Rust Language Cheat Sheet

10.12.2018

Contains clickable links to The Book ^BK, Rust by Example ^EX, Std Docs ^STD, Nomicon ^NOM, Reference ^REF. Furthermore, entries are marked as largely deprecated ^🗑️, have a minimum edition ^'18, or are bad ^⚡.

Data Structures

Define data types and memory locations, and use them.

Example	Explanation
`struct S {}`	Define a struct, ^BK ^EX ^STD ^REF with named fields.
`struct S` `()`	Define "tupled" struct with numbered fields `.0`, `.1`, ...
`struct S;`	Define zero sized unit struct.
`enum E {}`	Define an enum ^BK ^EX ^REF , c. algebraic data types, tagged unions.
`enum E { A, C {} }`	Define variants of enum; can be unit- `A`, tuple- `B` `()` and struct-like `C{}`.
`enum E { A = 1 }`	If variants are only unit-like, allow discriminant values, e.g., for FFI.
`union U {}`	Unsafe C-like union ^REF for FFI compatibility.
`static X: T = x;`	Global variable ^BK ^EX ^REF with `'static` lifetime, single memory location.
`const X: T = x;`	Define inlineable constant, ^BK ^EX ^REF. Inlined values are mutable!!!
`let x: T;`	Allocate `T` bytes on stack bound as `x`. Assignable once, not mutable.
`let mut x: T;`	Like `let`, but allow for mutability and mutable borrow.
`x = y`	Copy bytes at `y` to bytes at `x` if `T: Copy`. Compiler might optimize.
`x = y`	Same, but also invalidate `y` if `T` not `Copy`. Compiler might optimize.

Example	Explanation
`S { x: y }`	Create `struct S {}` or `use`'ed `enum E::S {}` with field `x` set to `y`.
`S { x }`	Same, but use local variable `x` for field `x`.
`S { ..s }`	Fill remaining fields from `s`, esp. useful with Default.
`S { 0: x }`	Like `S` `(x)` below, but set field `.0` with struct syntax.
`S` `(x)`	Create `struct S` `(T)` or `use`'ed `enum E::S` `()` with field `.0` set to `x`.
`S`	If `S` is unit `struct S;` or `use`'ed `enum E::S` create value of `S`.
`E::C { x: y }`	Create enum variant `C`. Other methods above also work.
`()`	Empty tuple, both literal and type, aka unit ^STD
`(x)`	Parenthesized expression.
`(x,)`	Single-element tuple expression. ^EX ^STD ^REF
`(T,)`	Single-element tuple type.
`[T]`	Array of unspecified length, i.e., slice. ^STD ^EX ^REF Can't live on stack.
`[T; n]`	Array type ^EX ^STD with `n` elements of type `T`.
`[x; n]`	Array with `n` copies of `x`. ^REF
`[x, y]`	Array with given elements.
`x[0]`	Collection indexing. Overloadable Index, IndexMut
`x[..]`	Collection slice-like indexing via RangeFull, c. slices.
`x[a..]`	Collection slice-like indexing via RangeFrom.
`x[..b]`	Collection slice-like indexing RangeTo.
`x[a..b]`	Collection slice-like indexing via Range.
`a..b`	Right-exclusive range ^REF creation, also seen as `..`, `a..`, `..b`.
`a..=b`	Inclusive range creation, also seen as `..=b`.
`x.i`	Member access. ^REF
`x.0`	Tuple access

References & Pointers

Granting access to un-owned memory. Also see section on Generics & Constraints.

Example	Explanation
`&T`	Immutable reference ^BK ^STD ^NOM ^REF (space for holding any `&t`).
`&[T]`	Special slice reference that contains address and length (like `&str`).
`&dyn T`	Special trait object ^BK reference that contains addr. of data and vtable.
`&mut T`	Like `&T` but allows mutability (also `&mut [T]`, `&mut dyn T`, ...)
`*const T`	Immutable raw pointer type ^BK ^STD ^REF, like `&T` but w/o compile safety.
`*mut T`	Mutable raw pointer type, like `&mut T` but w/o compile safety.
`&t`	Immutable borrow ^BK ^EX ^STD (on low level, address of this `t`, like `0x1234`).
`&mut t`	Borrow that allows mutability. ^EX
`ref t`	Bind by reference. ^BK ^EX ^🗑️
`*x`	Dereference. ^BK ^STD ^NOM
`'a`	A lifetime parameter, ^BK ^EX ^NOM ^REF, duration of a flow in static analysis.
`&'a T`	Place for an address of a `T`. Only accepts addr. living `'a` or longer.
`&'a mut T`	Same, but allow content of address to be changed.
`S<'a>`	Signals `S` will contain address with lifetime `'a`. Creator of `S` decides `'a`.
`fn f<'a>(t: &'a T)`	Same, for function. Caller decides `'a`.
`'static`	Special lifetime lasting the entire program execution.

