Tracking issue for proc_macro::is_available()

[dtolnay]

This is a tracking issue for proc_macro::is_available().
The feature gate for the issue is #![feature(proc_macro_is_available)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also uses as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement -- proc_macro::is_available() #71400
• Adjust documentation
• Stabilization PR (see instructions on rustc-dev-guide) -- Stabilize proc_macro::is_available #89735

Unresolved Questions

none yet

Implementation history

• Implemented in proc_macro::is_available() #71400

[Aaron1011]

Is anything blocking stabilization of this feature? It seems pretty uncontroversial - if the proc_macro crate ever gained the ability to be used outside of a procedural macro, this function could just unconditionally return true.

[dtolnay]

One non-uncontroversial part is how to react if proc_macro might not be all-or-nothing in the future. For example if the whole token API is made to work outside a proc macro, but the diagnostics API via Span (warn, error, note, etc) would not be available.

[bjorn3]

If the token api is made to work outside a proc macro, but the diagnostics api is not, I think users of proc_macro can just access the token api without checking is_available and maybe only check the rustc version. Usage of the diagnostics api should then still be behind a is_available check. Basically if it always works, is_available shouldn't be used, but if it only works inside a proc macro, is_available should be used.

[bjorn3]

I would really like for this feature to be stabilized. proc_macro2 is currently unusable with cg_clif outside of proc macros, as it checks if a proc macro function panics when being called. cg_clif doesn't yet support unwinding on panics. Also if the function proc_macro2 checks becomes available outside of a proc macro, that will break proc_macro2 usage outside of proc macros, so the sooner it migrates to is_available the better.

[smklein]

I would really like for this feature to be stabilized. proc_macro2 is currently unusable with cg_clif outside of proc macros, as it checks if a proc macro function panics when being called. cg_clif doesn't yet support unwinding on panics. Also if the function proc_macro2 checks becomes available outside of a proc macro, that will break proc_macro2 usage outside of proc macros, so the sooner it migrates to is_available the better.

Does the workaround described here not work for you? I'm running into a similar use-case, but the proc_macro2::fallback::force() function is a sufficient escape-hatch for now