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Merged
merged 4 commits into from
Feb 11, 2025
Merged

as cast improvements #1732

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e64212e
fixup list of valid `as` casts
WaffleLapkin Feb 7, 2025
7dc9ee3
use footnotes instead of `\*` in `as` description
WaffleLapkin Feb 7, 2025
cd3ec42
link from `as` cast table to semantic chapters
WaffleLapkin Feb 7, 2025
39d94e1
fixup footnotes
WaffleLapkin Feb 10, 2025
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54 changes: 27 additions & 27 deletions src/expressions/operator-expr.md
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Original file line number Diff line number Diff line change
Expand Up @@ -454,35 +454,35 @@ Any cast that does not fit either a coercion rule or an entry in the table is a
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` |
|-----------------------|-----------------------|----------------------------------|
| Integer or Float type | Integer or Float type | Numeric cast |
| Enumeration | Integer type | Enum cast |
| `bool` or `char` | Integer type | Primitive to integer cast |
| `u8` | `char` | `u8` to `char` cast |
| `*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 |
| `&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 |

\* or `T` and `V` are unsized types with compatible metadata:

* Both slice metadata (`*[u16]` -> `*[u8]`, `*str` -> `*(u8, [u32])`).
* Both the same trait object metadata, modulo dropping auto traits (`*dyn Debug` -> `*(u16, dyn Debug)`, `*dyn Debug + Send` -> `*dyn Debug`).
* **Note**: Adding auto traits is only allowed if the principal trait has the auto trait as a super trait (given `trait T: Send {}`, `*dyn T` -> `*dyn T + Send` is valid, but `*dyn Debug` -> `*dyn Debug + Send` is not).
* **Note**: Generics (including lifetimes) must match (`*dyn T<'a, A>` -> `*dyn T<'b, B>` requires `'a = 'b` and `A = B`).

\*\* only when `m₁` is `mut` or `m₂` is `const`. Casting `mut` reference to
| Type of `e` | `U` | Cast performed by `e as U` |
|-----------------------|-----------------------|-------------------------------------------------------|
| Integer or Float type | Integer or Float type | [Numeric cast][expr.as.numeric] |
| Enumeration | Integer type | [Enum cast][expr.as.enum] |
| `bool` or `char` | Integer type | [Primitive to integer cast][expr.as.bool-char-as-int] |
| `u8` | `char` | [`u8` to `char` cast][expr.as.u8-as-char] |
| `*T` | `*V` [^1] | [Pointer to pointer cast][expr.as.pointer] |
| `*T` where `T: Sized` | Integer type | [Pointer to address cast][expr.as.pointer-as-int] |
| Integer type | `*V` where `V: Sized` | [Address to pointer cast][expr.as.int-as-pointer] |
| `&m₁ [T; n]` | `*m₂ T` [^2] | Array to pointer cast |
| `*m₁ [T; n]` | `*m₂ T` [^2] | 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 [^3] | Function pointer | Closure to function pointer cast |

[^1]: where `T` and `V` have compatible metadata:
* `V: Sized`, or
* Both slice metadata (`*[u16]` -> `*[u8]`, `*str` -> `*(u8, [u32])`), or
* Both the same trait object metadata, modulo dropping auto traits (`*dyn Debug` -> `*(u16, dyn Debug)`, `*dyn Debug + Send` -> `*dyn Debug`)
* **Note**: *adding* auto traits is only allowed if the principal trait has the auto trait as a super trait (given `trait T: Send {}`, `*dyn T` -> `*dyn T + Send` is valid, but `*dyn Debug` -> `*dyn Debug + Send` is not)
* **Note**: Generics (including lifetimes) must match (`*dyn T<'a, A>` -> `*dyn T<'b, B>` requires `'a = 'b` and `A = B`)

[^2]: only when `m₁` is `mut` or `m₂` is `const`. Casting `mut` reference/pointer to
`const` pointer is allowed.

\*\*\* only for closures that do not capture (close over) any local variables
[^3]: only for closures that do not capture (close over) any local variables can be casted to function pointers.

### Semantics

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