pub trait CopyType {
type Copy: Boolean;
}Expand description
Marker trait for copy types.
The trait comes in two flavors: CopyType<Copy=True> and
CopyType<Copy=False>. In the first case, MemSize::mem_size can be computed on
arrays, vectors, and slices by multiplying the length or capacity
by the size of the element type; in the second case, it
is necessary to iterate on each element.
The trait is made necessary by the impossibility of checking that a type
implements Copy from a procedural macro.
Since we cannot use negative trait bounds, every type that is used as a parameter of
an array, vector, or slice must implement either CopyType<Copy=True> or
CopyType<Copy=False>. If you do not implement either of these traits,
you will not be able to compute the size of arrays, vectors, and slices but error
messages will be very unhelpful due to the contrived way we have to implement
mutually exclusive types working around the bug that prevents the compiler
from understanding that implementations for the two flavors of CopyType are mutually
exclusive.
If you use the provided derive macros all this logic will be hidden from you. You’ll
just have to add the attribute #[copy_type] to your structures if they
are Copy types and they do not contain references. We enforce this property by
adding a bound Copy + 'static to the type in the procedural macro.
Note that this approach forces us to compute the size of Copy types that contain
references by iteration even if you do not specify SizeFlags::FOLLOW_REFS.