Difference in error message when switching from proc-macro-error

[zhassan-aws]

Thank you for creating this fork of proc-macro-error. We've switched to proc-macro-error2 in model-checking/kani#3493, but we observed a difference in the emitted error.

With proc-macro-error, the error was:

error: Cannot derive `Arbitrary` for `Wrapper` union
 --> union.rs:5:10
  |
5 | #[derive(kani::Arbitrary)]
  |          ^^^^^^^^^^^^^^^
  |
note: `#[derive(Arbitrary)]` cannot be used for unions such as `Wrapper`
 --> union.rs:6:7
  |
6 | union Wrapper {
  |       ^^^^^^^
  = note: this error originates in the derive macro `kani::Arbitrary` (in Nightly builds, run with -Z macro-backtrace for more info)

With proc-macro-error2, the error is:

error: Cannot derive `Arbitrary` for `Wrapper` union
       
         = note: `#[derive(Arbitrary)]` cannot be used for unions such as `Wrapper`
       
 --> union.rs:5:10
  |
5 | #[derive(kani::Arbitrary)]
  |          ^^^^^^^^^^^^^^^
  |
  = note: this error originates in the derive macro `kani::Arbitrary` (in Nightly builds, run with -Z macro-backtrace for more info)

The two notable differences are:

1. The annotated item (union Wrapper) is no longer included in the error
2. The first note does not appear after the span.

Is the difference expected?

This is the code that emits the error:

https://github.com/model-checking/kani/blob/701f6fb8e26acf0c31f053cc1503a4139b277e29/library/kani_macros/src/derive.rs#L89

[GnomedDev]

The only thing I can think to have caused this is that it is no longer doing hacky nightly detection to use the unstable Diagnostic API. If you enable the nightly feature of this crate does it revert the error messages back?

I am personally against automatic nightly detection as it causes huge headaches for everyone in the ecosystem as people can unknowingly be using unstable features when just using the nightly toolchain for other reasons.

[zhassan-aws]

If you enable the nightly feature of this crate does it revert the error messages back?

Using the nightly feature does the trick!

$ diff -u without_nightly.log with_nightly.log 
--- without_nightly.log 2024-09-06 16:41:20.185980525 -0700
+++ with_nightly.log    2024-09-06 16:39:56.254197678 -0700
@@ -1,13 +1,15 @@
 Kani Rust Verifier 0.55.0 (standalone)
 error: Cannot derive `Arbitrary` for `Wrapper` union
-       
-         = note: `#[derive(Arbitrary)]` cannot be used for unions such as `Wrapper`
-       
  --> union.rs:5:10
   |
 5 | #[derive(kani::Arbitrary)]
   |          ^^^^^^^^^^^^^^^
   |
+note: `#[derive(Arbitrary)]` cannot be used for unions such as `Wrapper`
+ --> union.rs:6:7
+  |
+6 | union Wrapper {
+  |       ^^^^^^^
   = note: this error originates in the derive macro `kani::Arbitrary` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 1 previous error

Thanks! I'll close this issue.