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Keywords

Rust divides keywords into three categories:

Strict keywords

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

Lexer
STRICT_KEYWORDS →
      _
    | as
    | async
    | await
    | break
    | const
    | continue
    | crate
    | dyn
    | else
    | enum
    | extern
    | false
    | fn
    | for
    | if
    | impl
    | in
    | let
    | loop
    | match
    | mod
    | move
    | mut
    | pub
    | ref
    | return
    | self
    | Self
    | static
    | struct
    | super
    | trait
    | true
    | type
    | unsafe
    | use
    | where
    | while

STRICT_KEYWORDS _ as async await break const continue crate dyn else enum extern false fn for if impl in let loop match mod move mut pub ref return self Self static struct super trait true type unsafe use where while

2018 Edition differences

The following keywords were added in the 2018 edition:

  • async
  • await
  • dyn

Reserved keywords

Reserved 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
RESERVED_KEYWORDS →
      abstract
    | become
    | box
    | do
    | final
    | gen
    | macro
    | override
    | priv
    | try
    | typeof
    | unsized
    | virtual
    | yield

RESERVED_KEYWORDS abstract become box do final gen macro override priv try typeof unsized virtual yield

2018 Edition differences

The try keyword was added as a reserved keyword in the 2018 edition.

2024 Edition differences

The gen keyword was added as a reserved keyword in the 2024 edition.

Weak keywords

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

Lexer
WEAK_KEYWORDS →
      'static
    | macro_rules
    | raw
    | safe
    | union

WEAK_KEYWORDS 'static macro_rules raw safe union
  • macro_rules is used to create custom macros.
  • 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 }
  • safe is used for functions and statics, which has meaning in external blocks.
  • raw is used for raw borrow operators, and is only a keyword when matching a raw borrow operator form (such as &raw const expr or &raw mut expr).

2018 Edition differences

In the 2015 edition, dyn is a keyword when used in a type position followed by a path that does not start with :: or <, a lifetime, a question mark, a for keyword or an opening parenthesis.

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