Command-line option to list supported feature-gate names

[joshtriplett]

I'd like to have a command-line option to list all the supported feature-gate names in rustc. This would simplify the job of tools that want to detect the features supported by the installed version of rustc.

I don't know if introducing an option like this would require an RFC, or just a patch. Posting this as an issue to find that out before working on a patch.

[hanna-kruppe]

Is this about language features, library features, or both?

[est31]

I'm not sure whether this is what you want, but do you know of the output of x.py src/tools/tidy?

[joshtriplett]

@rkruppe Both, for as long as std and rustc remain coupled. Anything supported in a feature(...) declaration.

[joshtriplett]

@est31 I didn't. (I've seen that output before, just not the tool that generated it; thanks!) That would work, if available as a runtime option rather than a build-time command.

[hanna-kruppe]

Language features are listed in librustc/middle/lang_items.rs. Library features are a bit more difficult to enumerate because they're spread in annotations all over multiple crates (e.g., for the compiler to even know about the test feature, it must look at the test crate). Still, it ought to be feasible to load all relevant crates for --print features because the set is fixed and quite small. After all, third party crates can't (at least really really shouldn't) have stability annotations.

One wrinkle with the tidy tool is that it crawls the source code. So one would have to include its output into the binary as a static string, gross! Besides, it lists feature flags that aren't relevant to the current target. For example, on linux it lists various windows_* library features which are only applied to stuff behind a cfg(unix). This is the right choice for development but probably not for "what feature flags can I set?".

[steveklabnik]

#45223 was supposed to close this, but github didn’t for some reason. Closing!