Add FCW to disallow $crate in macro matcher

Co-authored-by: @CAD97

Author: nik-rev

compiler/rustc_expand/src/errors.rs

@@ -10,6 +10,10 @@ use rustc_span::{Ident, MacroRulesNormalizedIdent, Span, Symbol};
 #[diag("`#[cfg_attr]` does not expand to any attributes")]
 pub(crate) struct CfgAttrNoAttributes;
 
+#[derive(Diagnostic)]
+#[diag("usage of `$crate` in matcher")]
+pub(crate) struct DollarCrateInMatcher;
+
 #[derive(Diagnostic)]
 #[diag(
     "attempted to repeat an expression containing no syntax variables matched as repeating at this depth"

compiler/rustc_expand/src/mbe/quoted.rs

@@ -3,6 +3,7 @@ use rustc_ast::tokenstream::TokenStreamIter;
 use rustc_ast::{NodeId, tokenstream};
 use rustc_ast_pretty::pprust;
 use rustc_feature::Features;
+use rustc_lint_defs::builtin::DOLLAR_CRATE_IN_MATCHER;
 use rustc_session::Session;
 use rustc_session::parse::feature_err;
 use rustc_span::edition::Edition;
@@ -300,6 +301,16 @@ fn parse_tree<'a>(
                     let (ident, is_raw) = token.ident().unwrap();
                     let span = ident.span.with_lo(dollar_span.lo());
                     if ident.name == kw::Crate && matches!(is_raw, IdentIsRaw::No) {
+                        // FCW for `$crate` in matcher
+                        if part.is_pattern() {
+                            sess.psess.buffer_lint(
+                                DOLLAR_CRATE_IN_MATCHER,
+                                ident.span,
+                                node_id,
+                                crate::errors::DollarCrateInMatcher,
+                            );
+                        }
+
                         TokenTree::token(token::Ident(kw::DollarCrate, is_raw), span)
                     } else {
                         TokenTree::MetaVar(span, ident)

compiler/rustc_lint_defs/src/builtin.rs

@@ -39,6 +39,7 @@ declare_lint_pass! {
         DEPRECATED_IN_FUTURE,
         DEPRECATED_SAFE_2024,
         DEPRECATED_WHERE_CLAUSE_LOCATION,
+        DOLLAR_CRATE_IN_MATCHER,
         DUPLICATE_FEATURES,
         DUPLICATE_MACRO_ATTRIBUTES,
         ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
@@ -156,6 +157,33 @@ declare_lint_pass! {
     ]
 }
 
+declare_lint! {
+    /// The `dollar_crate_in_matcher` lint detects cases where `$crate` is used in the matcher.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(dollar_crate_in_matcher)]
+    ///
+    /// macro_rules! m {
+    ///     ($crate) => {};
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// `$crate` is inconsistent with the behavior of other keywords in matchers, namely
+    /// that other keywords work like any other identifier, and are currently not reserved in this position.
+    pub DOLLAR_CRATE_IN_MATCHER,
+    Warn,
+    "detects when `$crate` is used in matcher",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: fcw!(FutureReleaseError #155123),
+    };
+}
+
 declare_lint! {
     /// The `forbidden_lint_groups` lint detects violations of
     /// `forbid` applied to a lint group. Due to a bug in the compiler,

tests/ui/macros/dollar-crate-in-matcher.rs

@@ -0,0 +1,21 @@
+#![deny(dollar_crate_in_matcher)]
+
+macro_rules! x {
+    ($crate) => {};
+    //~^ ERROR usage of `$crate` in matcher [dollar_crate_in_matcher]
+    //~| WARNING this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+}
+
+macro_rules! y {
+    ($dollar:tt $krate:tt) => {
+        macro_rules! z {
+            ($dollar $krate) => {}
+        }
+    };
+}
+
+y!($crate);
+//~^ ERROR usage of `$crate` in matcher [dollar_crate_in_matcher]
+//~| WARNING this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+
+fn main() {}

tests/ui/macros/dollar-crate-in-matcher.stderr

@@ -0,0 +1,25 @@
+error: usage of `$crate` in matcher
+  --> $DIR/dollar-crate-in-matcher.rs:4:7
+   |
+LL |     ($crate) => {};
+   |       ^^^^^
+   |
+   = 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 #155123 <https://github.com/rust-lang/rust/issues/155123>
+note: the lint level is defined here
+  --> $DIR/dollar-crate-in-matcher.rs:1:9
+   |
+LL | #![deny(dollar_crate_in_matcher)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: usage of `$crate` in matcher
+  --> $DIR/dollar-crate-in-matcher.rs:17:5
+   |
+LL | y!($crate);
+   |     ^^^^^
+   |
+   = 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 #155123 <https://github.com/rust-lang/rust/issues/155123>
+
+error: aborting due to 2 previous errors
+