};
    = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>
 
 warning: missing fragment specifier
-  --> $DIR/macro-missing-fragment.rs:18:7
+  --> $DIR/macro-missing-fragment.rs:26:7
    |
 LL |     ( $name ) => {};
    |       ^^^^^
@@ -40,7 +40,7 @@ error: aborting due to 1 previous error; 3 warnings emitted
 
 Future incompatibility report: Future breakage diagnostic:
 warning: missing fragment specifier
-  --> $DIR/macro-missing-fragment.rs:4:20
+  --> $DIR/macro-missing-fragment.rs:9:20
    |
 LL |     ( $( any_token $field_rust_type )* ) => {};
    |                    ^^^^^^^^^^^^^^^^
@@ -48,14 +48,14 @@ LL |     ( $( any_token $field_rust_type )* ) => {};
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>
 note: the lint level is defined here
-  --> $DIR/macro-missing-fragment.rs:1:9
+  --> $DIR/macro-missing-fragment.rs:6:9
    |
 LL | #![warn(missing_fragment_specifier)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Future breakage diagnostic:
 warning: missing fragment specifier
-  --> $DIR/macro-missing-fragment.rs:12:7
+  --> $DIR/macro-missing-fragment.rs:19:7
    |
 LL |     ( $name ) => {};
    |       ^^^^^
@@ -63,14 +63,14 @@ LL |     ( $name ) => {};
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>
 note: the lint level is defined here
-  --> $DIR/macro-missing-fragment.rs:1:9
+  --> $DIR/macro-missing-fragment.rs:6:9
    |
 LL | #![warn(missing_fragment_specifier)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Future breakage diagnostic:
 warning: missing fragment specifier
-  --> $DIR/macro-missing-fragment.rs:18:7
+  --> $DIR/macro-missing-fragment.rs:26:7
    |
 LL |     ( $name ) => {};
    |       ^^^^^
@@ -78,7 +78,7 @@ LL |     ( $name ) => {};
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>
 note: the lint level is defined here
-  --> $DIR/macro-missing-fragment.rs:1:9
+  --> $DIR/macro-missing-fragment.rs:6:9
    |
 LL | #![warn(missing_fragment_specifier)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/tests/ui/macros/macro-missing-fragment.e2024.stderr b/tests/ui/macros/macro-missing-fragment.e2024.stderr
new file mode 100644
index 00000000000..3afa069c170
--- /dev/null
+++ b/tests/ui/macros/macro-missing-fragment.e2024.stderr
@@ -0,0 +1,47 @@
+error: missing fragment specifier
+  --> $DIR/macro-missing-fragment.rs:9:20
+   |
+LL |     ( $( any_token $field_rust_type )* ) => {};
+   |                    ^^^^^^^^^^^^^^^^
+   |
+   = note: fragment specifiers must be specified in the 2024 edition
+   = help: valid fragment specifiers are `ident`, `block`, `stmt`, `expr`, `expr_2021`, `pat`, `ty`, `lifetime`, `literal`, `path`, `meta`, `tt`, `item` and `vis`
+help: try adding a specifier here
+   |
+LL |     ( $( any_token $field_rust_type:spec )* ) => {};
+   |                                    +++++
+
+error: missing fragment specifier
+  --> $DIR/macro-missing-fragment.rs:19:7
+   |
+LL |     ( $name ) => {};
+   |       ^^^^^
+   |
+   = note: fragment specifiers must be specified in the 2024 edition
+   = help: valid fragment specifiers are `ident`, `block`, `stmt`, `expr`, `expr_2021`, `pat`, `ty`, `lifetime`, `literal`, `path`, `meta`, `tt`, `item` and `vis`
+help: try adding a specifier here
+   |
+LL |     ( $name:spec ) => {};
+   |            +++++
+
+error: missing fragment specifier
+  --> $DIR/macro-missing-fragment.rs:26:7
+   |
+LL |     ( $name ) => {};
+   |       ^^^^^
+   |
+   = note: fragment specifiers must be specified in the 2024 edition
+   = help: valid fragment specifiers are `ident`, `block`, `stmt`, `expr`, `expr_2021`, `pat`, `ty`, `lifetime`, `literal`, `path`, `meta`, `tt`, `item` and `vis`
+help: try adding a specifier here
+   |
+LL |     ( $name:spec ) => {};
+   |            +++++
+
+error: missing fragment specifier
+  --> $DIR/macro-missing-fragment.rs:9:20
+   |
+LL |     ( $( any_token $field_rust_type )* ) => {};
+   |                    ^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+
diff --git a/tests/ui/macros/macro-missing-fragment.rs b/tests/ui/macros/macro-missing-fragment.rs
index 210c85ebbf2..b7da87ae610 100644
--- a/tests/ui/macros/macro-missing-fragment.rs
+++ b/tests/ui/macros/macro-missing-fragment.rs
@@ -1,23 +1,32 @@
+//@ revisions: e2015 e2024
+//@[e2015] edition:2015
+//@[e2024] edition:2024
+//@[e2024] compile-flags: -Zunstable-options
+
 #![warn(missing_fragment_specifier)]
 
 macro_rules! used_arm {
     ( $( any_token $field_rust_type )* ) => {};
-    //~^ ERROR missing fragment
-    //~| WARN missing fragment
-    //~| WARN this was previously accepted
+    //[e2015]~^ ERROR missing fragment
+    //[e2015]~| WARN missing fragment
+    //[e2015]~| WARN this was previously accepted
+    //[e2024]~^^^^ ERROR missing fragment
+    //[e2024]~| ERROR missing fragment
 }
 
 macro_rules! used_macro_unused_arm {
     () => {};
     ( $name ) => {};
-    //~^ WARN missing fragment
-    //~| WARN this was previously accepted
+    //[e2015]~^ WARN missing fragment
+    //[e2015]~| WARN this was previously accepted
+    //[e2024]~^^^ ERROR missing fragment
 }
 
 macro_rules! unused_macro {
     ( $name ) => {};
-    //~^ WARN missing fragment
-    //~| WARN this was previously accepted
+    //[e2015]~^ WARN missing fragment
+    //[e2015]~| WARN this was previously accepted
+    //[e2024]~^^^ ERROR missing fragment
 }
 
 fn main() {