Appendix A: Keywords

The following list contains keywords that are reserved for current or future use by the Rust language. As such, they cannot be used as identifiers (except as raw identifiers as we’ll discuss in the “Raw Identifiers” section). Identifiers are names of functions, variables, parameters, struct fields, modules, crates, constants, macros, static values, attributes, types, traits, or lifetimes.

Keywords Currently in Use

The following is a list of keywords currently in use, with their functionality described.

• as - perform primitive casting, disambiguate the specific trait containing an item, or rename items in use statements
• async - return a Future instead of blocking the current thread
• await - suspend execution until the result of a Future is ready
• break - exit a loop immediately
• const - define constant items or constant raw pointers
• continue - continue to the next loop iteration
• crate - in a module path, refers to the crate root
• dyn - dynamic dispatch to a trait object
• else - fallback for if and if let control flow constructs
• enum - define an enumeration
• extern - link an external function or variable
• false - Boolean false literal
• fn - define a function or the function pointer type
• for - loop over items from an iterator, implement a trait, or specify a higher-ranked lifetime
• if - branch based on the result of a conditional expression
• impl - implement inherent or trait functionality
• in - part of for loop syntax
• let - bind a variable
• loop - loop unconditionally
• match - match a value to patterns
• mod - define a module
• move - make a closure take ownership of all its captures
• mut - denote mutability in references, raw pointers, or pattern bindings
• pub - denote public visibility in struct fields, impl blocks, or modules
• ref - bind by reference
• return - return from function
• Self - a type alias for the type we are defining or implementing
• self - method subject or current module
• static - global variable or lifetime lasting the entire program execution
• struct - define a structure
• super - parent module of the current module
• trait - define a trait
• true - Boolean true literal
• type - define a type alias or associated type
• union - define a union; is only a keyword when used in a union declaration
• unsafe - denote unsafe code, functions, traits, or implementations
• use - bring symbols into scope
• where - denote clauses that constrain a type
• while - loop conditionally based on the result of an expression

Keywords Reserved for Future Use

The following keywords do not yet have any functionality but are reserved by Rust for potential future use.

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

Raw Identifiers

Raw identifiers are the syntax that lets you use keywords where they wouldn’t normally be allowed. You use a raw identifier by prefixing a keyword with r#.

For example, match is a keyword. If you try to compile the following function that uses match as its name:

Filename: src/main.rs

fn match(needle: &str, haystack: &str) -> bool {
    haystack.contains(needle)
}

you’ll get this error:

error: expected identifier, found keyword `match`
 --> src/main.rs:4:4
  |
4 | fn match(needle: &str, haystack: &str) -> bool {
  |    ^^^^^ expected identifier, found keyword

The error shows that you can’t use the keyword match as the function identifier. To use match as a function name, you need to use the raw identifier syntax, like this:

Filename: src/main.rs

fn r#match(needle: &str, haystack: &str) -> bool {
    haystack.contains(needle)
}
fn main() {
    assert!(r#match("foo", "foobar"));
}

This code will compile without any errors. Note the r# prefix on the function name in its definition as well as where the function is called in main.

Raw identifiers allow you to use any word you choose as an identifier, even if that word happens to be a reserved keyword. This gives us more freedom to choose identifier names, as well as lets us integrate with programs written in a language where these words aren’t keywords. In addition, raw identifiers allow you to use libraries written in a different Rust edition than your crate uses. For example, try isn’t a keyword in the 2015 edition but is in the 2018 edition. If you depend on a library that’s written using the 2015 edition and has a try function, you’ll need to use the raw identifier syntax, r#try in this case, to call that function from your 2018 edition code. See Appendix E for more information on editions.