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.

<span class="caption">Table B-1: Operators</span>

| 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`, <code>&vert;...&vert; -> type</code> | 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` |
| <code>&vert;</code> | <code>pat &vert; pat</code> | Pattern alternatives | |
| <code>&vert;</code> | <code>expr &vert; expr</code> | Bitwise OR | `BitOr` |
| <code>&vert;=</code> | <code>var &vert;= expr</code> | Bitwise OR and assignment | `BitOrAssign` |
| <code>&vert;&vert;</code> | <code>expr &vert;&vert; expr</code> | 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.

<span class="caption">Table B-2: Stand-Alone Syntax</span>

| 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 |
| <code>&vert;...&vert; expr</code> | 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.

<span class="caption">Table B-3: Path-Related Syntax</span>

| 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.

<span class="caption">Table B-4: Generics</span>

| 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.

<span class="caption">Table B-5: Trait Bound Constraints</span>

| 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.

<span class="caption">Table B-6: Macros and Attributes</span>

| 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.

<span class="caption">Table B-7: Comments</span>

| 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.

<span class="caption">Table B-8: Tuples</span>

| 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`, etc. | Tuple indexing |

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

<span class="caption">Table B-9: Curly Brackets</span>

| Context | Explanation |
|---------|-------------|
| `{...}` | Block expression |
| `Type {...}` | `struct` literal |

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

<span class="caption">Table B-10: Square Brackets</span>

| 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” |