win: has("wsl") on Windows Subsystem for Linux #7330

Per CMAKE docs, CMAKE_HOST_SYSTEM_VERSION is the result of `uname -r`:
https://cmake.org/cmake/help/v3.4/variable/CMAKE_HOST_SYSTEM_VERSION.html?highlight=uname

    A numeric version string for the system. On systems that support
    uname, this variable is set to the output of uname -r. On other
    systems this is set to major-minor version numbers.

On Windows it is something like "6.1", so it won't match ".*-Microsoft".

Closes #7329

config/CMakeLists.txt

@@ -12,6 +12,11 @@ check_type_size("size_t" SIZEOF_SIZE_T)
 check_type_size("long long" SIZEOF_LONG_LONG)
 check_type_size("void *" SIZEOF_VOID_PTR)
 
+if (CMAKE_HOST_SYSTEM_VERSION MATCHES ".*-Microsoft")
+  # Windows Subsystem for Linux
+  set(HAVE_WSL 1)
+endif()
+
 check_symbol_exists(_NSGetEnviron crt_externs.h HAVE__NSGETENVIRON)
 
 # Headers

config/config.h.in

@@ -48,6 +48,7 @@
 #cmakedefine HAVE_UTIME_H
 #cmakedefine HAVE_UTIMES
 #cmakedefine HAVE_WORKING_LIBINTL
+#cmakedefine HAVE_WSL
 #cmakedefine UNIX
 #cmakedefine USE_FNAME_CASE
 #cmakedefine HAVE_SYS_UIO_H

runtime/doc/eval.txt

@@ -8445,6 +8445,7 @@ win32			Windows version of Vim (32 or 64 bit).
 winaltkeys		Compiled with 'winaltkeys' option.
 windows			Compiled with support for more than one window.
 writebackup		Compiled with 'writebackup' default on.
+wsl			WSL (Windows Subsystem for Linux) version of Vim.
 
 							*string-match*
 Matching a pattern in a String

src/nvim/eval.c

@@ -10671,6 +10671,9 @@ static void f_has(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     "windows",
     "winaltkeys",
     "writebackup",
+#if defined(HAVE_WSL)
+    "wsl",
+#endif
     "nvim",
   };
 

test/functional/eval/has_spec.lua

@@ -57,4 +57,10 @@ describe('has()', function()
       eq(0, funcs.has("unnamedplus"))
     end
   end)
+
+  it('"wsl"', function()
+    if 1 == funcs.has('win32') or 1 == funcs.has('mac') then
+      eq(0, funcs.has('wsl'))
+    end
+  end)
 end)