Move $$crate into its own feature gate

compiler/rustc_expand/src/mbe/quoted.rs

@@ -183,6 +183,17 @@ fn maybe_emit_macro_metavar_expr_concat_feature(features: &Features, sess: &Sess
     }
 }
 
+fn maybe_emit_macro_metavar_dollar_dollar_crate_feature(
+    features: &Features,
+    sess: &Session,
+    span: Span,
+) {
+    if !features.macro_metavar_expr_dollar_dollar_crate() {
+        let msg = "the `$$crate` meta-variable expression is unstable";
+        feature_err(sess, sym::macro_metavar_expr_dollar_dollar_crate, span, msg).emit();
+    }
+}
+
 /// Takes a `tokenstream::TokenTree` and returns a `self::TokenTree`. Specifically, this takes a
 /// generic `TokenTree`, such as is used in the rest of the compiler, and returns a `TokenTree`
 /// for use in parsing a macro.
@@ -319,6 +330,18 @@ fn parse_tree<'a>(
                     } else {
                         maybe_emit_macro_metavar_expr_feature(features, sess, dollar_span2);
                     }
+
+                    // Gate `$$crate`.
+                    if let Some(tokenstream::TokenTree::Token(token, _)) = iter.peek()
+                        && let Some((ident, is_raw)) = token.ident()
+                        && ident.name == kw::Crate
+                        && matches!(is_raw, IdentIsRaw::No)
+                    {
+                        maybe_emit_macro_metavar_dollar_dollar_crate_feature(
+                            features, sess, ident.span,
+                        );
+                    }
+
                     TokenTree::token(token::Dollar, dollar_span2)
                 }
 

compiler/rustc_feature/src/unstable.rs

@@ -577,6 +577,8 @@ declare_features! (
     (unstable, macro_metavar_expr, "1.61.0", Some(83527)),
     /// Provides a way to concatenate identifiers using metavariable expressions.
     (unstable, macro_metavar_expr_concat, "1.81.0", Some(124225)),
+    /// Allows using `$$crate`.
+    (incomplete, macro_metavar_expr_dollar_dollar_crate, "CURRENT_RUSTC_VERSION", Some(155111)),
     /// Allows `#[marker]` on certain traits allowing overlapping implementations.
     (unstable, marker_trait_attr, "1.30.0", Some(29864)),
     /// Enable mgca `type const` syntax before expansion.

compiler/rustc_span/src/symbol.rs

@@ -1201,6 +1201,7 @@ symbols! {
         macro_literal_matcher,
         macro_metavar_expr,
         macro_metavar_expr_concat,
+        macro_metavar_expr_dollar_dollar_crate,
         macro_reexport,
         macro_use,
         macro_vis_matcher,

tests/ui/feature-gates/feature-gate-macro-metavar-expr-dollar-dollar-crate.rs

@@ -0,0 +1,28 @@
+#![feature(macro_metavar_expr)]
+pub const IDX: usize = 1;
+
+macro_rules! _direct_usage_super_2 {
+    () => {
+        macro_rules! _direct_usage_sub_2 {
+            () => {
+                $$crate
+                //~^ ERROR the `$$crate` meta-variable expression is unstable
+            }
+        }
+    };
+}
+
+macro_rules! indirect_usage_crate {
+    ($d:tt) => {
+        const _FOO: usize = $d$d crate::IDX;
+        //~^ ERROR expected expression, found `$`
+    };
+}
+macro_rules! indirect_usage_use {
+    ($d:tt) => {
+        indirect_usage_crate!($d);
+    }
+}
+indirect_usage_use!($);
+
+fn main() {}

tests/ui/feature-gates/feature-gate-macro-metavar-expr-dollar-dollar-crate.stderr

@@ -0,0 +1,19 @@
+error[E0658]: the `$$crate` meta-variable expression is unstable
+  --> $DIR/feature-gate-macro-metavar-expr-dollar-dollar-crate.rs:8:19
+   |
+LL |                 $$crate
+   |                   ^^^^^
+   |
+   = note: see issue #155111 <https://github.com/rust-lang/rust/issues/155111> for more information
+   = help: add `#![feature(macro_metavar_expr_dollar_dollar_crate)]` 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: expected expression, found `$`
+  --> $DIR/feature-gate-macro-metavar-expr-dollar-dollar-crate.rs:17:29
+   |
+LL |         const _FOO: usize = $d$d crate::IDX;
+   |                             ^^ expected expression
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.