Functions & Behavior

Define units of code and their abstractions.

Sigil	Explanation
`trait T {}`	Define a trait. ^BK ^EX ^REF
`trait T : R {}`	`T` is subtrait of supertrait ^REF `R`. Any `S` must `impl R` before it can `impl T`.
`impl S {}`	Implementation ^REF of functionality for a type `S`.
`impl T for S {}`	Implement trait `T` for type `S`.
`impl !T for S {}`	Disable an automatically derived auto trait ^NOM ^REF.
`fn f() {}`	Definition of a function ^BK ^EX ^REF; or associated function if inside `impl`.
`fn f() -> T {}`	Same, returning a value of type T.
`fn f(&self) {}`	Define a method as part of an `impl`.
`const fn f() {}`	Constant `fn` for compile time compilations, e.g., `const X: u32 = f(Y)`. ^'18
`fn() -> T`	Function pointers, ^BK ^STD ^REF don't confuse with trait Fn.
`\|\| {}`	A closure ^BK ^EX ^REF that borrows its captures.
`\|x\| {}`	Closure with a bound parameter `x`.
`\|x\| x + x`	Closure without block expression.
`move \|x\| x + y`	Closure taking ownership of its captures.
`return \|\| true`	Closures may sometimes look like logical ORs (here: return a closure).
`f()`	Invoke callable `f` (e.g., a function, closure, function pointer, `Fn`, ...).
`x.f()`	Call member function, requires `f` takes `self`, `&self`, ... as first argument.
`X::f(x)`	Same as `x.f()`. Unless `impl Copy for X {}`, `f` can only be called once.
`X::f(&x)`	Same as `x.f()`.
`X::f(&mut x)`	Same as `x.f()`.
`S::f(&x)`	Same as `x.f()` if `X` derefs to `S` (i.e., `x.f()` finds methods of `S`).
`T::f(&x)`	Same as `x.f()` if `X impl T` (i.e., `x.f()` finds methods of `T` if in scope).
`X::f()`	Call associated function, e.g., `X::new()`.
`<X as T>::f()`	Call `T::f()` implemented for `X`.
`unsafe {}`	Marker for unsafe code ^BK ^EX ^NOM ^REF that will probably segfa#%$@.

Control Flow

Control execution within a function.

Sigil	Explanation
`while x {}`	Loop ^REF, run while expression `x` is true.
`loop {}`	Loop infinitely ^REF until `break`. Can yield value with `break x`.
`for x in iter {}`	Syntactic sugar to loop over iterators. ^BK ^STD ^REF
`if x {} else {}`	Conditional branch ^REF if expression is true.
`'label: loop {}`	Loop label ^EX ^REF, useful for flow control in nested loops.
`break`	Break expression ^REF to exit a loop.
`break x`	Same, but make `x` value of the loop expression (only in actual `loop`).
`break 'label`	Exit not only this loop, but the enclosing one marked with `'label`.
`continue`	Continue expression ^REF to the next loop iteration of this loop.
`continue 'label`	Same, but instead of enclosing loop marked with `'label`.
`return x`	Early return from function. More idiomatic way is to end with expression.
`x?`	If `x` is Err or None, return and propagate. ^BK ^EX ^STD ^REF

Organizing Code

Segment projects into smaller units and minimize dependencies.

Sigil	Explanation
`mod m {}`	Define a module. ^BK ^EX ^REF
`a::b`	Namespace path ^EX ^REF to element `b` within `a` (`mod`, `enum`, ...).
`::x`	Search `x` relative to crate root. ^🗑️
`crate::x`	Search `x` relative to crate root. ^'18
`self::x`	Search `x` relative to current module.
`super::x`	Search `x` relative to parent module.
`use a::b;`	Use ^EX ^REF `b` directly in this scope without requiring `a` anymore.
`use a::{b, c};`	Same, but bring `b` and `c` into scope.
`use a::*;`	Bring everything from `a` into scope and reexport.
`pub use a::b;`	Bring `a::b` into scope and reexport from here.
`pub T`	"Public if parent path public" visibility ^BK for `T`.
`pub(crate) T`	Visible at most in current crate.
`pub(self) T`	Visible at most in current module.
`pub(super) T`	Visible at most in parent.
`pub(in a::b) T`	Visible at most in `a::b`.
`extern crate x`;	Declare dependency on external crate ^BK ^EX ^REF ^🗑️ ; just `use x::f` in ^'18.
`extern "C" fn`	External dependency for FFI. ^BK ^EX ^NOM ^REF

Type Aliases and Casts

Short-hand names of types, and methods to convert one type to another.

