B - Operators and Symbols

2020-01-31

Appendix B: Operators and Symbols

This appendix contains a glossary of Rust’s syntax, including operators and other symbols that appear by themselves or in the context of paths, generics, trait bounds, macros, attributes, comments, tuples, and brackets.

Operators

Table B-1 contains the operators in Rust, an example of how the operator would appear in context, a short explanation, and whether that operator is overloadable. If an operator is overloadable, the relevant trait to use to overload that operator is listed.

Table B-1: Operators

Operator Example Explanation Overloadable?
! ident!(...), ident!{...}, ident![...] Macro expansion
! !expr Bitwise or logical complement Not
!= var != expr Nonequality comparison PartialEq
% expr % expr Arithmetic remainder Rem
%= var %= expr Arithmetic remainder and assignment RemAssign
& &expr, &mut expr Borrow
& &type, &mut type, &'a type, &'a mut type Borrowed pointer type
& expr & expr Bitwise AND BitAnd
&= var &= expr Bitwise AND and assignment BitAndAssign
&& expr && expr Logical AND
* expr * expr Arithmetic multiplication Mul
*= var *= expr Arithmetic multiplication and assignment MulAssign
* *expr Dereference
* *const type, *mut type Raw pointer
+ trait + trait, 'a + trait Compound type constraint
+ expr + expr Arithmetic addition Add
+= var += expr Arithmetic addition and assignment AddAssign
, expr, expr Argument and element separator
- - expr Arithmetic negation Neg
- expr - expr Arithmetic subtraction Sub
-= var -= expr Arithmetic subtraction and assignment SubAssign
-> fn(...) -> type, |...| -> type Function and closure return type
. expr.ident Member access
.. .., expr.., ..expr, expr..expr Right-exclusive range literal
..= ..=expr, expr..=expr Right-inclusive range literal
.. ..expr Struct literal update syntax
.. variant(x, ..), struct_type { x, .. } “And the rest” pattern binding
... expr...expr In a pattern: inclusive range pattern
/ expr / expr Arithmetic division Div
/= var /= expr Arithmetic division and assignment DivAssign
: pat: type, ident: type Constraints
: ident: expr Struct field initializer
: 'a: loop {...} Loop label
; expr; Statement and item terminator
; [...; len] Part of fixed-size array syntax
<< expr << expr Left-shift Shl
<<= var <<= expr Left-shift and assignment ShlAssign
< expr < expr Less than comparison PartialOrd
<= expr <= expr Less than or equal to comparison PartialOrd
= var = expr, ident = type Assignment/equivalence
== expr == expr Equality comparison PartialEq
=> pat => expr Part of match arm syntax
> expr > expr Greater than comparison PartialOrd
>= expr >= expr Greater than or equal to comparison PartialOrd
>> expr >> expr Right-shift Shr
>>= var >>= expr Right-shift and assignment ShrAssign
@ ident @ pat Pattern binding
^ expr ^ expr Bitwise exclusive OR BitXor
^= var ^= expr Bitwise exclusive OR and assignment BitXorAssign
| pat | pat Pattern alternatives
| expr | expr Bitwise OR BitOr
|= var |= expr Bitwise OR and assignment BitOrAssign
|| expr || expr Logical OR
? expr? Error propagation

Non-operator Symbols

The following list contains all non-letters that don’t function as operators; that is, they don’t behave like a function or method call.

Table B-2 shows symbols that appear on their own and are valid in a variety of locations.

Table B-2: Stand-Alone Syntax

Symbol Explanation
'ident Named lifetime or loop label
...u8, ...i32, ...f64, ...usize, etc. Numeric literal of specific type
"..." String literal
r"...", r#"..."#, r##"..."##, etc. Raw string literal, escape characters not processed
b"..." Byte string literal; constructs a [u8] instead of a string
br"...", br#"..."#, br##"..."##, etc. Raw byte string literal, combination of raw and byte string literal
'...' Character literal
b'...' ASCII byte literal
|...| expr Closure
! Always empty bottom type for diverging functions
_ “Ignored” pattern binding; also used to make integer literals readable

Table B-3 shows symbols that appear in the context of a path through the module hierarchy to an item.

