casting - `as` 允许进行哪些转换？

Question

The Rust Book section on the as operator目前说

The as keyword does basic casting:

let x: i32 = 5;

let y = x as i64;

It only allows certain kinds of casting, however.

那些某些类型允许的转换是什么？

此处已删除的答案解释说，有时您需要链接多个 as-casts 才能获得安全的结果，这不能在一个步骤中完成。什么时候需要？

Answer 1

我认为这没有很好的记录，但是 here是一些您可能会觉得有用的信息:

A cast e as U is valid if one of the following holds:

• e has type T and T coerces to U; coercion-cast
• e has type *T, U is *U_0, and either U_0: Sized or unsize_kind(T) = unsize_kind(U_0); ptr-ptr-cast
• e has type *T and U is a numeric type, while T: Sized; ptr-addr-cast
• e is an integer and U is *U_0, while U_0: Sized; addr-ptr-cast
• e has type T and T and U are any numeric types; numeric-cast
• e is a C-like enum and U is an integer type; enum-cast
• e has type bool or char and U is an integer; prim-int-cast
• e has type u8 and U is char; u8-char-cast
• e has type &[T; n] and U is *const T; array-ptr-cast
• e is a function pointer type and U has type *T, while T: Sized; fptr-ptr-cast
• e is a function pointer type and U is an integer; fptr-addr-cast

where &.T and *T are references of either mutability, and where unsize_kind(T) is the kind of the unsize info in T - the vtable for a trait definition (e.g. fmt::Display or Iterator, not Iterator<Item=u8>) or a length (or () if T: Sized).

Note that lengths are not adjusted when casting raw slices - T: *const [u16] as *const [u8] creates a slice that only includes half of the original memory.

Casting is not transitive, that is, even if e as U1 as U2 is a valid expression, e as U2 is not necessarily so (in fact it will only be valid if U1 coerces to U2).