Use DYLD_FALLBACK_LIBRARY_PATH for dylib_path_envvar on macOS

When loading and linking a dynamic library or bundle, dlopen
searches in LD_LIBRARY_PATH, DYLD_LIBRARY_PATH, PWD, and
DYLD_FALLBACK_LIBRARY_PATH.
In the Mach-O format, a dynamic library has an "install path."
Clients linking against the library record this path, and the
dynamic linker, dyld, uses it to locate the library.
dyld searches DYLD_LIBRARY_PATH *before* the install path.
dyld searches DYLD_FALLBACK_LIBRARY_PATH only if it cannot
find the library in the install path.
Setting DYLD_LIBRARY_PATH can easily have unintended
consequences.

See https://users.rust-lang.org/t/subprocess-and-dynamic-library-linking-problem-interaction/7873/10
See https://trac.macports.org/ticket/57692
See https://bugzilla.mozilla.org/show_bug.cgi?id=1536486

This is the same change as was applied to cargo in
rust-lang/cargo#6355

src/toolchain.rs

@@ -380,17 +380,43 @@ impl<'a> Toolchain<'a> {
  }
 
  pub fn set_ldpath(&self, cmd: &mut Command) {
- let new_path = self.path.join("lib");
+ let mut new_path = vec![self.path.join("lib")];
 
  #[cfg(not(target_os = "macos"))]
  mod sysenv {
  pub const LOADER_PATH: &str = "LD_LIBRARY_PATH";
  }
  #[cfg(target_os = "macos")]
  mod sysenv {
- pub const LOADER_PATH: &str = "DYLD_LIBRARY_PATH";
+ // When loading and linking a dynamic library or bundle, dlopen
+ // searches in LD_LIBRARY_PATH, DYLD_LIBRARY_PATH, PWD, and
+ // DYLD_FALLBACK_LIBRARY_PATH.
+ // In the Mach-O format, a dynamic library has an "install path."
+ // Clients linking against the library record this path, and the
+ // dynamic linker, dyld, uses it to locate the library.
+ // dyld searches DYLD_LIBRARY_PATH *before* the install path.
+ // dyld searches DYLD_FALLBACK_LIBRARY_PATH only if it cannot
+ // find the library in the install path.
+ // Setting DYLD_LIBRARY_PATH can easily have unintended
+ // consequences.
+ pub const LOADER_PATH: &str = "DYLD_FALLBACK_LIBRARY_PATH";
  }
- env_var::prepend_path(sysenv::LOADER_PATH, vec![new_path.clone()], cmd);
+ #[cfg(target_os = "macos")]
+ if env::var_os(sysenv::LOADER_PATH)
+ .filter(|x| x.len() > 0)
+ .is_none()
+ {
+ // These are the defaults when DYLD_FALLBACK_LIBRARY_PATH isn't
+ // set or set to an empty string. Since we are explicitly setting
+ // the value, make sure the defaults still work.
+ if let Some(home) = env::var_os("HOME") {
+ new_path.push(PathBuf::from(home).join("lib"));
+ }
+ new_path.push(PathBuf::from("/usr/local/lib"));
+ new_path.push(PathBuf::from("/usr/lib"));
+ }
+
+ env_var::prepend_path(sysenv::LOADER_PATH, new_path, cmd);
 
  // Prepend CARGO_HOME/bin to the PATH variable so that we're sure to run
  // cargo/rustc via the proxy bins. There is no fallback case for if the