Track proc-macro-error2 future-incompatibility warning

[DCjanus]

Summary

cargo test currently records a future-incompatibility warning from an indirect dependency:

cang-jie
└── jieba-rs 0.9.0
    └── include-flate 0.3.3
        └── include-flate-codegen 0.3.3
            └── proc-macro-error2 2.0.1

The warning is emitted when proc-macro-error2 is compiled:

warning[E0365]: extern crate `proc_macro` is private and cannot be re-exported

This is currently only a warning, but Rust marks it as future-incompatible, so a future compiler release may turn it into a hard error.

Current upstream status

• Rust tracking issue: Tracking issue for future-incompatibility lint pub_use_of_private_extern_crate rust-lang/rust#127909
• proc-macro-error2 issue: Crate emits future incompatibility warning GnomedDev/proc-macro-error-2#13
• proc-macro-error2 fix PR: Fix future incompatibility GnomedDev/proc-macro-error-2#14

As of now, proc-macro-error2 latest is still 2.0.1, so there is no released fix to pick up.

How to reproduce

Run a command that needs to compile the dependency chain, for example:

cargo test --workspace

If the warning has already been recorded by Cargo, inspect it with:

cargo report future-incompatibilities --id 1

Follow-up

• Track the upstream proc-macro-error2 fix.
• Once a fixed version is released, check whether a normal dependency refresh is enough for cang-jie.
• If the warning becomes a hard error before then, consider a temporary workaround such as pinning the Rust toolchain or patching the dependency.