Sigil	Explanation
`type T = S;`	Create a type alias ^BK ^REF, i.e., another name for `S`.
`Self`	Type alias for implementing type ^REF, e.g. `fn new() -> Self`.
`self`	Method subject in `fn f(self) {}`, same as `fn f(self: Self) {}`.
`&self`	Same, but refers to self as borrowed, same as `f(self: &Self)`
`&mut self`	Same, but mutably borrowed, same as `f(self: &mut Self)`
`self: Box<Self>`	Arbitrary self type, add methods to smart pointers (`my_box.f_of_self()`).
`S as T`	Disambiguate ^BK ^REF type `S` as trait `T`.
`x as u32`	Primitive cast ^EX ^REF, may truncate and be a bit surprising. ^NOM

Code Generation

Constructs expanded before the actual compilation happens.

Example	Explanation
`m!()`	Macro ^BK ^STD ^REF invocation, also `m!{}`, `m![]` (depending on macro).
`$x:ty`	Macro capture, also `$x:expr`, `$x:ty`, `$x:path`, ... ^REF
`$x`	Macro substitution in macros by example. ^BK ^EX ^REF
`$(x),*`	Macro repetition "zero or more times" in macros by example.
`$(x),+`	Same, but "one or more times".
`$(x)<<+`	In fact separators other than `,` are also accepted. Here: `<<`.
`$crate`	Special hygiene variable, crate where macros is defined. ^?
`#[attr]`	Outer attribute. ^EX ^REF, annotating the following item.
`#![attr]`	Inner attribute, annotating the surrounding item.

Pattern Matching

These constructs are found in match or let expressions.

Example	Explanation
`match m {}`	Initiate pattern matching. ^BK ^EX ^REF
`E::A => {}`	Match enum variant `A`, c. pattern matching. ^BK ^EX ^REF
`E::B ( .. ) => {}`	Match enum tuple variant `B`, wildcard any index.
`E::C { .. } => {}`	Match enum struct variant `C`, wildcard any field.
`S { x: 0, y: 1 } => {}`	Match struct with specific params.
`S { x, y } => {}`	Match struct with any values for fields `x` and `y`.
`S { .. } => {}`	Match struct with any values.
`D => {}`	Match enum variant `E::D` if `D` in `use`.
`D => {}`	Match anything, bind `D`; ⚡ possibly false friend of `E::D` if `D` not in `use`.
`_ => {}`	Proper wildcard that matches anything / "all the rest".
`[a, 0] => {}`	Match array with any value for `a` and `0` for second.
`(a, 0) => {}`	Match tuple with any value for `a` and `0` for second.
`x @ 1 .. 5 => {}`	Bind matched to `x`; pattern binding ^BK ^EX.
`0 \| 1 => {}`	Pattern alternatives (or-patterns).
`S { x } if x > 10`	Pattern match guards. ^BK ^EX

Example	Explanation
`let Some(x) = Some(5)`	Notably, `let` also pattern matches similar to the table above.
`let S { x } = s`	Only `x` will be bound to value `s.x`.
`let (_, b, _) = abc`	Only `b` will be bound to value `abc.1`.
`let (a, ..) = abc`	Ignoring 'the rest' also works.
`let Some(x) = get()`	⚡ Will not work if pattern can be 'refuted', use `if let` instead.
`if let Some(x) = get()`	Branch if pattern can actually be assigned ^REF (e.g., `enum` variant).
`fn f(S { x }: S)`	Function parameters also work like `let`, here `x` bound to `s.x` of `f(s)`.

Generics & Constraints

Generics combine with many other constructs such as struct S<T>, fn f<T>(), ...

