Allow creation of UniquePtrs to trivial aliased types.

[adetaylor]

Fixes #360.

@dtolnay the runtime check is out of character for cxx, but we can't do it at:

• generation time, because we don't know if an ExternType has claimed a given type to be trivial;
• build time, because what matters is whether the UniquePtr::new method is called rather than whether it exists.

I think it would be possible to make the check at build-time by splitting UniquePtrTarget into UniquePtrTarget and CreatableUniquePtrTarget (such that the new method simply doesn't exist for some subset of UniquePtrs) but that's significantly beyond my bravery level.

[dtolnay]

it would be possible to make the check at build-time by splitting UniquePtrTarget into UniquePtrTarget and CreatableUniquePtrTarget (such that the new method simply doesn't exist for some subset of UniquePtrs)

For making the new method not exist for some subset of unique ptrs, a trait bound would be a better way I think.

cxx/src/unique_ptr.rs

Line 35 in e35673d

pub fn new(value: T) -> Self {

pub fn new(value: T) -> Self
where
    T: ExternType<Kind = Trivial>,
{...}

[adetaylor]

Ah yes, I'll give that a go.

[dtolnay]

Nice!