Fix default DYLD_FALLBACK_LIBRARY_PATH on MacOS.

src/cargo/core/compiler/compilation.rs

@@ -196,6 +196,17 @@ impl<'cfg> Compilation<'cfg> {
         };
 
         search_path.extend(util::dylib_path().into_iter());
+        if cfg!(target_os = "macos") {
+            // 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.
+            if let Ok(home) = env::var("HOME") {
+                search_path.push(PathBuf::from(home).join("lib"));
+            }
+            search_path.push(PathBuf::from("/usr/local/lib"));
+            search_path.push(PathBuf::from("/lib"));
+            search_path.push(PathBuf::from("/usr/lib"));
+        }
         let search_path = join_paths(&search_path, util::dylib_path_envvar())?;
 
         cmd.env(util::dylib_path_envvar(), &search_path);

src/cargo/util/paths.rs

@@ -45,6 +45,10 @@ pub fn dylib_path_envvar() -> &'static str {
         // find the library in the install path.
         // Setting DYLD_LIBRARY_PATH can easily have unintended
         // consequences.
+        //
+        // Also, DYLD_LIBRARY_PATH appears to have significant performance
+        // penalty starting in 10.13. Cargo's testsuite ran more than twice as
+        // slow with it on CI.
         "DYLD_FALLBACK_LIBRARY_PATH"
     } else {
         "LD_LIBRARY_PATH"

tests/testsuite/run.rs

@@ -1136,3 +1136,88 @@ fn default_run_workspace() {
         .with_stdout("run-a")
         .run();
 }
+
+#[test]
+#[cfg(target_os = "macos")]
+fn run_link_system_path_macos() {
+    use crate::support::paths::{self, CargoPathExt};
+    use std::fs;
+    // Check that the default system library path is honored.
+    // First, build a shared library that will be accessed from
+    // DYLD_FALLBACK_LIBRARY_PATH.
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [project]
+            name = "foo"
+            version = "0.0.1"
+            [lib]
+            crate-type = ["cdylib"]
+            "#,
+        )
+        .file(
+            "src/lib.rs",
+            "#[no_mangle] pub extern fn something_shared() {}",
+        )
+        .build();
+    p.cargo("build").run();
+
+    // This is convoluted. Since this test can't modify things in /usr,
+    // this needs to dance around to check that things work.
+    //
+    // The default DYLD_FALLBACK_LIBRARY_PATH is:
+    //      $(HOME)/lib:/usr/local/lib:/lib:/usr/lib
+    //
+    // This will make use of ~/lib in the path, but the default cc link
+    // path is /usr/lib:/usr/local/lib. So first need to build in one
+    // location, and then move it to ~/lib.
+    //
+    // 1. Build with rustc-link-search pointing to libfoo so the initial
+    //    binary can be linked.
+    // 2. Move the library to ~/lib
+    // 3. Run `cargo run` to make sure it can still find the library in
+    //    ~/lib.
+    //
+    // This should be equivalent to having the library in /usr/local/lib.
+    let p2 = project()
+        .at("bar")
+        .file("Cargo.toml", &basic_bin_manifest("bar"))
+        .file(
+            "src/main.rs",
+            r#"
+            extern {
+                fn something_shared();
+            }
+            fn main() {
+                unsafe { something_shared(); }
+            }
+            "#,
+        )
+        .file(
+            "build.rs",
+            &format!(
+                r#"
+            fn main() {{
+                println!("cargo:rustc-link-lib=foo");
+                println!("cargo:rustc-link-search={}");
+            }}
+            "#,
+                p.target_debug_dir().display()
+            ),
+        )
+        .build();
+    p2.cargo("build").run();
+    p2.cargo("test").run();
+
+    let libdir = paths::home().join("lib");
+    fs::create_dir(&libdir).unwrap();
+    fs::rename(
+        p.target_debug_dir().join("libfoo.dylib"),
+        libdir.join("libfoo.dylib"),
+    )
+    .unwrap();
+    p.root().rm_rf();
+    p2.cargo("run").run();
+    p2.cargo("test").run();
+}