Appendix A: Keywords

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), including names of functions, variables, parameters, struct fields, modules, crates, constants, macros, static values, attributes, types, traits, or lifetimes.

Keywords Currently in Use

The following keywords currently have the functionality described.

• as - perform primitive casting, disambiguate the specific trait containing an item, or rename items in use and extern crate statements
• break - exit a loop immediately
• const - define constant items or constant raw pointers
• continue - continue to the next loop iteration
• crate - link an external crate or a macro variable representing the crate in which the macro is defined
• 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 crate, 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 implementing a trait
• 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
• 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 have any functionality but are reserved by Rust for potential future use.

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

Raw Identifiers

Raw identifiers let you use keywords where they would not normally be allowed by prefixing them with r#.

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

<span class="filename">Filename: src/main.rs</span>

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 says that you can’t use the keyword match as the function identifier. You can use match as a function name by using a raw identifier:

<span class="filename">Filename: src/main.rs</span>

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 both 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. In addition, raw identifiers allow you to use libraries written in a different Rust edition than your crate uses. For example, try is not a keyword in the 2015 edition but is in the 2018 edition. If you depend on a library that is written using the 2015 edition and has a try function, to call that function from your 2018 edition code, you’ll need to use the raw identifier syntax, r#try in this case. See Appendix E for more information on editions.