2024: Add macro_rules expr and expr_2021

Author: ehuss

src/macros-by-example.md

@@ -27,7 +27,7 @@
 > &nbsp;&nbsp; | `$` `(` _MacroMatch_<sup>+</sup> `)` _MacroRepSep_<sup>?</sup> _MacroRepOp_
 >
 > _MacroFragSpec_ :\
-> &nbsp;&nbsp; &nbsp;&nbsp; `block` | `expr` | `ident` | `item` | `lifetime` | `literal`\
+> &nbsp;&nbsp; &nbsp;&nbsp; `block` | `expr` | `expr_2021` | `ident` | `item` | `lifetime` | `literal`\
 > &nbsp;&nbsp; | `meta` | `pat` | `pat_param` | `path` | `stmt` | `tt` | `ty` | `vis`
 >
 > _MacroRepSep_ :\
@@ -127,6 +127,7 @@ fragment specifiers are:
   * `pat_param`: a [_PatternNoTopAlt_]
   * `pat`: at least any [_PatternNoTopAlt_], and possibly more depending on edition
   * `expr`: an [_Expression_]
+  * `expr_2021` an [_Expression_] except [_UnderscoreExpression_] and [_ConstBlockExpression_]
   * `ty`: a [_Type_]
   * `ident`: an [IDENTIFIER_OR_KEYWORD] or [RAW_IDENTIFIER]
   * `path`: a [_TypePath_] style path
@@ -142,16 +143,13 @@ the syntax element that matched them. The keyword metavariable `$crate` can be
 used to refer to the current crate; see [Hygiene] below. Metavariables can be
 transcribed more than once or not at all.
 
-For reasons of backwards compatibility, though `_` [is also an
-expression][_UnderscoreExpression_], a standalone underscore is not matched by
-the `expr` fragment specifier. However, `_` is matched by the `expr` fragment
-specifier when it appears as a subexpression.
-For the same reason, a standalone [const block] is not matched but it is matched when appearing as a subexpression.
-
 > **Edition Differences**: Starting with the 2021 edition, `pat` fragment-specifiers match top-level or-patterns (that is, they accept [_Pattern_]).
 >
 > Before the 2021 edition, they match exactly the same fragments as `pat_param` (that is, they accept [_PatternNoTopAlt_]).
 >
+> Before the 2024 edition, `expr` fragment specifiers do not match [_UnderscoreExpression_] or [_ConstBlockExpression_] at the top level. They are allowed within subexpressions.
+> The `expr_2021` fragment specifier exists as a way to maintain backwards compatibility if needed.
+>
 > The relevant edition is the one in effect for the `macro_rules!` definition.
 
 ## Repetitions
@@ -493,7 +491,7 @@ expansions, taking separators into account. This means:
 
 For more detail, see the [formal specification].
 
-[const block]: expressions/block-expr.md#const-blocks
+[_ConstBlockExpression_]: expressions/block-expr.md#const-blocks
 [Hygiene]: #hygiene
 [IDENTIFIER]: identifiers.md
 [IDENTIFIER_OR_KEYWORD]: identifiers.md