rust: pin-init: internal: use `HasInitData` to provide inference help only

Currently, type inference of the initialized type is done by use the
function calling syntax `PATH::__pin_data` or `PATH::__init_data` to obtain
the required init data type, without needing the user to explicitly mention
all generics. Switch to always use `HasInitData::__init_data` to provide
that inference, which will not produce method-not-found errors as it is
always available.

Change `__pin_data` to take `InitData<Self>` as the receiver, so for types
that require `HasPinData`, `HasPinData::__pin_data(data)` can be used to
obtain the needed type without having to mention the user-provided path
again. The expanded code also uses the span of the type, so diagnostics
will hint to user that the type is causing the issue.

With this change, the error message when `HasPinData` is not implemented
changes from

    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`

to

    error[E0277]: the trait bound `Foo: pin_init::__internal::HasPinData` is not satisfied
     --> 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`
     --> tests/ui/compile-fail/init/missing_pin_data.rs:3:1
      |
    3 | struct Foo {
      | ^^^^^^^^^^

Link: https://patch.msgid.link/20260923-dev-pin-data-diag-v1-3-e4d7193569e5@garyguo.net
Signed-off-by: Gary Guo <gary@garyguo.net>

0 files changed, 0 insertions, 0 deletions