When rust supports it, use `#[diagnostic::on_unimplemented]` for common usage errors in check/check_key

[antifuchs]

Since it's confusing to figure out how to get a rate limiter that supports check or check_key sometimes:

Maybe you could use #[diagnostic::on_unimplemented] to make the error message less confusing?

Originally posted by @kornelski in #274

[kornelski]

It's available since 1.78.0. Current MSRV is 1.76.

[antifuchs]

...for trait definitions. It isn't available on direct impl blocks:

:;    rustc --version
rustc 1.88.0 (6b00bc388 2025-06-23)
:;    git diff | cat
diff --git a/governor/src/state/direct.rs b/governor/src/state/direct.rs
index 9a599cf..5ae31e8 100644
--- a/governor/src/state/direct.rs
+++ b/governor/src/state/direct.rs
@@ -70,6 +70,7 @@ where
     ///
     /// If the rate limit is reached, `check` returns information about the earliest
     /// time that a cell might be allowed through again.
+    #[diagnostic::on_unimplemented(message = "hi there")]
     pub fn check(&self) -> Result<MW::PositiveOutcome, MW::NegativeOutcome> {
         self.gcra.test_and_update::<NotKeyed, C::Instant, S, MW>(
             self.start,
:;    cargo test
   Compiling governor v0.10.0 (/Users/asf/Hacks/governor/governor)
error: `#[diagnostic::on_unimplemented]` can only be applied to trait definitions
  --> governor/src/state/direct.rs:73:5
   |
73 |     #[diagnostic::on_unimplemented(message = "hi there")]
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: the lint level is defined here
  --> governor/src/lib.rs:33:9
   |
33 | #![deny(warnings)]
   |         ^^^^^^^^
   = note: `#[deny(unknown_or_malformed_diagnostic_attributes)]` implied by `#[deny(warnings)]`

error: could not compile `governor` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...```