rust: lower compile error on non-linux platforms to a warning

As discussed on D9671#146704 this lowers the error to a warning.

Differential Revision: https://phab.mercurial-scm.org/D9852

rust/hg-core/src/lib.rs

@@ -30,14 +30,6 b' pub mod config;'
 pub mod operations;
 pub mod utils;
 
-// Remove this to see (potential) non-artificial compile failures. MacOS
-// *should* compile, but fail to compile tests for example as of 2020-03-06
-#[cfg(not(target_os = "linux"))]
-compile_error!(
-    "`hg-core` has only been tested on Linux and will most \
-     likely not behave correctly on other platforms."
-);
-
 use crate::utils::hg_path::{HgPathBuf, HgPathError};
 pub use filepatterns::{
     parse_pattern_syntax, read_pattern_file, IgnorePattern,

setup.py

@@ -609,6 +609,12 b' class hgbuildext(build_ext):'
         # and its build is not explictely disabled (for external build
         # as Linux distributions would do)
         if self.distribution.rust and self.rust:
+            if not sys.platform.startswith('linux'):
+                self.warn(
+                    "rust extensions have only been tested on Linux "
+                    "and may not behave correctly on other platforms"
+                )
+
             for rustext in ruststandalones:
                 rustext.build('' if self.inplace else self.build_lib)