Extern blob is duplicated per platform

[klutzy]

Usually C bindings requires different calling conventions per platform. For example, linux-32 usually uses cdecl, win-32 uses stdcall, and win-64 uses win64-specific convention.
So if we want to bind a function universally, we have to duplicate extern ... {} block for each platform, e.g.

#[cfg(target_os = "linux")]
extern "cdecl" {
    fn func();
    ...
}
#[cfg(target_os = "win32", target_arch = "x86")]
extern "stdcall" {
    fn func();
    ...
}
#[cfg(target_os = "win32", target_arch = "x86_64")]
extern { // this is identical to extern "C" or extern "cdecl"
    fn func();
    ...
}

which is inconvenient and fragile. (I actually copied large extern blob for Mingw-w64 support (#8488).)

Other option is to generate binding code per platform from template. lifthrasiir/rust-opengles-angle uses the approach for egl binding.

I think it's good to add default calling convention keyword e.g. "platform" so that

extern "platform" {
    fn func();
    ...
}

where "platform" is interpreted as cdecl for linux-32, stdcall for win32, and so on. It would solve the issue for most cases.

EDIT: in irc @luqmana suggested to change extern {} (no abi specified) as platform-native callconv. (It's currently interpreted as "cdecl".) I think it's the best option.

[luqmana]

cc @nikomatsakis

[brson]

I don't think it's generally true that a cdecl function on unix would be a stdcall function on win32. The win32 API specifically is stdcall, but the C api is still cdecl on windows. The win32/win64 split seems to be the place where this is a problem.

This specific case could be solved with a macro.

[brson]

I wonder if MSVC uses the stdcall abi by default for normal user code.

[brson]

Making "stdcall" just mean 'the x86_64 abi ' on 64-bit platforms (the same way "cdecl" does) would also solve this problem, though I don't think we should.

[nikomatsakis]

In fact, the ABI strings were designed to accommodate this. They can handle "platform-specific" ABIs, and trans will select the most appropriate. The current rules may need some tweaking, I don't remember how stdcall fits in etc.

[nikomatsakis]

Hmm, well, the ABI strings aren't quite able to handle this example because they only distinguish by architecture, not by O/S. But perhaps we should fix this. In that case, one could put extern "stdcall cdecl" and it would select stdcall on Windows. but I imagine the reason I didn't do this is that sometimes on linux code still uses the pascal calling convention. This could be accommodated by having another name for pascal calling convention on linux; I'm not sure what the standard names in use are though. Otherwise, I tend to agree with @brson; a macro might help here. (It's quite possible the current ABI sets should just go away in favor of such macros as well)

[klutzy]

#10367 added extern "system". solved!