keywords.md

Keywords

Rust divides keywords into three categories:

• strict
• reserved
• weak

Strict keywords

These keywords can only be used in their correct contexts. They cannot be used as the names of:

• Items
• Variables and function parameters
• Fields and variants
• Type parameters
• Lifetime parameters or loop labels
• Macros or attributes
• Macro placeholders
• Crates

^Lexer:
KW_AS : as
KW_BREAK : break
KW_CONST : const
KW_CONTINUE : continue
KW_CRATE : crate
KW_ELSE : else
KW_ENUM : enum
KW_EXTERN : extern
KW_FALSE : false
KW_FN : fn
KW_FOR : for
KW_IF : if
KW_IMPL : impl
KW_IN : in
KW_LET : let
KW_LOOP : loop
KW_MATCH : match
KW_MOD : mod
KW_MOVE : move
KW_MUT : mut
KW_PUB : pub
KW_REF : ref
KW_RETURN : return
KW_SELFVALUE : self
KW_SELFTYPE : Self
KW_STATIC : static
KW_STRUCT : struct
KW_SUPER : super
KW_TRAIT : trait
KW_TRUE : true
KW_TYPE : type
KW_UNSAFE : unsafe
KW_USE : use
KW_WHERE : where
KW_WHILE : while

The following keywords were added beginning in the 2018 edition.

^{Lexer 2018+}
KW_ASYNC : async
KW_AWAIT : await
KW_DYN : dyn

Reserved keywords

These keywords aren't used yet, but they are reserved for future use. They have the same restrictions as strict keywords. The reasoning behind this is to make current programs forward compatible with future versions of Rust by forbidding them to use these keywords.

^Lexer
KW_ABSTRACT : abstract
KW_BECOME : become
KW_BOX : box
KW_DO : do
KW_FINAL : final
KW_MACRO : macro
KW_OVERRIDE : override
KW_PRIV : priv
KW_TYPEOF : typeof
KW_UNSIZED : unsized
KW_VIRTUAL : virtual
KW_YIELD : yield

The following keywords are reserved beginning in the 2018 edition.

^{Lexer 2018+}
KW_TRY : try

Weak keywords

These keywords have special meaning only in certain contexts. For example, it is possible to declare a variable or method with the name union.

• union is used to declare a union and is only a keyword when used in a union declaration.

• 'static is used for the static lifetime and cannot be used as a generic lifetime parameter or loop label

  // error[E0262]: invalid lifetime parameter name: `'static`
  fn invalid_lifetime_parameter<'static>(s: &'static str) -> &'static str { s }
  

• In the 2015 edition, dyn is a keyword when used in a type position followed by a path that does not start with ::.

  Beginning in the 2018 edition, dyn has been promoted to a strict keyword.

^Lexer
KW_UNION : union
KW_STATICLIFETIME : 'static

^{Lexer 2015}
KW_DYN : dyn