Re: [PATCH 0/5] rust: pin-init: improve diagnostics of missing `#[pin_data]` and `#[pin]`

["Gary Guo" <gary@garyguo.net>]

On Wed Sep 23, 2026 at 7:20 PM BST, Gary Guo wrote:
> 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`

Applied to pin-init-next, with typo pointed out by Sashiko fixed.

Best,
Gary

>
>  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>