Change underscore to be a keyword #2049
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This changes underscore from being a punctuation character to a keyword. This is intended to help better align with proc-macros, which treat it as an [`Ident`](https://doc.rust-lang.org/proc_macro/struct.Ident.html). Note one unusual rule is inline assembly `ParamName` which is `IDENTIFIER_OR_KEYWORD`. From what I can tell, it does accept `_`, but the fmt template does not. Templates are not specified in great detail in the std docs, and don't touch on this fact. Closes rust-lang#1236 Closes rust-lang#2020
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| RAW_IDENTIFIER -> `r#` IDENTIFIER_OR_KEYWORD _except `crate`, `self`, `super`, `Self`, `_`_ | ||
| NON_KEYWORD_IDENTIFIER -> IDENTIFIER_OR_KEYWORD _except a [strict][lex.keywords.strict] or [reserved][lex.keywords.reserved] keyword_ | ||
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RESERVED_RAW_IDENTIFIER a few lines below may be redundant now.
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Hm, I spent some time thinking about this and how to correctly express that these keywords are rejected. The current except clause didn't express that in the way that I was intending. I pushed up a commit that instead of removing the reserved rule, it moves the except part into the reserved rule.
Or, to put it in another way, r#crate is a token, it's just rejected as an error. The previous lexical grammar wasn't really conveying that.
| * `expr`: an [Expression] | ||
| * `expr_2021`: an [Expression] except [UnderscoreExpression] and [ConstBlockExpression] (see [macro.decl.meta.edition2024]) | ||
| * `ident`: an [IDENTIFIER_OR_KEYWORD], [RAW_IDENTIFIER], or [`$crate`] | ||
| * `ident`: an [IDENTIFIER_OR_KEYWORD] except `_`, [RAW_IDENTIFIER], or [`$crate`] |
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Pre-existing, but this doesn't seem correct, ident accepts raw identifiers and expanded $crate (and unexpanded $crate is two tokens and not IDENTIFIER_OR_KEYWORD).
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petrochenkov
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Oct 14, 2025
This reworks the reserved raw identifier and lifetimes to hopefully more clearly express what they mean. The "except" clauses in the raw identifier were intended to mean a set subtraction, not an explicit "and it is an error if it is specified". Using set subtraction isn't correct because that would mean `r#crate` would be interpreted as 3 tokens (since RAW_IDENTIFIER did not match it, but IDENTIFIER_OR_KEYWORD PUNCTUATION IDENTIFIER_OR_KEYWORD would). I also reordered Token, since the intent is that the first production in an alternation that matches wins. The idea here is to make the reserved tokens high priority, so that they clearly match first and cause an error. (I did not exhaustively analyze the rest of the rules to see if they follow that behavior, that is for another day.) It could be said it would be nice to document the rationale for the restrictions (rust-lang#2042), but that is a bigger ask.
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This changes underscore from being a punctuation character to a keyword. This is intended to help better align with proc-macros, which treat it as an
Ident.Note one unusual rule is inline assembly
ParamNamewhich isIDENTIFIER_OR_KEYWORD. From what I can tell, it does accept_, but the fmt template does not. Templates are not specified in great detail in the std docs, and don't touch on this fact.Closes #1236
Closes #2020
cc @petrochenkov @mattheww