rust-course/writing-material/posts/operators.md

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	Short-circuiting logical AND
*	expr * expr	Arithmetic multiplication	Mul
*=	var *= expr	Arithmetic multiplication and assignment	MulAssign
*	*expr	Dereference	Deref
*	*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	PartialOrd
..=	..=expr, expr..=expr	Right-inclusive range literal	PartialOrd
..	..expr	Struct literal update syntax
..	variant(x, ..), struct_type { x, .. }	“And the rest” pattern binding
...	expr...expr	(Deprecated, use ..= instead) 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	Short-circuiting 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
ident!(...), ident!{...}, ident![...]	Macro invocation

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
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”