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Add expr_2021 macro fragment specifier #1580

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merged 2 commits into from
Oct 2, 2024
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Add expr_2021 macro fragment specifier #1580

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e4ba8b0
Add `expr_2021` macro fragment specifier
ehuss Oct 2, 2024
a5f20e3
Add expr_2021 to FOLLOW set rules
ehuss Aug 20, 2024
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2 changes: 1 addition & 1 deletion src/macro-ambiguity.md
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Expand Up @@ -325,7 +325,7 @@ represent simple nonterminals with the given fragment specifier.

* FOLLOW(pat) = {`=>`, `,`, `=`, `|`, `if`, `in`}`.

* FOLLOW(expr) = FOLLOW(stmt) = {`=>`, `,`, `;`}`.
* FOLLOW(expr) = FOLLOW(expr_2021) = FOLLOW(stmt) = {`=>`, `,`, `;`}`.

* FOLLOW(ty) = FOLLOW(path) = {`{`, `[`, `,`, `=>`, `:`, `=`, `>`, `>>`, `;`,
`|`, `as`, `where`, block nonterminals}.
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8 changes: 6 additions & 2 deletions src/macros-by-example.md
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Expand Up @@ -30,7 +30,7 @@ r[macro.decl.syntax]
> &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_ :\
Expand Down Expand Up @@ -143,7 +143,8 @@ Valid fragment specifiers are:
statements that require semicolons)
* `pat_param`: a [_PatternNoTopAlt_]
* `pat`: at least any [_PatternNoTopAlt_], and possibly more depending on edition
* `expr`: an [_Expression_]
* `expr`: an [_Expression_] except [_UnderscoreExpression_] and [_ConstBlockExpression_] (see [macro.decl.meta.expr-underscore])
* `expr_2021`: same as `expr` (see [macro.decl.meta.edition2021])
* `ty`: a [_Type_]
* `ident`: an [IDENTIFIER_OR_KEYWORD] or [RAW_IDENTIFIER]
* `path`: a [_TypePath_] style path
Expand Down Expand Up @@ -175,6 +176,8 @@ r[macro.decl.meta.edition2021]
> Before the 2021 edition, they match exactly the same fragments as `pat_param` (that is, they accept [_PatternNoTopAlt_]).
>
> The relevant edition is the one in effect for the `macro_rules!` definition.
>
> The `expr_2021` fragment specifier exists to maintain backwards compatibility with editions before 2024.

## Repetitions

Expand Down Expand Up @@ -572,6 +575,7 @@ For more detail, see the [formal specification].
[Repetitions]: #repetitions
[_Attr_]: attributes.md
[_BlockExpression_]: expressions/block-expr.md
[_ConstBlockExpression_]: expressions/block-expr.md#const-blocks
[_DelimTokenTree_]: macros.md
[_Expression_]: expressions.md
[_Item_]: items.md
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