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Document array expression with a const. #914

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Dec 29, 2020
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Document array expression with a const. #914

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Move links to bottom of the file.
ehuss Dec 28, 2020
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Document array expression with a const.
ehuss Dec 22, 2020
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Document [a; 0] behavior.
ehuss Dec 28, 2020
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ehuss Dec 29, 2020
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43 changes: 28 additions & 15 deletions src/expressions/array-expr.md
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Expand Up @@ -10,26 +10,35 @@
> &nbsp;&nbsp; &nbsp;&nbsp; [_Expression_] ( `,` [_Expression_] )<sup>\*</sup> `,`<sup>?</sup>\
> &nbsp;&nbsp; | [_Expression_] `;` [_Expression_]

An _[array](../types/array.md) 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.
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 semi-colon. The expression after the `;` must have type
`usize` and be a [constant expression],
such as a [literal](../tokens.md#literals) or a [constant
item](../items/constant-items.md). `[a; b]` creates an array containing `b`
copies of the value of `a`. If the expression after the semi-colon has a value
greater than 1 then this requires that the type of `a` is
[`Copy`](../special-types-and-traits.md#copy).
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.

> **Note:** In the case where `b` is 0, and `a` is a non-constant item, there
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Can you use the warning for this?

<div class="warning">

Warning: This is an example warning.

</div>

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Done!

> is currently a bug in `rustc` where the value `a` is evaluated but not
> dropped, thus causing a leak. See [issue
> #74836](https://github.com/rust-lang/rust/issues/74836).

```rust
[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
Expand All @@ -44,10 +53,9 @@ expressions].
> _IndexExpression_ :\
> &nbsp;&nbsp; [_Expression_] `[` [_Expression_] `]`

[Array](../types/array.md) and [slice](../types/slice.md)-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.
[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
Expand Down Expand Up @@ -81,11 +89,16 @@ 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.

[`Copy`]: ../special-types-and-traits.md#copy
[IndexMut]: ../../std/ops/trait.IndexMut.html
[Index]: ../../std/ops/trait.Index.html
[Inner attributes]: ../attributes.md
[_Expression_]: ../expressions.md
[_InnerAttribute_]: ../attributes.md
[array]: ../types/array.md
[attributes on block expressions]: block-expr.md#attributes-on-block-expressions
[constant expression]: ../const_eval.md#constant-expressions
[constant item]: ../items/constant-items.md
[literal]: ../tokens.md#literals
[memory location]: ../expressions.md#place-expressions-and-value-expressions
[slice]: ../types/slice.md