Avoid adding DYLD_FALLBACK_LIBRARY_PATH defaults when it is set

The macos dynamic linker behavior wrt DYLD_FALLBACK_LIBRARY_PATH is to
use the value it is set with, and if there is no such value (the
environment variable is either not set or set but empty), it uses a
default value of $HOME/lib:/usr/local/lib:/usr/lib.

Currently, cargo takes the value of DYLD_FALLBACK_LIBRARY_PATH, prepends
its paths to it, and then unconditionally adds
$HOME/lib:/usr/local/lib:/usr/lib, which in principle, shouldn't happen
if DYLD_FALLBACK_LIBRARY_PATH was set originally.

src/cargo/core/compiler/compilation.rs

@@ -178,11 +178,13 @@ impl<'cfg> Compilation<'cfg> {
             search_path
         };
 
-        search_path.extend(util::dylib_path().into_iter());
-        if cfg!(target_os = "macos") {
+        let dylib_path = util::dylib_path();
+        let dylib_path_is_empty = dylib_path.is_empty();
+        search_path.extend(dylib_path.into_iter());
+        if cfg!(target_os = "macos") && dylib_path_is_empty {
             // These are the defaults when DYLD_FALLBACK_LIBRARY_PATH isn't
-            // set. Since Cargo is explicitly setting the value, make sure the
-            // defaults still work.
+            // set or set to an empty string. Since Cargo is explicitly setting
+            // the value, make sure the defaults still work.
             if let Some(home) = env::var_os("HOME") {
                 search_path.push(PathBuf::from(home).join("lib"));
             }

tests/testsuite/run.rs

@@ -1228,4 +1228,8 @@ fn run_link_system_path_macos() {
     // was set by the cargo that invoked the test.
     p2.cargo("run").env_remove(VAR).run();
     p2.cargo("test").env_remove(VAR).run();
+    // Ensure this still works when DYLD_FALLBACK_LIBRARY_PATH has
+    // a value set.
+    p2.cargo("run").env(VAR, &libdir).run();
+    p2.cargo("test").env(VAR, &libdir).run();
 }