Add parsing for builtin # in expression and item context

compiler/rustc_parse/messages.ftl

@@ -257,6 +257,10 @@ parse_invalid_literal_suffix_on_tuple_index = suffixes on a tuple index are inva
     .tuple_exception_line_2 = on proc macros, you'll want to use `syn::Index::from` or `proc_macro::Literal::*_unsuffixed` for code that will desugar to tuple field access
     .tuple_exception_line_3 = see issue #60210 <https://github.com/rust-lang/rust/issues/60210> for more information
 
+parse_expected_builtin_ident = expected identifier after `builtin #`
+
+parse_unknown_builtin_construct = unknown `builtin #` construct `{$name}`
+
 parse_non_string_abi_literal = non-string ABI literal
     .suggestion = specify the ABI with a string literal
 

compiler/rustc_parse/src/errors.rs

@@ -2644,3 +2644,18 @@ pub(crate) struct MalformedCfgAttr {
     pub span: Span,
     pub sugg: &'static str,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_unknown_builtin_construct)]
+pub(crate) struct UnknownBuiltinConstruct {
+    #[primary_span]
+    pub span: Span,
+    pub name: Symbol,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_expected_builtin_ident)]
+pub(crate) struct ExpectedBuiltinIdent {
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_parse/src/parser/expr.rs

@@ -1300,6 +1300,8 @@ fn parse_expr_bottom(&mut self) -> PResult<'a, P<Expr>> {
             })
         } 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 @@ fn parse_expr_yield(&mut self) -> PResult<'a, P<Expr>> {
         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 @@ fn check_let_expr(expr: &Expr) -> (bool, bool) {
         })
     }
 
+    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()?;

compiler/rustc_parse/src/parser/item.rs

@@ -265,6 +265,9 @@ fn parse_item_kind(
             // 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 @@ fn recover_missing_kw_before_item(&mut self) -> PResult<'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`

compiler/rustc_parse/src/parser/stmt.rs

@@ -90,7 +90,11 @@ pub(crate) fn parse_stmt_without_recovery(
                 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.

compiler/rustc_span/src/symbol.rs

@@ -95,6 +95,7 @@
 
         // Weak keywords, have special meaning only in specific contexts.
         Auto:               "auto",
+        Builtin:            "builtin",
         Catch:              "catch",
         Default:            "default",
         MacroRules:         "macro_rules",

tests/ui/parser/builtin-syntax.rs

@@ -0,0 +1,7 @@
+fn main() {
+    builtin # foobar(); //~ ERROR unknown `builtin #` construct
+}
+
+fn not_identifier() {
+    builtin # {}(); //~ ERROR expected identifier after
+}

tests/ui/parser/builtin-syntax.stderr

@@ -0,0 +1,14 @@
+error: unknown `builtin #` construct `foobar`
+  --> $DIR/builtin-syntax.rs:2:5
+   |
+LL |     builtin # foobar();
+   |     ^^^^^^^^^^^^^^^^
+
+error: expected identifier after `builtin #`
+  --> $DIR/builtin-syntax.rs:6:15
+   |
+LL |     builtin # {}();
+   |               ^
+
+error: aborting due to 2 previous errors
+