Compression warnings are disabled by default, including on nightly

[DCjanus]

Background

After #38 was merged, I checked proc-macro-error3 because @SOF3 mentioned a concern about the reliability of the new crate.

I compared its source with proc-macro-error2. I did not find any suspicious changes, and the source published on crates.io matches the GitHub source. During this check, I noticed that its nightly diagnostics require an explicit Cargo feature.

Current behavior

On master at commit 4118d8e, include-flate uses emit_warning! from proc-macro-error3, but does not enable its nightly feature.

The result is:

proc-macro-error3/nightly	stable	nightly
disabled	No compression warnings	No compression warnings
enabled	Compilation fails with E0554	Warnings are shown with the correct source spans

This means compression warnings are never shown with the default configuration, even when the compiler is nightly.

Why this happens

proc-macro-error3 only enables proc_macro::Diagnostic when its nightly feature is enabled. Without that feature, emit_warning! is a no-op.

Cargo features cannot be enabled only for a nightly compiler. If include-flate enables this dependency feature by default, stable builds will fail because proc_macro_diagnostic is unstable.

Possible directions

I see three possible options:

1. Remove compression warnings if this behavior is no longer useful.
2. Remove proc-macro-error3, detect nightly with rustversion, and call proc_macro::Diagnostic directly. Stable builds would skip the warning.
3. Add an explicit include-flate feature that enables proc-macro-error3/nightly. Users could opt in on nightly, but enabling it on stable would fail to compile.

Which behavior would be preferred?

[SOF3]

tbh the most reasonable direction would be to add a nightly feature, but it feels weird to expect someone to enable a nightly feature just so that it shows a potential warning.