| # Casts |
| |
| Casts are a superset of coercions: every coercion can be explicitly invoked via a cast. |
| However some conversions require a cast. |
| While coercions are pervasive and largely harmless, these "true casts" are rare and potentially dangerous. |
| As such, casts must be explicitly invoked using the `as` keyword: `expr as Type`. |
| |
| You can find an exhaustive list of [all the true casts][cast list] and [casting semantics][semantics list] on the reference. |
| |
| ## Safety of casting |
| |
| True casts generally revolve around raw pointers and the primitive numeric types. |
| Even though they're dangerous, these casts are infallible at runtime. |
| If a cast triggers some subtle corner case no indication will be given that this occurred. |
| The cast will simply succeed. |
| That said, casts must be valid at the type level, or else they will be prevented statically. |
| For instance, `7u8 as bool` will not compile. |
| |
| That said, casts aren't `unsafe` because they generally can't violate memory safety *on their own*. |
| For instance, converting an integer to a raw pointer can very easily lead to terrible things. |
| However the act of creating the pointer itself is safe, because actually using a raw pointer is already marked as `unsafe`. |
| |
| ## Some notes about casting |
| |
| ### Lengths when casting raw slices |
| |
| Note that lengths are not adjusted when casting raw slices; `*const [u16] as *const [u8]` creates a slice that only includes half of the original memory. |
| |
| ### Transitivity |
| |
| Casting is not transitive, that is, even if `e as U1 as U2` is a valid expression, `e as U2` is not necessarily so. |
| |
| [cast list]: ../reference/expressions/operator-expr.html#type-cast-expressions |
| [semantics list]: ../reference/expressions/operator-expr.html#semantics |
