Improve the diagnostics of missing `#[pin_data]` and `#[pin]`, by adding
diagnostics attributes. For `#[pin_data]`, we have to change the expansion
to retrieve `PinData` in a two-step process, so we change the error message
from "method exists but trait bound not satisfied" to "trait not
implemented".

This is the new error message for missing `#[pin_data]`:

    error[E0277]: `Foo` cannot be used with `pin_init!` macro
     --> tests/ui/compile-fail/init/missing_pin_data.rs:9:19
      |
    9 |         pin_init!(Self { a: 42 })
      |                   ^^^^ unsatisfied trait bound
      |
    help: the trait `pin_init::__internal::HasPinData` is not implemented for `Foo`

where it was previously

    error[E0599]: no associated function or constant named `__pin_data` found for struct `Foo` in the current scope
     --> tests/ui/compile-fail/init/missing_pin_data.rs:9:9
      |
    3 | struct Foo {
      | ---------- associated function or constant `__pin_data` not found for this struct
    ...
    9 |         pin_init!(Self { a: 42 })
      |         ^^^^^^^^^^^^^^^^^^^^^^^^^ associated function or constant not found in `Foo`
      |
      = help: items from traits can only be used if the trait is implemented and in scope
      = note: the following trait defines an item `__pin_data`, perhaps you need to implement it:
              candidate #1: `pin_init::__internal::HasPinData`

and this is the new error message for missing `#[pin]`:

    error[E0277]: `impl PinInit<usize>` cannot be used to movably initialize `usize` with error `_`
      --> tests/ui/compile-fail/pin_data/missing_pin.rs:12:18
       |
    12 |             a <- a,
       |             -----^
       |             |    |
       |             |    the trait `Init<usize, _>` is not implemented for `impl PinInit<usize>`
       |             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

where it was previously

    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

Signed-off-by: Gary Guo <gary@garyguo.net>
---
Gary Guo (5):
      rust: pin-init: internal: make `__init_data` and `__pin_data` safe
      rust: pin-init: internal: remove associated type of `HasInitData`
      rust: pin-init: internal: use `HasInitData` to provide inference help only
      rust: pin-init: internal: add custom diagnostic when `#[pin_data]` is not implemented
      rust: pin-init: add diagnostic attribute for `PinInit` and `Init`

 rust/pin-init/internal/src/init.rs     | 25 +++++++++---------
 rust/pin-init/internal/src/pin_data.rs |  2 +-
 rust/pin-init/src/__internal.rs        | 48 +++++++++++++++-------------------
 rust/pin-init/src/lib.rs               | 11 +++++++-
 4 files changed, 45 insertions(+), 41 deletions(-)
---
base-commit: dfb6a037fd586f1ffcba3b143dab58f5c88294d1
change-id: 20260923-dev-pin-data-diag-65209ab751fc

Best regards,
--  
Gary Guo <gary@garyguo.net>