Don't use posix_fadvise on Mac

Fixes #1.

src/files.rs

@@ -16,8 +16,6 @@ use std::os::unix::io::*;
 
 use bytesize::ByteSize;
 use fasthash::{FastHasher, HasherExt, Murmur3Hasher, t1ha2::Hasher128};
-#[cfg(unix)]
-use nix::fcntl::*;
 use serde::*;
 use smallvec::alloc::fmt::Formatter;
 use smallvec::alloc::str::FromStr;
@@ -309,9 +307,16 @@ fn to_off_t(offset: u64) -> i64 {
 /// Wrapper for `posix_fadvise`. Ignores errors.
 /// This method is used to advise the system, so its failure is not critical to the result of
 /// the program. At worst, failure could hurt performance.
-#[cfg(unix)]
-fn fadvise(file: &File, offset: FilePos, len: FileLen, advice: PosixFadviseAdvice) {
-    let _ = posix_fadvise(file.as_raw_fd(),
+#[cfg(any(
+    target_os = "linux",
+    target_os = "android",
+    target_os = "emscripten",
+    target_os = "fuchsia",
+    any(target_os = "wasi", target_env = "wasi"),
+    target_env = "uclibc",
+    target_env = "freebsd"))]
+fn fadvise(file: &File, offset: FilePos, len: FileLen, advice: nix::fcntl::PosixFadviseAdvice) {
+    let _ = nix::fcntl::posix_fadvise(file.as_raw_fd(),
                           to_off_t(offset.into()),
                           to_off_t(len.into()),
                           advice);
@@ -322,7 +327,14 @@ fn fadvise(file: &File, offset: FilePos, len: FileLen, advice: PosixFadviseAdvic
 /// On non-Unix systems, does nothing.
 /// Failures are not signalled to the caller, but a warning is printed to stderr.
 fn configure_readahead(file: &File, offset: FilePos, len: FileLen, cache_policy: Caching) {
-    if cfg!(unix) {
+    if cfg!(any(target_os = "linux",
+                target_os = "android",
+                target_os = "emscripten",
+                target_os = "fuchsia",
+                any(target_os = "wasi", target_env = "wasi"),
+                target_env = "uclibc",
+                target_env = "freebsd")) {
+        use nix::fcntl::*;
         let advise = |advice: PosixFadviseAdvice| {
             fadvise(file, offset, len, advice)
         };
@@ -337,7 +349,14 @@ fn configure_readahead(file: &File, offset: FilePos, len: FileLen, cache_policy:
 /// Tells the system to remove given file fragment from the page cache.
 /// On non-Unix systems, does nothing.
 fn evict_page_cache(file: &File, offset: FilePos, len: FileLen) {
-    if cfg!(unix) {
+    if cfg!(any(target_os = "linux",
+                target_os = "android",
+                target_os = "emscripten",
+                target_os = "fuchsia",
+                any(target_os = "wasi", target_env = "wasi"),
+                target_env = "uclibc",
+                target_env = "freebsd")) {
+        use nix::fcntl::*;
         fadvise(file, offset, len, PosixFadviseAdvice::POSIX_FADV_DONTNEED);
     }
 }