rust: pin-init: add diagnostic attribute for `PinInit` and `Init`

Give a slightly more useful error message when these traits are not
implemented.

Before this change:

    error[E0277]: the trait bound `impl pin_init::PinInit<Bar>: Init<Bar, _>` is not satisfied
      --> tests/ui/compile-fail/init/invalid_init.rs:19:16
       |
    19 |         bar <- Bar::new(),
       |         -------^^^^^^^^^^
       |         |      |
       |         |      the trait `Init<Bar, _>` is not implemented for `impl pin_init::PinInit<Bar>`
       |         required by a bound introduced by this call

After this change:

    error[E0277]: `impl pin_init::PinInit<Bar>` cannot be used to movably initialize `Bar` with error `_`
      --> tests/ui/compile-fail/init/invalid_init.rs:19:16
       |
    19 |         bar <- Bar::new(),
       |         -------^^^^^^^^^^
       |         |      |
       |         |      the trait `Init<Bar, _>` is not implemented for `impl pin_init::PinInit<Bar>`
       |         required by a bound introduced by this call
       |
       = note: if your type implements `PinInit` but not `Init`, you might be forgetting a `#[pin]` annotation on fields

Ideally, we would generate differnet message when `PinInit` is implemented
but not `Init`; that is not feasible with today's
`diagnostics::on_unimplemented` attribute. However, the added note is worth
it because in `init!()` is more rarely used compared to `pin_init!()`; in
most cases `Init` being unimplemented is the result of missing `#[pin]`.

Link: https://patch.msgid.link/20260923-dev-pin-data-diag-v1-5-e4d7193569e5@garyguo.net
Signed-off-by: Gary Guo <gary@garyguo.net>

0 files changed, 0 insertions, 0 deletions