A - Keywords - The Rust Programming Language

## 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), 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
- `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` - import 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`
- `alignof`
- `become`
- `box`
- `do`
- `final`
- `macro`
- `offsetof`
- `override`
- `priv`
- `proc`
- `pure`
- `sizeof`
- `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:

    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

You can write this with a raw identifier:

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

    fn main() {
        assert!(r#match("foo", "foobar"));
    }

Note the `r#` prefix on both the function name as well as the call.

#### Motivation

This feature is useful for a few reasons, but the primary motivation was inter-edition situations. For example, `try` is not a keyword in the 2015 edition, but is in the 2018 edition. So if you have a library that is written in Rust 2015 and has a `try` function, to call it in Rust 2018, you'll need to use the raw identifier.