[swift-dev] [RFC] Finer grained OS checks

[Saleem Abdulrasool]

Hi,

Id like to revive the discussion around OS "variants".  I've been doing
some work to bring up Windows without any emulation layer (MSVCRT based) as
a viable host environment.  This work is bringing to light the need for
more finer grained OS checks.

Currently, we have the `os` compilation condition.  However, this doesn't
provide sufficiently detailed information for Windows.  On Windows, we have
at least 4 different "variants":

- "msvc" (Microsoft's environment)
- "itanium" (MS ABI for C, Itanium ABI for C++)
- "gnu" (MinGW)
- "cygnus" (cygwin)

Each one of these is slightly different and requires particular handling in
the runtime.  However, the OS for each one of these is windows, and so
`os(Windows)` yields true on all of them.

This is not a problem strictly limited to Windows.  It also appears in
other OSes.  As a concrete example, Linux has traditionally had the "gnu"
environment (libc).  However, there is also "uclibc" which is pretty
common, and these days, "musl" as different targets.  Swift also supports
android, which is a Linux environment variant.

As deeper system integration occurs with swift, the need for finer grained
os detection logic is probably going to be needed.

To keep things simple, I would like to propose the "environment"
conditional compilation directive.  It would yield true for the appropriate
environment with one of the previously mentioned values for windows and
true for "android" on Android.  It would sit as a peer to `os` and allow
for finer grained querying of the host environment.

-- 
Saleem Abdulrasool
compnerd (at) compnerd (dot) org
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