Make the expansion of guard metavars begin guard non-terminals

compiler/rustc_parse/src/parser/expr.rs

@@ -3464,11 +3464,7 @@ pub(super) fn parse_arm(&mut self) -> PResult<'a, Arm> {
     }
 
     pub(crate) fn eat_metavar_guard(&mut self) -> Option<Box<Guard>> {
-        self.eat_metavar_seq_with_matcher(
-            |mv_kind| matches!(mv_kind, MetaVarKind::Guard),
-            |this| this.parse_match_arm_guard(),
-        )
-        .flatten()
+        self.eat_metavar_seq(MetaVarKind::Guard, |this| this.parse_match_arm_guard()).flatten()
     }
 
     fn parse_match_arm_guard(&mut self) -> PResult<'a, Option<Box<Guard>>> {

compiler/rustc_parse/src/parser/nonterminal.rs

@@ -105,7 +105,10 @@ fn may_be_ident(kind: MetaVarKind) -> bool {
                 token::Lifetime(..) | token::NtLifetime(..) => true,
                 _ => false,
             },
-            NonterminalKind::Guard => token.is_keyword(kw::If),
+            NonterminalKind::Guard => match token.kind {
+                token::OpenInvisible(InvisibleOrigin::MetaVar(MetaVarKind::Guard)) => true,
+                _ => token.is_keyword(kw::If),
+            },
             NonterminalKind::TT | NonterminalKind::Item | NonterminalKind::Stmt => {
                 token.kind.close_delim().is_none()
             }