`Pin<Box<impl Default>>` should `impl Default`

[Nemo157]

I tried this code (playground):

use std::pin::Pin;

#[derive(Default)]
pub struct Foo {
    pub bar: Pin<Box<i32>>,
}

I expected to see this happen: it compile.

Instead, this happened:

error[E0277]: the trait bound `Pin<Box<i32>>: Default` is not satisfied
 --> src/lib.rs:5:5
  |
3 | #[derive(Default)]
  |          ------- in this derive macro expansion
4 | pub struct Foo {
5 |     pub bar: Pin<Box<i32>>,
  |     ^^^^^^^^^^^^^^^^^^^^^^ the trait `Default` is not implemented for `Pin<Box<i32>>`

I believe this impl should have been provided:

impl<T: Default> Default for Pin<Box<T>>

Unfortunately because Box is #[fundamental] I believe this can't be added now, this code is currently accepted and would have an overlapping implementation (playground):

use std::pin::Pin;

pub struct Bar;

impl Default for Bar {
    fn default() -> Self {
        Self
    }
}

impl Default for Pin<Box<Bar>> {
    fn default() -> Self {
        Box::pin(Bar)
    }
}

#[derive(Default)]
pub struct Foo {
    pub bar: Pin<Box<Bar>>,
}

Unless potentially something to do with specialization would allow this overlap in the future? Also, AFAICT nobody else has cared enough about this to open an issue in the 6 years Pin has been stable, so maybe it can just be closed as an unfortunate API limitation.

[Jules-Bertholet]

Despite this technically being breaking, maybe we could get away with it: https://github.com/search?q=%2Fimpl%28%3C.*%3E%29%3F+Default+for+Pin%3C%2F+path%3A*.rs&type=code