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array-expr.md

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Array and array index expressions

Array expressions

Syntax
ArrayExpression :
   [ InnerAttribute* ArrayElements? ]

ArrayElements :
      Expression ( , Expression )* ,?
   | Expression ; Expression

An array expression can be written by enclosing zero or more comma-separated expressions of uniform type in square brackets. This produces an array containing each of these values in the order they are written.

Alternatively there can be exactly two expressions inside the brackets, separated by a semicolon. The expression after the ; must have type usize and be a constant expression, such as a literal or a constant item. [a; b] creates an array containing b copies of the value of a. If the expression after the semicolon has a value greater than 1 then this requires that the type of a is Copy, or a must be a path to a constant item.

When the repeat expression a is a constant item, it is evaluated b times. If b is 0, the constant item is not evaluated at all. For expressions that are not a constant item, it is evaluated exactly once, and then the result is copied b times.

Warning: In the case where b is 0, and a is a non-constant item, there is currently a bug in rustc where the value a is evaluated but not dropped, thus causing a leak. See issue #74836.

[1, 2, 3, 4];
["a", "b", "c", "d"];
[0; 128];              // array with 128 zeros
[0u8, 0u8, 0u8, 0u8,];
[[1, 0, 0], [0, 1, 0], [0, 0, 1]]; // 2D array
const EMPTY: Vec<i32> = Vec::new();
[EMPTY; 2];

Array expression attributes

Inner attributes are allowed directly after the opening bracket of an array expression in the same expression contexts as attributes on block expressions.

Array and slice indexing expressions

Syntax
IndexExpression :
   Expression [ Expression ]

Array and slice-typed expressions can be indexed by writing a square-bracket-enclosed expression of type usize (the index) after them. When the array is mutable, the resulting memory location can be assigned to.

For other types an index expression a[b] is equivalent to *std::ops::Index::index(&a, b), or *std::ops::IndexMut::index_mut(&mut a, b) in a mutable place expression context. Just as with methods, Rust will also insert dereference operations on a repeatedly to find an implementation.

Indices are zero-based for arrays and slices. Array access is a constant expression, so bounds can be checked at compile-time with a constant index value. Otherwise a check will be performed at run-time that will put the thread in a panicked state if it fails.

// lint is deny by default.
#![warn(unconditional_panic)]

([1, 2, 3, 4])[2];        // Evaluates to 3

let b = [[1, 0, 0], [0, 1, 0], [0, 0, 1]];
b[1][2];                  // multidimensional array indexing

let x = (["a", "b"])[10]; // warning: index out of bounds

let n = 10;
let y = (["a", "b"])[n];  // panics

let arr = ["a", "b"];
arr[10];                  // warning: index out of bounds

The array index expression can be implemented for types other than arrays and slices by implementing the Index and IndexMut traits.