Rollup merge of #126700 - compiler-errors:fragment, r=fmease

Make edition dependent `:expr` macro fragment act like the edition-dependent `:pat` fragment does

Parse the `:expr` fragment as `:expr_2021` in editions <=2021, and as `:expr` in edition 2024. This is similar to how we parse `:pat` as `:pat_param` in edition <=2018 and `:pat_with_or` in >=2021, and means we can get rid of a span dependency from `nonterminal_may_begin_with`.

Specifically, this fixes a theoretical regression since the `expr_2021` macro fragment previously would allow `const {}` if the *caller* is edition 2024. This is inconsistent with the way that the `pat` macro fragment was upgraded, and also leads to surprising behavior when a macro *caller* crate upgrades to edtion 2024, since they may have parsing changes that they never asked for (with no way of opting out of it).

This PR also allows using `expr_2021` in all editions. Why was this was disallowed in the first place? It's purely additive, and also it's still feature gated?

r? ```@fmease``` ```@eholk``` cc ```@vincenzopalazzo```
cc #123865

Tracking:

- https://github.com/rust-lang/rust/issues/123742

compiler/rustc_ast/src/token.rs

@@ -900,7 +900,11 @@ pub enum NonterminalKind {
     PatWithOr,
     Expr,
     /// Matches an expression using the rules from edition 2021 and earlier.
-    Expr2021,
+    Expr2021 {
+        /// Keep track of whether the user used `:expr` or `:expr_2021` and we inferred it from the
+        /// edition of the span. This is used for diagnostics AND feature gating.
+        inferred: bool,
+    },
     Ty,
     Ident,
     Lifetime,
@@ -929,8 +933,13 @@ impl NonterminalKind {
                 Edition::Edition2021 | Edition::Edition2024 => NonterminalKind::PatWithOr,
             },
             sym::pat_param => NonterminalKind::PatParam { inferred: false },
-            sym::expr => NonterminalKind::Expr,
-            sym::expr_2021 if edition().at_least_rust_2021() => NonterminalKind::Expr2021,
+            sym::expr => match edition() {
+                Edition::Edition2015 | Edition::Edition2018 | Edition::Edition2021 => {
+                    NonterminalKind::Expr2021 { inferred: true }
+                }
+                Edition::Edition2024 => NonterminalKind::Expr,
+            },
+            sym::expr_2021 => NonterminalKind::Expr2021 { inferred: false },
             sym::ty => NonterminalKind::Ty,
             sym::ident => NonterminalKind::Ident,
             sym::lifetime => NonterminalKind::Lifetime,
@@ -949,8 +958,8 @@ impl NonterminalKind {
             NonterminalKind::Stmt => sym::stmt,
             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::Expr | NonterminalKind::Expr2021 { inferred: true } => sym::expr,
+            NonterminalKind::Expr2021 { inferred: false } => sym::expr_2021,
             NonterminalKind::Ty => sym::ty,
             NonterminalKind::Ident => sym::ident,
             NonterminalKind::Lifetime => sym::lifetime,