Enable dyld interposing on Apple platforms when overriding

Enables the MI_OSX_ZONE and MI_OSX_INTERPOSE flags. These allow using
`malloc`/`free` on pointers returned from dynamic libraries (which will
otherwise fail, as the dynamic libraries use the System allocator).

These are enabled by default in mimalloc's CMakeLists.txt, but since we
build without that, we need to specify these flags ourselves.

Author: madsmtm

libmimalloc-sys/build.rs

@@ -15,6 +15,7 @@ fn main() {
 
     let target_os = env::var("CARGO_CFG_TARGET_OS").expect("target_os not defined!");
     let target_family = env::var("CARGO_CFG_TARGET_FAMILY").expect("target_family not defined!");
+    let target_vendor = env::var("CARGO_CFG_TARGET_VENDOR").expect("target_vendor not defined!");
     let target_arch = env::var("CARGO_CFG_TARGET_ARCH").expect("target_arch not defined!");
 
     if target_family != "windows" {
@@ -27,6 +28,10 @@ fn main() {
         if target_family != "windows" {
             build.define("MI_MALLOC_OVERRIDE", None);
         }
+        if target_vendor == "apple" {
+            build.define("MI_OSX_ZONE", Some("1"));
+            build.define("MI_OSX_INTERPOSE", Some("1"));
+        }
     }
 
     if env::var_os("CARGO_FEATURE_SECURE").is_some() {

test-override-with-dylib/src/main.rs

@@ -49,5 +49,15 @@ fn main() {
     // Extra check that the symbol was actually from the same place.
     let dep = unsafe { CStr::from_ptr(dep_lookup_malloc_address()) };
     let here = unsafe { CStr::from_ptr(lookup_malloc_address()) };
-    assert_eq!(dep, here);
+
+    if cfg!(target_vendor = "apple") {
+        // macOS / Mach-O symbols are not overriden in dependencies, they are
+        // hooked into with `zone_register`.
+        assert_eq!(
+            dep.to_str().unwrap(),
+            "/usr/lib/system/libsystem_malloc.dylib"
+        );
+    } else {
+        assert_eq!(dep, here);
+    }
 }