make format_args! only depend on libcore

[thchittenden]

Currently format_args! relies on format stuff in libstd, all of which is present in libcore. In order to use format_args! in a #[no_std] context you have to reexport core as std which is pretty hacky.

[alexcrichton]

This is a general issue with macro hygiene. There is no dependency by format_args! on anything other than libcore other than names. Additionally, this isn't quite as easy as a renaming because libcore isn't injected into crates, only libstd is.

Basically, fixing this issue will likely require a significant amount of work in terms of macro hygiene across crates. I would love to have it fixed though...

[sfackler]

Is there any reason to not just add extern crate core to std_inject?

[alexcrichton]

I think of core as an implementation detail of std that shouldn't be exposed to users in terms of backwards compatibility. While it would certainly fix this problem for now to inject extern crate core, I don't think we want to do that in the long term.

[kmcallister]

I'm working on a fix for #16803 which will also solve this problem. We tell libsyntax whether the crate is #![no_std] and then the extensions produce references to std or core etc. as appropriate.

[sfackler]

We could potentially avoid the back-compat hazards by importing core under a gensymed name so user code wouldn't be able to see that it exists. It would be a bit hacky, though.

[kmcallister]

I'm also making Vec<u8> into a FormatWriter that works without libstd.