Change underscore to be a keyword by ehuss · Pull Request #2049 · rust-lang/reference

ehuss · 2025-10-12T15:50:29Z

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 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#1236
Closes#2020

cc @petrochenkov @mattheww

petrochenkov · 2025-10-14T15:35:08Z

+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_



RESERVED_RAW_IDENTIFIER a few lines below may be redundant now.

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.

petrochenkov · 2025-10-14T15:38:33Z

 * `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`]


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).

Yea, we have a few issues related to this (like #588 and #587). I was also uneasy adding this in the first place.
I don't know the right way to document that the expanded $crate can be accepted by ident. Perhaps this needs to just be more explicit what it means?

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. Closesrust-lang#1236Closesrust-lang#2020

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.

rustbot · 2025-10-22T00:14:14Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

Update books ## rust-lang/book 6 commits in af415fc6c8a6823dfb4595074f27d5a3e9e2fe49..f660f341887c8bbcd6c24fbfdf5d2a262f523965 2025-10-28 01:41:51 UTC to 2025-10-24 15:39:51 UTC - Update to Rust 1.90 (rust-lang/book#4545) - Update to Rust 1.89 (rust-lang/book#4544) - Update to Rust 1.88 (rust-lang/book#4543) - Update to Rust 1.87 (rust-lang/book#4542) - Update to Rust 1.86 (rust-lang/book#4541) - Remove unused subheadings (rust-lang/book#4538) ## rust-lang/edition-guide 1 commits in e2ed891f00361efc26616d82590b1c85d7a8920e..5c621253d8f2a5a4adb64a6365905db67dffe3a2 2025-10-23 14:13:01 UTC to 2025-10-23 14:13:01 UTC - Add back the link for never_type_fallback_flowing_into_unsafe (rust-lang/edition-guide#378) ## rust-lang/reference 12 commits in 752eab01cebdd6a2d90b53087298844c251859a1..e122eefff3fef362eb7e0c08fb7ffbf5f9461905 2025-10-28 20:52:27 UTC to 2025-10-21 19:42:06 UTC - Remove custom redirect scripts (rust-lang/reference#2065) - Avoid specifying "last import wins" for MBEs (rust-lang/reference#2057) - Add caveats about mutable references in consts (rust-lang/reference#2058) - Move punctuation index to an appendix, and expand for other syntaxes (rust-lang/reference#2061) - Use American spelling for "labeled" (rust-lang/reference#2066) - Remove rules from names intro (rust-lang/reference#2060) - Minor adjustments to inner/outer attribute definitions (rust-lang/reference#2059) - Change underscore to be a keyword (rust-lang/reference#2049) - Update `proc_macro_attribute` to use the attribute template (rust-lang/reference#1889) - Update `proc_macro` to use the attribute template (rust-lang/reference#1887) - Update `macro_use` to use the attribute template (rust-lang/reference#1886) - Update `macro_export` to use the attribute template (rust-lang/reference#1885) ## rust-lang/rust-by-example 7 commits in 2c9b490d70e535cf166bf17feba59e594579843f..160e6bbca70b0c01aa4de88d19db7fc5ff8447c3 2025-11-03 12:26:45 UTC to 2025-10-22 13:19:06 UTC - Fix typos in flow_control/match/binding (rust-lang/rust-by-example#1971) - add clarification on overflowing_literals lint (rust-lang/rust-by-example#1970) - Update enum_use.md (rust-lang/rust-by-example#1960) - Remove weird extra spaces in code (rust-lang/rust-by-example#1962) - Include a link to The Rust Reference in flow_control/match/destructuring (rust-lang/rust-by-example#1963) - Add an example showing pattern binding when matching several values in a match arm (rust-lang/rust-by-example#1966) - Fix typo in linked list length calculation comment (rust-lang/rust-by-example#1968)

