3 files changed, 55 insertions, 1 deletions
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 61396ee0d4a..844cf335962 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -1300,6 +1300,8 @@ impl<'a> Parser<'a> {
             })
         } else if self.check(&token::OpenDelim(Delimiter::Bracket)) {
             self.parse_expr_array_or_repeat(Delimiter::Bracket)
+        } else if self.is_builtin() {
+            self.parse_expr_builtin()
         } else if self.check_path() {
             self.parse_expr_path_start()
         } else if self.check_keyword(kw::Move)
@@ -1755,6 +1757,42 @@ impl<'a> Parser<'a> {
         self.maybe_recover_from_bad_qpath(expr)
     }
 
+    /// Parse `builtin # ident(args,*)`.
+    fn parse_expr_builtin(&mut self) -> PResult<'a, P<Expr>> {
+        self.parse_builtin(|_this, _lo, _ident| {
+            Ok(None)
+        })
+    }
+
+    pub(crate) fn parse_builtin<T>(
+        &mut self,
+        parse: impl FnOnce(&mut Parser<'a>, Span, Ident) -> PResult<'a, Option<T>>,
+    ) -> PResult<'a, T> {
+        let lo = self.token.span;
+
+        self.bump(); // `builtin`
+        self.bump(); // `#`
+
+        let Some((ident, false)) = self.token.ident() else {
+            let err = errors::ExpectedBuiltinIdent { span: self.token.span }
+                .into_diagnostic(&self.sess.span_diagnostic);
+            return Err(err);
+        };
+        self.bump();
+
+        self.expect(&TokenKind::OpenDelim(Delimiter::Parenthesis))?;
+        let ret = if let Some(res) = parse(self, lo, ident)? {
+            Ok(res)
+        } else {
+            let err = errors::UnknownBuiltinConstruct { span: lo.to(ident.span), name: ident.name }
+                .into_diagnostic(&self.sess.span_diagnostic);
+            return Err(err);
+        };
+        self.expect(&TokenKind::CloseDelim(Delimiter::Parenthesis))?;
+
+        ret
+    }
+
     /// Returns a string literal if the next token is a string literal.
     /// In case of error returns `Some(lit)` if the next token is a literal with a wrong kind,
     /// and returns `None` if the next token is not literal at all.
@@ -2824,6 +2862,10 @@ impl<'a> Parser<'a> {
         })
     }
 
+    pub(crate) fn is_builtin(&self) -> bool {
+        self.token.is_keyword(kw::Builtin) && self.look_ahead(1, |t| *t == token::Pound)
+    }
+
     /// Parses a `try {...}` expression (`try` token already eaten).
     fn parse_try_block(&mut self, span_lo: Span) -> PResult<'a, P<Expr>> {
         let (attrs, body) = self.parse_inner_attrs_and_block()?;
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 6ca88200dc5..edfe316ec95 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -265,6 +265,9 @@ impl<'a> Parser<'a> {
             // UNION ITEM
             self.bump(); // `union`
             self.parse_item_union()?
+        } else if self.is_builtin() {
+            // BUILTIN# ITEM
+            return self.parse_item_builtin();
         } else if self.eat_keyword(kw::Macro) {
             // MACROS 2.0 ITEM
             self.parse_item_decl_macro(lo)?
@@ -434,6 +437,11 @@ impl<'a> Parser<'a> {
         }
     }
 
+    fn parse_item_builtin(&mut self) -> PResult<'a, Option<ItemInfo>> {
+        // To be expanded
+        return Ok(None);
+    }
+
     /// Parses an item macro, e.g., `item!();`.
     fn parse_item_macro(&mut self, vis: &Visibility) -> PResult<'a, MacCall> {
         let path = self.parse_path(PathStyle::Mod)?; // `foo::bar`
diff --git a/compiler/rustc_parse/src/parser/stmt.rs b/compiler/rustc_parse/src/parser/stmt.rs
index 1c17de337e8..a0bd086053e 100644
--- a/compiler/rustc_parse/src/parser/stmt.rs
+++ b/compiler/rustc_parse/src/parser/stmt.rs
@@ -90,7 +90,11 @@ impl<'a> Parser<'a> {
                 attrs,
                 errors::InvalidVariableDeclarationSub::UseLetNotVar,
             )?
-        } else if self.check_path() && !self.token.is_qpath_start() && !self.is_path_start_item() {
+        } else if self.check_path()
+            && !self.token.is_qpath_start()
+            && !self.is_path_start_item()
+            && !self.is_builtin()
+        {
             // We have avoided contextual keywords like `union`, items with `crate` visibility,
             // or `auto trait` items. We aim to parse an arbitrary path `a::b` but not something
             // that starts like a path (1 token), but it fact not a path.