Rollup merge of #146762 - madsmtm:test-apple-sim, r=jieyouxu

Fix and provide instructions for running test suite on Apple simulators

The following now works:

```sh
./x test --host='' --target aarch64-apple-ios-sim --skip tests/debuginfo
./x test --host='' --target aarch64-apple-tvos-sim --skip tests/debuginfo
./x test --host='' --target aarch64-apple-watchos-sim --skip tests/debuginfo
./x test --host='' --target aarch64-apple-visionos-sim --skip tests/debuginfo
```

I have documented the setup I used [in the `rustc-dev-guide`](https://rustc-dev-guide.rust-lang.org/tests/running.html#testing-on-emulators), it's fairly standard use of `remote-test-server` (with a small fix to library load paths which I've made in the first commit).

I first tried the somewhat simpler `target.aarch64-apple-ios-sim.runner = "xcrun simctl spawn $UDID"`, but that doesn't work as required libraries etc. also need to be copied to the device.

The debuginfo tests fail, I think because the debug info in `.dSYM` isn't available. I am yet unsure exactly how to fix this, either we need to copy that directory to the target as well, or we need to configure `lldb` somehow to read it from the host.

I decided to not add this to our CI, since I suspect we wouldn't gain much from it? Running on the simulator still uses the host Darwin kernel, it's basically just configured to run in another mode with more restricted permissions and different system libraries.

r? jieyouxu
CC ``@simlay,`` you're a lot more familiar with `xcrun simctl` than I.

1 files changed, 15 insertions, 4 deletions

diff --git a/src/tools/remote-test-server/src/main.rs b/src/tools/remote-test-server/src/main.rs
index 67a7ad6f3b4..5ec5e6e2898 100644
--- a/src/tools/remote-test-server/src/main.rs
+++ b/src/tools/remote-test-server/src/main.rs
@@ -53,6 +53,10 @@ impl Config {
             batch: false,
             bind: if cfg!(target_os = "android") || cfg!(windows) {
                 ([0, 0, 0, 0], 12345).into()
+            } else if cfg!(target_env = "sim") {
+                // iOS/tvOS/watchOS/visionOS simulators share network device
+                // with the host machine.
+                ([127, 0, 0, 1], 12345).into()
             } else {
                 ([10, 0, 2, 15], 12345).into()
             },
@@ -262,10 +266,17 @@ fn handle_run(socket: TcpStream, work: &Path, tmp: &Path, lock: &Mutex<()>, conf
     cmd.args(args);
     cmd.envs(env);
 
-    // On windows, libraries are just searched in the executable directory,
-    // system directories, PWD, and PATH, in that order. PATH is the only one
-    // we can change for this.
-    let library_path = if cfg!(windows) { "PATH" } else { "LD_LIBRARY_PATH" };
+    let library_path = if cfg!(windows) {
+        // On windows, libraries are just searched in the executable directory,
+        // system directories, PWD, and PATH, in that order. PATH is the only
+        // one we can change for this.
+        "PATH"
+    } else if cfg!(target_vendor = "apple") {
+        // On Apple platforms, the environment variable is named differently.
+        "DYLD_LIBRARY_PATH"
+    } else {
+        "LD_LIBRARY_PATH"
+    };
 
     // Support libraries were uploaded to `work` earlier, so make sure that's
     // in `LD_LIBRARY_PATH`. Also include our own current dir which may have