Auto merge of #115131 - frank-king:feature/unnamed-fields-lite, r=petrochenkov

Parse unnamed fields and anonymous structs or unions (no-recovery)

It is part of #114782 which implements #49804. Only parse anonymous structs or unions in struct field definition positions.

r? `@petrochenkov`

compiler/rustc_parse/src/parser/ty.rs

@@ -136,6 +136,17 @@ impl<'a> Parser<'a> {
         )
     }
 
+    /// Parse a type suitable for a field defintion.
+    /// The difference from `parse_ty` is that this version
+    /// allows anonymous structs and unions.
+    pub fn parse_ty_for_field_def(&mut self) -> PResult<'a, P<Ty>> {
+        if self.can_begin_anon_struct_or_union() {
+            self.parse_anon_struct_or_union()
+        } else {
+            self.parse_ty()
+        }
+    }
+
     /// Parse a type suitable for a function or function pointer parameter.
     /// The difference from `parse_ty` is that this version allows `...`
     /// (`CVarArgs`) at the top level of the type.
@@ -336,6 +347,36 @@ impl<'a> Parser<'a> {
         if allow_qpath_recovery { self.maybe_recover_from_bad_qpath(ty) } else { Ok(ty) }
     }
 
+    /// Parse an anonymous struct or union (only for field definitions):
+    /// ```ignore (feature-not-ready)
+    /// #[repr(C)]
+    /// struct Foo {
+    ///     _: struct { // anonymous struct
+    ///         x: u32,
+    ///         y: f64,
+    ///     }
+    ///     _: union { // anonymous union
+    ///         z: u32,
+    ///         w: f64,
+    ///     }
+    /// }
+    /// ```
+    fn parse_anon_struct_or_union(&mut self) -> PResult<'a, P<Ty>> {
+        assert!(self.token.is_keyword(kw::Union) || self.token.is_keyword(kw::Struct));
+        let is_union = self.token.is_keyword(kw::Union);
+
+        let lo = self.token.span;
+        self.bump();
+
+        let (fields, _recovered) =
+            self.parse_record_struct_body(if is_union { "union" } else { "struct" }, lo, false)?;
+        let span = lo.to(self.prev_token.span);
+        self.sess.gated_spans.gate(sym::unnamed_fields, span);
+        // These can be rejected during AST validation in `deny_anon_struct_or_union`.
+        let kind = if is_union { TyKind::AnonUnion(fields) } else { TyKind::AnonStruct(fields) };
+        Ok(self.mk_ty(span, kind))
+    }
+
     /// Parses either:
     /// - `(TYPE)`, a parenthesized type.
     /// - `(TYPE,)`, a tuple with a single field of type TYPE.
@@ -696,6 +737,11 @@ impl<'a> Parser<'a> {
         Ok(bounds)
     }
 
+    pub(super) fn can_begin_anon_struct_or_union(&mut self) -> bool {
+        (self.token.is_keyword(kw::Struct) || self.token.is_keyword(kw::Union))
+            && self.look_ahead(1, |t| t == &token::OpenDelim(Delimiter::Brace))
+    }
+
     /// Can the current token begin a bound?
     fn can_begin_bound(&mut self) -> bool {
         // This needs to be synchronized with `TokenKind::can_begin_bound`.