SmallVec can not coerce lifetimes of wrapped value

[Luro02]

I could not get the following code to compile with SmallVec (I heavily simplified the code):

use std::borrow::Cow;

pub struct SmallVec<T> {
    data: *mut T,
}

pub struct Example<'a>(SmallVec<[Cow<'a, str>; 2]>);

impl<'a> Example<'a> {
    pub fn into_owned(self) -> Example<'static> { unimplemented!() }
}

fn transform<'a>(input: Example<'a>) -> Example<'a> {
    input.into_owned()
}

fn main() {}

playground

error[E0308]: mismatched types
  --> src/main.rs:14:5
   |
14 |     input.into_owned()
   |     ^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected struct `Example<'a>`
              found struct `Example<'static>`
note: the lifetime `'a` as defined on the function body at 13:14...
  --> src/main.rs:13:14
   |
13 | fn transform<'a>(input: Example<'a>) -> Example<'a> {
   |              ^^
   = note: ...does not necessarily outlive the static lifetime

error: aborting due to previous error

which does compile when you replace SmallVec with Vec.

This works with Vec, because internally the pointer is wrapped in Unique<T>:

https://github.com/rust-lang/rust/blob/fb5615a4771ea3d54256f969dc84d2dfd38d812c/src/liballoc/raw_vec.rs#L47

whereas SmallVec contains a raw pointer:

rust-smallvec/lib.rs

Line 370 in a541e2d

Heap((*mut A::Item, usize)),

This issue can be solved by changing *mut A::Item to Option<NonNull<A::Item>>, because NonNull implements the CoerceUnsized trait.

This might be related to #146