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Update type casts section #961

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Mar 26, 2021
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Update type casts section #961

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13 changes: 9 additions & 4 deletions src/expressions/operator-expr.md
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Expand Up @@ -307,7 +307,8 @@ fn average(values: &[f64]) -> f64 {

`as` can be used to explicitly perform [coercions](../type-coercions.md), as well as the following additional casts.
Any cast that does not fit either a coercion rule or an entry in the table is a compiler error.
Here `*T` means either `*const T` or `*mut T`.
Here `*T` means either `*const T` or `*mut T`. `m` stands for optional `mut` in
reference types and `mut` or `const` in pointer types.

| Type of `e` | `U` | Cast performed by `e as U` |
|-----------------------|-----------------------|----------------------------------|
Expand All @@ -318,17 +319,21 @@ Here `*T` means either `*const T` or `*mut T`.
| `*T` | `*V` where `V: Sized` \* | Pointer to pointer cast |
| `*T` where `T: Sized` | Integer type | Pointer to address cast |
| Integer type | `*V` where `V: Sized` | Address to pointer cast |
| `&[T; n]` | `*const T` | Array to pointer cast |
| `&m₁ T` | `*m₂ T` \*\* | Reference to pointer cast |
| `&m₁ [T; n]` | `*m₂ T` \*\* | Array to pointer cast |
| [Function item] | [Function pointer] | Function item to function pointer cast |
| [Function item] | `*V` where `V: Sized` | Function item to pointer cast |
| [Function item] | Integer | Function item to address cast |
| [Function pointer] | `*V` where `V: Sized` | Function pointer to pointer cast |
| [Function pointer] | Integer | Function pointer to address cast |
| Closure \*\* | Function pointer | Closure to function pointer cast |
| Closure \*\*\* | Function pointer | Closure to function pointer cast |

\* or `T` and `V` are compatible unsized types, e.g., both slices, both the same trait object.

\*\* only for closures that do not capture (close over) any local variables
\*\* only when `m₁` is `mut` or `m₂` is `const`. Casting `mut` reference to
`const` pointer is allowed.

\*\*\* only for closures that do not capture (close over) any local variables

### Semantics

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