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