[PATCH 5/5] rust: pin-init: add diagnostic attribute for `PinInit` and `Init`

[Gary Guo <gary@garyguo.net>]

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]`.

Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/pin-init/src/lib.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index 319485dc18ce..d1ed0561bb27 100644
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/src/lib.rs
@@ -966,6 +966,9 @@ macro_rules! assert_pinned {
 #[cfg_attr(not(kernel), doc = "[`Arc<T>`]: alloc::alloc::sync::Arc")]
 #[cfg_attr(not(kernel), doc = "[`Box<T>`]: alloc::alloc::boxed::Box")]
 #[must_use = "An initializer must be used in order to create its value."]
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` cannot be used to initialize `{T}` with error `{E}`"
+)]
 pub unsafe trait PinInit<T: ?Sized, E = Infallible>: Sized {
     /// Alias of [`PinInit::__init`].
     ///
@@ -1100,6 +1103,11 @@ unsafe fn __init(self, slot: *mut T) -> Result<(), E> {
 #[cfg_attr(not(kernel), doc = "[`Arc<T>`]: alloc::alloc::sync::Arc")]
 #[cfg_attr(not(kernel), doc = "[`Box<T>`]: alloc::alloc::boxed::Box")]
 #[must_use = "An initializer must be used in order to create its value."]
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` cannot be used to movably initialize `{T}` with error `{E}`",
+    note = "if your type implements `PinInit` but not `Init`, \
+            you might be forgetting a `#[pin]` annotation on fields"
+)]
 pub unsafe trait Init<T: ?Sized, E = Infallible>: PinInit<T, E> {
     /// First initializes the value using `self` then calls the function `f` with the initialized
     /// value.

-- 
2.54.0