Rollup merge of #148448 - rustbot:docs-update, r=ehuss Update books ## rust-lang/book 6 commits in af415fc6c8a6823dfb4595074f27d5a3e9e2fe49..f660f341887c8bbcd6c24fbfdf5d2a262f523965 2025-10-28 01:41:51 UTC to 2025-10-24 15:39:51 UTC - Update to Rust 1.90 (rust-lang/book#4545) - Update to Rust 1.89 (rust-lang/book#4544) - Update to Rust 1.88 (rust-lang/book#4543) - Update to Rust 1.87 (rust-lang/book#4542) - Update to Rust 1.86 (rust-lang/book#4541) - Remove unused subheadings (rust-lang/book#4538) ## rust-lang/edition-guide 1 commits in e2ed891f00361efc26616d82590b1c85d7a8920e..5c621253d8f2a5a4adb64a6365905db67dffe3a2 2025-10-23 14:13:01 UTC to 2025-10-23 14:13:01 UTC - Add back the link for never_type_fallback_flowing_into_unsafe (rust-lang/edition-guide#378) ## rust-lang/reference 12 commits in 752eab01cebdd6a2d90b53087298844c251859a1..e122eefff3fef362eb7e0c08fb7ffbf5f9461905 2025-10-28 20:52:27 UTC to 2025-10-21 19:42:06 UTC - Remove custom redirect scripts (rust-lang/reference#2065) - Avoid specifying "last import wins" for MBEs (rust-lang/reference#2057) - Add caveats about mutable references in consts (rust-lang/reference#2058) - Move punctuation index to an appendix, and expand for other syntaxes (rust-lang/reference#2061) - Use American spelling for "labeled" (rust-lang/reference#2066) - Remove rules from names intro (rust-lang/reference#2060) - Minor adjustments to inner/outer attribute definitions (rust-lang/reference#2059) - Change underscore to be a keyword (rust-lang/reference#2049) - Update `proc_macro_attribute` to use the attribute template (rust-lang/reference#1889) - Update `proc_macro` to use the attribute template (rust-lang/reference#1887) - Update `macro_use` to use the attribute template (rust-lang/reference#1886) - Update `macro_export` to use the attribute template (rust-lang/reference#1885) ## rust-lang/rust-by-example 7 commits in 2c9b490d70e535cf166bf17feba59e594579843f..160e6bbca70b0c01aa4de88d19db7fc5ff8447c3 2025-11-03 12:26:45 UTC to 2025-10-22 13:19:06 UTC - Fix typos in flow_control/match/binding (rust-lang/rust-by-example#1971) - add clarification on overflowing_literals lint (rust-lang/rust-by-example#1970) - Update enum_use.md (rust-lang/rust-by-example#1960) - Remove weird extra spaces in code (rust-lang/rust-by-example#1962) - Include a link to The Rust Reference in flow_control/match/destructuring (rust-lang/rust-by-example#1963) - Add an example showing pattern binding when matching several values in a match arm (rust-lang/rust-by-example#1966) - Fix typo in linked list length calculation comment (rust-lang/rust-by-example#1968)

This tries to clarify a few things regarding fmt syntax: - The comment on `Parser::word` seems to be wrong, as that underscore-prefixed words are just fine. This was changed in rust-lang#66847. - I struggled to follow the description of the width argument. It referred to a "second argument", but I don't know what second argument it is referring to (which is the first?). Either way, I rewrote the paragraph to try to be a little more explicit, and to use shorter sentences. - The description of the precision argument wasn't really clear about the distinction of an Nth argument and a named argument. I added a sentence to try to emphasize the difference. - `IDENTIFIER_OR_KEYWORD` was changed recently in rust-lang/reference#2049 to include bare `_`. But fmt named arguments are not allowed to be a bare `_`.

Add some clarifications and fixes for fmt syntax This tries to clarify a few things regarding fmt syntax: - The comment on `Parser::word` seems to be wrong, as that underscore-prefixed words are just fine. This was changed in rust-lang#66847. - I struggled to follow the description of the width argument. It referred to a "second argument", but I don't know what second argument it is referring to (which is the first?). Either way, I rewrote the paragraph to try to be a little more explicit, and to use shorter sentences. - The description of the precision argument wasn't really clear about the distinction of an Nth argument and a named argument. I added a sentence to try to emphasize the difference. - `IDENTIFIER_OR_KEYWORD` was changed recently in rust-lang/reference#2049 to include bare `_`. But fmt named arguments are not allowed to be a bare `_`.

Rollup merge of #151013 - ehuss:fmt-clarification, r=joboet Add some clarifications and fixes for fmt syntax This tries to clarify a few things regarding fmt syntax: - The comment on `Parser::word` seems to be wrong, as that underscore-prefixed words are just fine. This was changed in #66847. - I struggled to follow the description of the width argument. It referred to a "second argument", but I don't know what second argument it is referring to (which is the first?). Either way, I rewrote the paragraph to try to be a little more explicit, and to use shorter sentences. - The description of the precision argument wasn't really clear about the distinction of an Nth argument and a named argument. I added a sentence to try to emphasize the difference. - `IDENTIFIER_OR_KEYWORD` was changed recently in rust-lang/reference#2049 to include bare `_`. But fmt named arguments are not allowed to be a bare `_`.

rustbot added the S-waiting-on-review Status: The marked PR is awaiting review from a maintainer label Oct 12, 2025

petrochenkov reviewed Oct 14, 2025
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ehuss force-pushed the underscore-keyword branch from fe4d139 to 46088a7Compare October 14, 2025 18:44

ehuss mentioned this pull request Oct 14, 2025
Document $crate in macro matcher #588
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ehuss added 2 commits October 22, 2025 00:14

traviscross force-pushed the underscore-keyword branch from 46088a7 to 0234184Compare October 22, 2025 00:14

traviscross approved these changes Oct 22, 2025
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traviscross added this pull request to the merge queue Oct 22, 2025

Merged via the queue into rust-lang:master with commit cd8d3d5Oct 22, 2025
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rustbot removed the S-waiting-on-review Status: The marked PR is awaiting review from a maintainer label Oct 22, 2025

rustbot mentioned this pull request Nov 3, 2025
Update books rust-lang/rust#148448
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ehuss mentioned this pull request Jan 12, 2026
Add some clarifications and fixes for fmt syntax rust-lang/rust#151013
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ehuss added this to the 1.93.0 milestone Jun 16, 2026

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Change underscore to be a keyword - #2049

Change underscore to be a keyword#2049
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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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