Table B-3: Path-Related Syntax

Symbol Explanation
ident::ident Namespace path
::path Path relative to the crate root (i.e., an explicitly absolute path)
self::path Path relative to the current module (i.e., an explicitly relative path).
super::path Path relative to the parent of the current module
type::ident, <type as trait>::ident Associated constants, functions, and types
<type>::... Associated item for a type that cannot be directly named (e.g., <&T>::..., <[T]>::..., etc.)
trait::method(...) Disambiguating a method call by naming the trait that defines it
type::method(...) Disambiguating a method call by naming the type for which it’s defined
<type as trait>::method(...) Disambiguating a method call by naming the trait and type

Table B-4 shows symbols that appear in the context of using generic type parameters.

Table B-4: Generics

Symbol Explanation
path<...> Specifies parameters to generic type in a type (e.g., Vec<u8>)
path::<...>, method::<...> Specifies parameters to generic type, function, or method in an expression; often referred to as turbofish (e.g., "42".parse::<i32>())
fn ident<...> ... Define generic function
struct ident<...> ... Define generic structure
enum ident<...> ... Define generic enumeration
impl<...> ... Define generic implementation
for<...> type Higher-ranked lifetime bounds
type<ident=type> A generic type where one or more associated types have specific assignments (e.g., Iterator<Item=T>)

Table B-5 shows symbols that appear in the context of constraining generic type parameters with trait bounds.

Table B-5: Trait Bound Constraints

Symbol Explanation
T: U Generic parameter T constrained to types that implement U
T: 'a Generic type T must outlive lifetime 'a (meaning the type cannot transitively contain any references with lifetimes shorter than 'a)
T : 'static Generic type T contains no borrowed references other than 'static ones
'b: 'a Generic lifetime 'b must outlive lifetime 'a
T: ?Sized Allow generic type parameter to be a dynamically sized type
'a + trait, trait + trait Compound type constraint

Table B-6 shows symbols that appear in the context of calling or defining macros and specifying attributes on an item.

Table B-6: Macros and Attributes

Symbol Explanation
#[meta] Outer attribute
#![meta] Inner attribute
$ident Macro substitution
$ident:kind Macro capture
$(…)… Macro repetition

Table B-7 shows symbols that create comments.

Table B-7: Comments

Symbol Explanation
// Line comment
//! Inner line doc comment
/// Outer line doc comment
/*...*/ Block comment
/*!...*/ Inner block doc comment
/**...*/ Outer block doc comment

Table B-8 shows symbols that appear in the context of using tuples.

Table B-8: Tuples

Symbol Explanation
() Empty tuple (aka unit), both literal and type
(expr) Parenthesized expression
(expr,) Single-element tuple expression
(type,) Single-element tuple type
(expr, ...) Tuple expression
(type, ...) Tuple type
expr(expr, ...) Function call expression; also used to initialize tuple structs and tuple enum variants
ident!(...), ident!{...}, ident![...] Macro invocation
expr.0, expr.1, etc. Tuple indexing

Table B-9 shows the contexts in which curly braces are used.

Table B-9: Curly Brackets

Context Explanation
{...} Block expression
Type {...} struct literal

Table B-10 shows the contexts in which square brackets are used.

Table B-10: Square Brackets

Context Explanation
[...] Array literal
[expr; len] Array literal containing len copies of expr
[type; len] Array type containing len instances of type
expr[expr] Collection indexing. Overloadable (Index, IndexMut)
expr[..], expr[a..], expr[..b], expr[a..b] Collection indexing pretending to be collection slicing, using Range, RangeFrom, RangeTo, or RangeFull as the “index”