# Glossary

### Abstract syntax tree

An ‘abstract syntax tree’, or ‘AST’, is an intermediate representation of the structure of the program when the compiler is compiling it.

### Alignment

The alignment of a value specifies what addresses values are preferred to start at. Always a power of two. References to a value must be aligned. More.

### Arity

Arity refers to the number of arguments a function or operator takes. For some examples, f(2, 3) and g(4, 6) have arity 2, while h(8, 2, 6) has arity 3. The ! operator has arity 1.

### Array

An array, sometimes also called a fixed-size array or an inline array, is a value describing a collection of elements, each selected by an index that can be computed at run time by the program. It occupies a contiguous region of memory.

### Associated item

An associated item is an item that is associated with another item. Associated items are defined in implementations and declared in traits. Only functions, constants, and type aliases can be associated. Contrast to a free item.

### Blanket implementation

Any implementation where a type appears uncovered. impl<T> Foo for T, impl<T> Bar<T> for T, impl<T> Bar<Vec<T>> for T, and impl<T> Bar<T> for Vec<T> are considered blanket impls. However, impl<T> Bar<Vec<T>> for Vec<T> is not a blanket impl, as all instances of T which appear in this impl are covered by Vec.

### Bound

Bounds are constraints on a type or trait. For example, if a bound is placed on the argument a function takes, types passed to that function must abide by that constraint.

### Combinator

Combinators are higher-order functions that apply only functions and earlier defined combinators to provide a result from its arguments. They can be used to manage control flow in a modular fashion.

### Dispatch

Dispatch is the mechanism to determine which specific version of code is actually run when it involves polymorphism. Two major forms of dispatch are static dispatch and dynamic dispatch. While Rust favors static dispatch, it also supports dynamic dispatch through a mechanism called ‘trait objects’.

### Dynamically sized type

A dynamically sized type (DST) is a type without a statically known size or alignment.

### Expression

An expression is a combination of values, constants, variables, operators and functions that evaluate to a single value, with or without side-effects.

For example, 2 + (3 * 4) is an expression that returns the value 14.

### Free item

An item that is not a member of an implementation, such as a free function or a free const. Contrast to an associated item.

### Fundamental traits

A fundamental trait is one where adding an impl of it for an existing type is a breaking change. The Fn traits and Sized are fundamental.

### Fundamental type constructors

A fundamental type constructor is a type where implementing a blanket implementation over it is a breaking change. &, &mut, Box, and Pin are fundamental.

Any time a type T is considered local, &T, &mut T, Box<T>, and Pin<T> are also considered local. Fundamental type constructors cannot cover other types. Any time the term “covered type” is used, the T in &T, &mut T, Box<T>, and Pin<T> is not considered covered.

### Inhabited

A type is inhabited if it has constructors and therefore can be instantiated. An inhabited type is not “empty” in the sense that there can be values of the type. Opposite of Uninhabited.

### Inherent implementation

An implementation that applies to a nominal type, not to a trait-type pair. More.

### Inherent method

A method defined in an inherent implementation, not in a trait implementation.

### Initialized

A variable is initialized if it has been assigned a value and hasn’t since been moved from. All other memory locations are assumed to be uninitialized. Only unsafe Rust can create such a memory without initializing it.

### Local trait

A trait which was defined in the current crate. A trait definition is local or not independent of applied type arguments. Given trait Foo<T, U>, Foo is always local, regardless of the types substituted for T and U.

### Local type

A struct, enum, or union which was defined in the current crate. This is not affected by applied type arguments. struct Foo is considered local, but Vec<Foo> is not. LocalType<ForeignType> is local. Type aliases do not affect locality.

### Nominal types

Types that can be referred to by a path directly. Specifically enums, structs, unions, and trait objects.

### Object safe traits

Traits that can be used as trait objects. Only traits that follow specific rules are object safe.

### Prelude

Prelude, or The Rust Prelude, is a small collection of items - mostly traits - that are imported into every module of every crate. The traits in the prelude are pervasive.

### Scrutinee

A scrutinee is the expression that is matched on in match expressions and similar pattern matching constructs. For example, in match x { A => 1, B => 2 }, the expression x is the scrutinee.

### Size

The size of a value has two definitions.

The first is that it is how much memory must be allocated to store that value.

The second is that it is the offset in bytes between successive elements in an array with that item type.

It is a multiple of the alignment, including zero. The size can change depending on compiler version (as new optimizations are made) and target platform (similar to how usize varies per-platform).

More.

### Slice

A slice is dynamically-sized view into a contiguous sequence, written as [T].

It is often seen in its borrowed forms, either mutable or shared. The shared slice type is &[T], while the mutable slice type is &mut [T], where T represents the element type.

### Statement

A statement is the smallest standalone element of a programming language that commands a computer to perform an action.

### String literal

A string literal is a string stored directly in the final binary, and so will be valid for the 'static duration.

Its type is 'static duration borrowed string slice, &'static str.

### String slice

A string slice is the most primitive string type in Rust, written as str. It is often seen in its borrowed forms, either mutable or shared. The shared string slice type is &str, while the mutable string slice type is &mut str.

Strings slices are always valid UTF-8.

### Trait

A trait is a language item that is used for describing the functionalities a type must provide. It allows a type to make certain promises about its behavior.

Generic functions and generic structs can use traits to constrain, or bound, the types they accept.

### Uncovered type

A type which does not appear as an argument to another type. For example, T is uncovered, but the T in Vec<T> is covered. This is only relevant for type arguments.

### Undefined behavior

Compile-time or run-time behavior that is not specified. This may result in, but is not limited to: process termination or corruption; improper, incorrect, or unintended computation; or platform-specific results. More.

### Uninhabited

A type is uninhabited if it has no constructors and therefore can never be instantiated. An uninhabited type is “empty” in the sense that there are no values of the type. The canonical example of an uninhabited type is the never type !, or an enum with no variants enum Never { }. Opposite of Inhabited.