The Periodic Table of Rust Types

Immutable Pointer Mutable Pointer Owned Pointer Bare Unsized
Raw *T
Immutable raw pointer
*mut T
Mutable raw pointer
Raw pointers do not have ownership, *T or *mut T should be used as appropriate.
Simple &'r T
Immutable borrowed reference
&'r mut T
Mutable borrowed reference
Owned pointer
Primitive type, struct, enum and so on
Trait &'r Trait:K
Immutable borrowed trait object
&'r mut Trait:K
Mutable borrowed trait object
Owned trait object
Unsized trait type
Impossible today, the trait should be wrapped inside a box.
Vector &'r [T]
Immutable borrowed vector slice
&'r mut [T]
Mutable borrowed vector slice
Owned vector
[T, ..n]
Fixed-size vector
Unsized vector type
Impossible today, the vector should know its size at compile time or be wrapped inside a box.
String &'r str
Immutable borrowed string slice
&'r mut str
Mutable borrowed string slice
Impossible today, there are few operations available to the fixed-size mutable slice.
Owned string
Fixed sized storage is impractical for the variable length UTF-8 encoding.
Unsized string type
Impossible today, the string should be wrapped inside a box.
&'r Fn<T…, U>:K
Closure with immutable environment
Impossible today, closures always assume a mutable environment.
'r |T…|:K -> U
≡ &'r mut FnMut<T…, U>:K
Closure with mutable environment
~Fn<T…, U>:K
~FnMut<T…, U>:K
Closure with owned environment
Impossible today, closures cannot be sent.
extern "ABI"
fn(T…) -> U

Bare function type
Procedures should be able to be consumed.
Procedures should be able to be consumed.
proc(T…) -> U
≡ ~FnOnce<T…, U>:K
Procedure with owned environment
Supported as of 0.10
Trait bounds
Function arguments
-> U
Function return
extern "ABI"
ABI definition

What is this?

This "periodic table" is a cheatsheet for various Rust types. Rust programming language has a versatile type system that avoids a certain class of memory error in the safe context, and consequently has somewhat many types at the first glance. This table organizes them into an orthogonal tabular form, making them easier to understand and reason. I also hope that this makes very obvious why Rust needs such seemingly many types.

The periodic table was made by Kang Seonghoon as a thought experiment, and then... it have got redditted unexpectedly :p Henceforth I've made the URL permanent and added some descriptions. Some post-reddit changes include:

Discussion: /r/rust, /r/programming, Hacker News


Columns indicate the ownership. There are two big groups from left to right: indirect (i.e. pointers) and direct. Particularly indirect types can be coerced horizontally: ~T can be coerced to &mut T or &T and &mut T can be coerced to &T.

Rows indicate the different kind of types. There are three big groups from top to bottom: unsafe dereference, safe dereference and callable.

Colored backgrounds (sorry the accessibility!) indicate the current availability. Black background means the type is plain absurd and prohibited. Gray background means the type makes some sense but it is not yet in the language. Fortunately we have two proposals that cover all the missing types now:

There are some optional syntactic parts possible in types. Many of them are turned off by default since they are normally verbose, but you can turn them on if you want.

Copyright © 2014, Kang Seonghoon. This work is licensed under a Creative Commons Attribution 4.0 International License.

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