identifiers.md

r[ident]

Identifiers

r[ident.syntax]

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#_`

r[ident.unicode] Identifiers follow the specification in Unicode Standard Annex #31 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.

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. Two identifiers are equal if their NFC forms are equal.

Procedural and declarative 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].