Example	Explanation
`S<T>`	A generic ^BK ^EX type with a type parameter (`T` is placeholder name).
`S<T: R>`	Type short hand trait bound ^BK ^EX specification (`R` must be trait).
`T: R + S`	Compound type bound ^BK ^EX, also seen as `T: R + 'a`
`T: ?Sized`	Opt out of a pre-defined trait bound Sized. ^?
`T: 'a`	Type lifetime bound ^EX, all references in T must outlive `'a`.
`'b: 'a`	Lifetime `'b` must live at least as long as (i.e., outlives) `'a` bound.
`S<T> where T: R`	Same as `S<T: R>` but easier for longer bounds.
`S<T = R>`	Default type parameter ^BK for associated type.
`S<'_>`	Inferred anonymous lifetime. ^BK
`S<_>`	Inferred anonymous type. ^?
`S::<T>`	Turbofish ^STD call site type disambiguation, e.g. `f::<u32>()`.
`trait T<X> {}`	A trait generic over `X`. Can have multiple `impl T for S` (one per `X`).
`trait T { type X; }`	Defines associated type ^BK ^REF `X`. Only one `impl T for S` possible.
`type X = R;`	Set associated type within `impl T for S { type X = R; }`.
`impl<T> S<T> {}`	Implement functionality for any `T` in `S<T>`.
`impl S<T> {}`	Implement functionality for exactly `S<T>` (e.g., `S<u32>`).
`fn f() -> impl T`	Existential types ^BK, returns an unknown-to-caller `S` that `impl T`.
`fn f(x: &impl T)`	Trait bound,"impl traits" ^BK, somewhat similar to `fn f<S:T>(x: &S)`.
`fn f(x: &dyn T)`	Marker for dynamic dispatch ^BK ^REF, `f` will not be monomorphized.
`for<'a>`	Higher-rank trait bounds. ^NOM ^REF

Strings & Chars

Rust has several ways to create string or char literals, depending on your needs.

Example	Explanation
`"..."`	String literal ^REF, will escape `\n`, ...
`r"..."`,	Raw string literal. ^REF, won't escape `\n`, ...
`r#"..."#`, etc.	Raw string literal, but can also contain `"`.
`b"..."`	Byte string literal ^REF; constructs ASCII `[u8]`, not a string.
`br"..."`, `br#"..."#`, etc.	Raw byte string literal, combination of the above.
`'🦀'`	Character literal ^REF, can contain unicode.
`b'x'`	ASCII byte literal. ^REF

Comments

No comment.

Example	Explanation
`//`	Line comment.
`//!`	Inner line doc comment. ^BK ^EX ^REF
`///`	Outer line doc comment.
`/.../`	Block comment.
`/!.../`	Inner block doc comment.
`/*.../`	Outer block doc comment.

Miscellaneous

These sigils did not fit any other category but are good to know nonetheless.

Example	Explanation
`!`	Always empty never type. ^BK ^EX ^STD ^REF
`_`	Unnamed variable binding, e.g., `\|x, _\| {}`.
`_x`	Variable binding explicitly marked as unused.
`1_234_567`	Numeric separator for visual clarity.
`1u8`	Type specifier for numeric literals ^EX ^REF (also `i8`, `u16`, ...).
`r#foo`	A raw identifier ^BK ^EX for edition compatibility.
`x;`	Statement ^REF terminator, c. expressions ^EX ^REF

Common Operators

Rust supports all common operators you would expect to find in a language (+, *, %, =, ==...). Since they behave no differently in Rust we do not list them here. For some of them Rust also support operator overloading. ^STD

Invisible Sugar

If something works that "shouldn't work now that you think about it", it might be due to one of these.

Name	Description
Coercions ^NOM	'Weaken' types to match signature, e.g., `&mut T` to `&T`.
Deref ^NOM	Deref `x: T` until `x`, `*x`, ... compatible with some target `S`.
Prelude ^STD	Automatic import of basic types.
Reborrow	Since `x: &mut T` can't be copied; move new `&mut *x` instead.
Lifetime Elision ^BK ^NOM ^REF	Automatically annotate `f(x: &T)` to `f(x: &'a T)`.
Method Resolution ^REF	Deref or borrow `x` until `x.f()` works.

Idiomatic Rust

If you are used to programming Java or C, consider these.

