book/nostarch/appendix_b.md

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## 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` |
| `!=` | `expr != 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 tables contain all symbols 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`, and so on | Numeric literal of
specific type |
| `"..."` | String literal |
| `r"..."`, `r#"..."#`, `r##"..."##`, and so on | Raw string literal; escape
characters not processed |
| `b"..."` | Byte string literal; constructs an array of bytes instead of a
string |
| `br"..."`, `br#"..."#`, `br##"..."##`, and so on | 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 (that is, an explicitly absolute
path) |
| `self::path` | Path relative to the current module (that is, 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 (for
example, `<&T>::...`, `<[T]>::...`, and so on) |
| `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 a generic type in a type (for example,
`Vec<u8>`) |
| `path::<...>, method::<...>` | Specifies parameters to a generic type,
function, or method in an expression; often referred to as turbofish (for
example, `"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 (for example, `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
`struct`s and tuple `enum` variants |
| `expr.0`, `expr.1`, and so on | Tuple indexing |

Table B-9 shows the contexts in which curly brackets 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” |