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Add identifier syntax to identifiers.md #1583

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Oct 17, 2024
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Add identifier syntax to identifiers.md #1583

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3778433
Add identifier syntax to identifiers.md
chorman0773 Aug 22, 2024
c4d99b2
Switch to normalization
ehuss Sep 17, 2024
02596d2
Remove ascii-limitations specific rules
ehuss Sep 17, 2024
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24 changes: 24 additions & 0 deletions src/identifiers.md
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Original file line number Diff line number Diff line change
@@ -1,5 +1,8 @@
# Identifiers

r[ident]

r[ident.syntax]
> **<sup>Lexer:<sup>**\
> IDENTIFIER_OR_KEYWORD :\
> &nbsp;&nbsp; &nbsp;&nbsp; XID_Start XID_Continue<sup>\*</sup>\
Expand All @@ -13,6 +16,7 @@
> NON_KEYWORD_IDENTIFIER | RAW_IDENTIFIER

<!-- When updating the version, update the UAX links, too. -->
r[ident.unicode]
Identifiers follow the specification in [Unicode Standard Annex #31][UAX31] for Unicode version 15.0, with the additions described below. Some examples of identifiers:

* `foo`
Expand All @@ -21,6 +25,7 @@ Identifiers follow the specification in [Unicode Standard Annex #31][UAX31] for
* `Москва`
* `東京`

r[ident.profile]
The profile used from UAX #31 is:

* Start := [`XID_Start`], plus the underscore character (U+005F)
Expand All @@ -31,28 +36,47 @@ with the additional constraint that a single underscore character is not an iden

> **Note**: Identifiers starting with an underscore are typically used to indicate an identifier that is intentionally unused, and will silence the unused warning in `rustc`.

r[ident.keyword]
Identifiers may not be a [strict] or [reserved] keyword without the `r#` prefix described below in [raw identifiers](#raw-identifiers).

r[ident.zero-width-chars]
Zero width non-joiner (ZWNJ U+200C) and zero width joiner (ZWJ U+200D) characters are not allowed in identifiers.

r[ident.ascii-limitations]
Identifiers are restricted to the ASCII subset of [`XID_Start`] and [`XID_Continue`] in the following situations:

r[ident.ascii-extern-crate]
* [`extern crate`] declarations

r[ident.ascii-extern-prelude]
* External crate names referenced in a [path]

r[ident.ascii-outlined-module]
* [Module] names loaded from the filesystem without a [`path` attribute]

r[ident.ascii-no_mangle]
* [`no_mangle`] attributed items

r[ident.ascii-extern-item]
* Item names in [external blocks]

## Normalization

r[ident.normalization]

Identifiers are normalized using Normalization Form C (NFC) as defined in [Unicode Standard Annex #15][UAX15]. Two identifiers are equal if their NFC forms are equal.

[Procedural][proc-macro] and [declarative][mbe] macros receive normalized identifiers in their input.

## Raw identifiers

r[ident.raw]

r[ident.raw.intro]
A raw identifier is like a normal identifier, but prefixed by `r#`. (Note that
the `r#` prefix is not included as part of the actual identifier.)

r[ident.raw.allowed]
Unlike a normal identifier, a raw identifier may be any strict or reserved
keyword except the ones listed above for `RAW_IDENTIFIER`.

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