Add rust test for /usr/bin/ld symlink

This test will fail if the symlink from `/usr/bin/ld` to the llvm
toolchain directory is not made.

WORKSPACE

@@ -138,3 +138,18 @@ http_archive(
         "https://ftp.pcre.org/pub/pcre/pcre-8.43.tar.gz",
     ],
 )
+
+# Rust rules (for testing)
+
+http_archive(
+    name = "rules_rust",
+    sha256 = "82b30cbb46c61a9014de0a8d0443a45e6eb6bd9add35ab421cfb1943dc3271f5",
+    strip_prefix = "rules_rust-e589105b4e8181dd1d0d8ccaa0cf3267efb06e86",
+    urls = [
+        "https://github.com/bazelbuild/rules_rust/archive/e589105b4e8181dd1d0d8ccaa0cf3267efb06e86.tar.gz",
+    ],
+)
+
+load("@rules_rust//rust:repositories.bzl", "rust_repositories")
+
+rust_repositories()

tests/rust/.gitignore

@@ -0,0 +1 @@
+target/

tests/rust/BUILD.bazel

@@ -0,0 +1,13 @@
+load("@rules_rust//rust:rust.bzl", "rust_binary", "rust_test")
+load("@rules_rust//cargo:cargo_build_script.bzl", "cargo_build_script")
+
+cargo_build_script(
+    name = "build_script",
+    srcs = ["build.rs"],
+)
+
+rust_test(
+    name = "rust_test",
+    srcs = ["src/rust_test.rs"],
+    deps = [":build_script"],
+)

tests/rust/README.md

@@ -0,0 +1,5 @@
+*This example project is used to test Rust's compatibility (on Mac) with a custom llvm_toolchain.*
+
+Rust's `rustc` expects to find the `ld` executable in the same directory as the compiler executable, which is not (currently) the case for Mac compilation with `llvm_toolchain` as LLVM's `ld.lld` linker is still experimental on MacOS and therefore the default linker at `/usr/bin/ld` is used. When compiling Rust code we encounter an `executable "ld" doesn't exist` error. The current fix is to manually symlink `/usr/bin/ld` into the llvm toolchain's `bin` directory to ensure it is found. This is necessary to make compilation work for both normal rust crates and crates which use build scripts (`build.rs`).
+
+This project contains a simple build script and rust executable to test that both work well with `/usr/bin/ld` into the llvm toolchain's `bin` directory. If the build succeeds it means `rustc` found `ld` both in the build script and rust binary invocations.

tests/rust/build.rs

@@ -0,0 +1,17 @@
+use std::env;
+use std::fs;
+use std::path::Path;
+
+fn main() {
+    let out_dir = env::var("OUT_DIR").unwrap();
+    let out = Path::new(&out_dir).join("hello.rs");
+
+    let _ = fs::write(
+        &out,
+        "pub fn hello() -> &'static str {
+            \"Hello, world!\"
+        }
+        ");
+
+    println!("cargo:rerun-if-changed=build.rs");
+}

tests/rust/src/rust_test.rs

@@ -0,0 +1,11 @@
+include!(concat!(env!("OUT_DIR"), "/hello.rs"));
+
+#[cfg(test)]
+mod test {
+    use crate::hello;
+
+    #[test]
+    fn test_hello() {
+        assert_eq!("Hello, world!", hello());
+    }
+}

tests/scripts/run_external_tests.sh

@@ -46,5 +46,6 @@ fi
 "${bazel}" --bazelrc=/dev/null test "${test_args[@]}" \
   //tests/foreign:pcre \
   @openssl//:libssl \
+  //tests/rust:rust_test \
   $("${bazel}" query 'attr(timeout, short, tests(@com_google_absl//absl/...))') \
   $("${bazel}" query 'tests(@io_bazel_rules_go//tests/core/cgo:all) except set(@io_bazel_rules_go//tests/core/cgo:cc_libs_test @io_bazel_rules_go//tests/core/cgo:opts_test)')