| r[ident] |
| # Identifiers |
| |
| r[ident.syntax] |
| ```grammar,lexer |
| IDENTIFIER_OR_KEYWORD -> |
| XID_Start XID_Continue* |
| | `_` XID_Continue+ |
| |
| XID_Start -> <`XID_Start` defined by Unicode> |
| |
| XID_Continue -> <`XID_Continue` defined by Unicode> |
| |
| 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_ |
| |
| IDENTIFIER -> NON_KEYWORD_IDENTIFIER | RAW_IDENTIFIER |
| |
| RESERVED_RAW_IDENTIFIER -> `r#_` |
| ``` |
| |
| <!-- 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 16.0, with the additions described below. Some examples of identifiers: |
| |
| * `foo` |
| * `_identifier` |
| * `r#true` |
| * `Москва` |
| * `東京` |
| |
| r[ident.profile] |
| The profile used from UAX #31 is: |
| |
| * Start := [`XID_Start`], plus the underscore character (U+005F) |
| * Continue := [`XID_Continue`] |
| * Medial := empty |
| |
| with the additional constraint that a single underscore character is not an identifier. |
| |
| > [!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: |
| |
| * [`extern crate`] declarations (except the [AsClause] identifier) |
| * External crate names referenced in a [path] |
| * [Module] names loaded from the filesystem without a [`path` attribute] |
| * [`no_mangle`] attributed items |
| * Item names in [external blocks] |
| |
| r[ident.normalization] |
| ## 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. |
| |
| r[ident.raw] |
| ## Raw identifiers |
| |
| 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`. |
| |
| r[ident.raw.reserved] |
| It is an error to use the [RESERVED_RAW_IDENTIFIER] token `r#_` in order to avoid confusion with the [WildcardPattern]. |
| |
| [`extern crate`]: items/extern-crates.md |
| [`no_mangle`]: abi.md#the-no_mangle-attribute |
| [`path` attribute]: items/modules.md#the-path-attribute |
| [`XID_Continue`]: http://unicode.org/cldr/utility/list-unicodeset.jsp?a=%5B%3AXID_Continue%3A%5D&abb=on&g=&i= |
| [`XID_Start`]: http://unicode.org/cldr/utility/list-unicodeset.jsp?a=%5B%3AXID_Start%3A%5D&abb=on&g=&i= |
| [external blocks]: items/external-blocks.md |
| [mbe]: macros-by-example.md |
| [module]: items/modules.md |
| [path]: paths.md |
| [proc-macro]: procedural-macros.md |
| [reserved]: keywords.md#reserved-keywords |
| [strict]: keywords.md#strict-keywords |
| [UAX15]: https://www.unicode.org/reports/tr15/tr15-56.html |
| [UAX31]: https://www.unicode.org/reports/tr31/tr31-41.html |
