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.
 

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