hg-core: add a compilation error if trying to compile outside of Linux

For now, we can only provide support for Linux in `hg-core`, so we have to be
explicit about it in case anyone wonders why their Dirstate is suddenly broken,
or why the crate does not compile (on Windows for example).

Differential Revision: https://phab.mercurial-scm.org/D8246

rust/hg-core/src/lib.rs

@@ -23,6 +23,14 b' pub mod revlog;'
 pub use revlog::*;
 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;
 pub use filepatterns::{
     build_single_regex, read_pattern_file, PatternSyntax, PatternTuple,