Add expr_2021 nonterminal and feature flag

This commit adds a new nonterminal `expr_2021` in macro patterns, and
`expr_fragment_specifier_2024` feature flag. For now, `expr` and
`expr_2021` are treated the same, but in future PRs we will update
`expr` to match to new grammar.

Co-authored-by: Vincezo Palazzo <vincenzopalazzodev@gmail.com>

compiler/rustc_ast/src/token.rs

@@ -878,6 +878,8 @@ pub enum NonterminalKind {
     },
     PatWithOr,
     Expr,
+    /// Matches an expression using the rules from edition 2021 and earlier.
+    Expr2021,
     Ty,
     Ident,
     Lifetime,
@@ -907,6 +909,7 @@ impl NonterminalKind {
             },
             sym::pat_param => NonterminalKind::PatParam { inferred: false },
             sym::expr => NonterminalKind::Expr,
+            sym::expr_2021 if edition() >= Edition::Edition2024 => NonterminalKind::Expr2021,
             sym::ty => NonterminalKind::Ty,
             sym::ident => NonterminalKind::Ident,
             sym::lifetime => NonterminalKind::Lifetime,
@@ -926,6 +929,7 @@ impl NonterminalKind {
             NonterminalKind::PatParam { inferred: false } => sym::pat_param,
             NonterminalKind::PatParam { inferred: true } | NonterminalKind::PatWithOr => sym::pat,
             NonterminalKind::Expr => sym::expr,
+            NonterminalKind::Expr2021 => sym::expr_2021,
             NonterminalKind::Ty => sym::ty,
             NonterminalKind::Ident => sym::ident,
             NonterminalKind::Lifetime => sym::lifetime,