Idiom	Code
Think in Expressions	`x = if x { a } else { b };`
	`x = loop { break 5 };`
	`fn f() -> u32 { 0 }`
Think in Iterators	`(1..10).map(f).collect()`
	`names.iter().filter(\|x\| x.starts_with("A"))`
Handle Absence with `?`	`x = try_something()?;`
	`get_option()?.run()?`
Use Strong Types	`enum E { Invalid, Valid { ... } }` over `ERROR_INVALID = -1`
	`enum E { Visible, Hidden }` over `visible: bool`
	`struct Charge(f32)` over `f32`
Provide Builders	`Car::new("X").hp(5).run();`
Split Implementations	Generic types `S<T>` can have a separate `impl` per `T`.
	Rust doesn't have OO, but with separate `impl` you can get specialization.
Unsafe	Avoid `unsafe {}` code unless doing FFI.
Implement Traits	`#[derive(Debug, Copy, ...)]` and custom `impl` where needed.
Tooling	With clippy you can improve your code quality.
	Formatting with rustfmt helps others to read your code.
	Annotate your APIs with doc comments that can show up on docs.rs.
	Add doc tests ^BK (``` my_api::f() ```) to ensure docs match code.
	Add unit tests ^BK (`#[test]`) to ensure your code works.
	Follow the API Guidelines to make your API feel Rustic.

A Guide to Reading Lifetimes

Lifetimes can be overwhelming at times. Here is a simplified guide on how to read and interpret constructs containing lifetimes if you are familiar with C.

Note: This section is work in progress. For the time being, use with a grain of salt. Feedback very welcome!

Construct	How to read
`let t: T = T(0)`	A location that is `T`-sized, named `t`, and contains the value `T(0)`.
	If declared with `let`, that location lives on the stack.
	Generally, `t` can mean location of `t`, and value within `t`.
	As a location, `t = T(1)` means, assign value `T(1)` to location `t`.
	As a value, `f(t)` means call `f` with value inside of `t`.
	To explicitly talk about its location (address) we do `&t`.
	To explicitly talk about a location that can hold such a location we do `&T`.
`&'a T`	A `&T` is a location that can hold an address (i.e., reference).
	Any address stored in here must be that of a valid `T`.
	Any address stored must live at least for (outlive) duration `'a`.
	That means during `'a` memory targeted by `&T` can't be invalidated.
	Also, this `&T` must be stopped being used before `'a` ends.
	Duration of `'a` is purely compile time view, based on static analysis.
`&T`	Sometimes `'a` might be elided (or can't be specified) but it still exists.
	Within methods bodies, lifetimes are determined automatically.
	Within signatures, lifetimes may be 'elided' (annotated automatically).
`&t`	This will produce the actual address of location `t`, called 'borrow'.
	The moment `&t` is produced, location `t` is put into a borrowed state.
	As long as any `&t` could be around, `t` cannot be altered directly.
	This analysis is based on all possible address propagation paths.
	For example, in `let a = &t; let b = a;`, also `b` needs to go.
	Borrowing of `t` stops once last `&t` is last used, not when `&t` dropped.
`&mut t`	Same, but will produce a mutable borrow.
	A `&mut` will allow the owner of the borrow (address) to change `t`'s content.
	This reiterates that not the value in `t`, but `t`'s location is borrowed.
`S<'a> {}`	Signals that `S` will hold an address (i.e., reference).
	`'a` will be determined automatically by the user of this struct.
	`'a` will be chosen as small as possible.
`f<'a>(x: &'a T)`	Signals this function will accept an address (i.e., reference).
`-> &'a S`	... and that it returns one.
	`'a` will be determined automatically by the caller.
	`'a` will be chosen as small as possible.
	`'a` will be picked so that it satisfies input and output at call site.
	`'a` is mix of where `x` comes from and `f(x)` goes.
	In addition, propagate borrow state according to lifetime names!
	So while result address with `'a` is used, input address with `'a` is locked.
	Here: while `s` from `let s = f(&x)` is around, `x` counts as 'borrowed'.

Tooling

Some commands and tools that are good to know.

Command	Description
`cargo init`	Create a new project for the latest edition.
`cargo build`	Build the project in debug mode (`--release` for all optimization).
`cargo check`	Check if project would compile (much faster).
`cargo rustc -- -Zunpretty=X`	Show more desugared Rust code, in particular with X being:
	`expanded` Show with expanded macros, ...
`rustup docs`	Open offline Rust documentation, good on a plane!

These are 3rd party tools and need to be installed with cargo install cargo-[tool] first. They often require unstable and are subject to break.

Command	Description
`cargo asm`	Show generated assembly instructions for code.

More Cheats

These are other great visual guides and tables.

Containers

Macro Railroad

Lifetimes

Cheat Sheet	Description
Rust Learning⭐	Probably the best collection of links about learning Rust.
String Conversions	How to get type of string from another.
API Guidelines Checklist	How to design your own APIs.
Periodic Table of Types	How various types and references correlate.
Futures	How to construct and work with futures.