[PATCH 4/5] rust: pin-init: internal: add custom diagnostic when `#[pin_data]` is not implemented

[Gary Guo <gary@garyguo.net>]

Produce a more helpful error message when `#[pin_data]` is omitted. The
error message changes from

    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 {
      | ^^^^^^^^^^

to

    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`
     --> tests/ui/compile-fail/init/missing_pin_data.rs:3:1
      |
    3 | struct Foo {
      | ^^^^^^^^^^
      = note: did you forget to add `#[pin_data]` attribute to the struct?

Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/pin-init/src/__internal.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/rust/pin-init/src/__internal.rs b/rust/pin-init/src/__internal.rs
index f236db47d18b..cb7a0c3066b6 100644
--- a/rust/pin-init/src/__internal.rs
+++ b/rust/pin-init/src/__internal.rs
@@ -83,6 +83,10 @@ pub unsafe fn new() -> Self {
 ///
 /// `pin-init` relies on the correctness of the helper functions defined on `PinData`.
 /// Thus, only the `#[pin_data]` can implement this trait.
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` cannot be used with `pin_init!` macro",
+    note = "did you forget to add `#[pin_data]` attribute to the struct?"
+)]
 pub unsafe trait HasPinData {
     type PinData;
 

-- 
2.54.0