Implement deref patterns via builtin#

compiler/rustc_builtin_macros/src/lib.rs

@@ -37,7 +37,6 @@ mod compile_error;
 mod concat;
 mod concat_bytes;
 mod concat_idents;
-mod deref_pat;
 mod derive;
 mod deriving;
 mod edition_panic;
@@ -85,7 +84,6 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
         concat_idents: concat_idents::expand_concat_idents,
         const_format_args: format::expand_format_args,
         core_panic: edition_panic::expand_panic,
-        deref: deref_pat::expand_deref_pat,
         env: env::expand_env,
         file: source_util::expand_file,
         format_args: format::expand_format_args,

compiler/rustc_parse/src/parser/expr.rs

@@ -1939,11 +1939,10 @@ impl<'a> Parser<'a> {
     /// Parse `builtin # ident(args,*)`.
     fn parse_expr_builtin(&mut self) -> PResult<'a, P<Expr>> {
         self.parse_builtin(|this, lo, ident| {
-            if ident.name == sym::offset_of {
-                return Ok(Some(this.parse_expr_offset_of(lo)?));
-            }
-
-            Ok(None)
+            Ok(match ident.name {
+                sym::offset_of => Some(this.parse_expr_offset_of(lo)?),
+                _ => None,
+            })
         })
     }
 

compiler/rustc_parse/src/parser/pat.rs

@@ -498,11 +498,14 @@ impl<'a> Parser<'a> {
             } else {
                 PatKind::Lit(const_expr)
             }
+        } else if self.is_builtin() {
+            self.parse_pat_builtin()?
+        }
         // Don't eagerly error on semantically invalid tokens when matching
         // declarative macros, as the input to those doesn't have to be
         // semantically valid. For attribute/derive proc macros this is not the
         // case, so doing the recovery for them is fine.
-        } else if self.can_be_ident_pat()
+        else if self.can_be_ident_pat()
             || (self.is_lit_bad_ident().is_some() && self.may_recover())
         {
             // Parse `ident @ pat`
@@ -1119,6 +1122,21 @@ impl<'a> Parser<'a> {
         .contains(&self.token.kind)
     }
 
+    fn parse_pat_builtin(&mut self) -> PResult<'a, PatKind> {
+        self.parse_builtin(|self_, _lo, ident| {
+            Ok(match ident.name {
+                // builtin#deref(PAT)
+                sym::deref => Some(ast::PatKind::Deref(self_.parse_pat_allow_top_alt(
+                    None,
+                    RecoverComma::Yes,
+                    RecoverColon::Yes,
+                    CommaRecoveryMode::LikelyTuple,
+                )?)),
+                _ => None,
+            })
+        })
+    }
+
     /// Parses `box pat`
     fn parse_pat_box(&mut self) -> PResult<'a, PatKind> {
         let box_span = self.prev_token.span;

library/core/src/macros/mod.rs

@@ -1716,14 +1716,14 @@ pub(crate) mod builtin {
 
     #[cfg(not(bootstrap))]
     /// Unstable placeholder for type ascription.
-    #[rustc_builtin_macro]
+    #[allow_internal_unstable(builtin_syntax)]
     #[unstable(
         feature = "deref_patterns",
         issue = "87121",
         reason = "placeholder syntax for deref patterns"
     )]
     pub macro deref($pat:pat) {
-        /* compiler built-in */
+        builtin # deref($pat)
     }
 
     /// Unstable implementation detail of the `rustc` compiler, do not use.