rust: fix building on macOS (issue6801)

The VFS change is copied over from Cargo, and likely to apply to other
platforms as well.

The dirstate change is essentially a replay of 440972d2175d, which was
reverted in e98fd81bb151, part of !383, to silence some clippy
warnings.

rust/hg-core/src/dirstate_tree/on_disk.rs

@@ -415,9 +415,9 b' impl Node {'
 
     fn synthesize_unix_mode(&self) -> u32 {
         let file_type = if self.flags().contains(Flags::MODE_IS_SYMLINK) {
-            libc::S_IFLNK
+            libc::S_IFLNK as u32
         } else {
-            libc::S_IFREG
+            libc::S_IFREG as u32
         };
         let permissions = if self.flags().contains(Flags::MODE_EXEC_PERM) {
             0o755

rust/hg-core/src/vfs.rs

@@ -193,3 +193,13 b' pub(crate) fn is_on_nfs_mount(path: impl'
         r == 0 && buf.f_type as u32 == libc::NFS_SUPER_MAGIC as u32
     }
 }
+
+/// Similar to what Cargo does; although detecting NFS (or non-local
+/// file systems) _should_ be possible on other operating systems,
+/// we'll just assume that mmap() works there, for now; after all,
+/// _some_ functionality is better than a compile error, i.e. none at
+/// all
+#[cfg(not(target_os = "linux"))]
+pub(crate) fn is_on_nfs_mount(_path: impl AsRef<Path>) -> bool {
+    false
+}