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.

6 files changed, 66 insertions, 45 deletions

diff --git a/src/doc/rustc-dev-guide/src/tests/running.md b/src/doc/rustc-dev-guide/src/tests/running.md
index 317b65f98cd..482f3c42578 100644
--- a/src/doc/rustc-dev-guide/src/tests/running.md
+++ b/src/doc/rustc-dev-guide/src/tests/running.md
@@ -339,9 +339,34 @@ results.  The Docker image is set up to launch `remote-test-server` and the
 build tools use `remote-test-client` to communicate with the server to
 coordinate running tests (see [src/bootstrap/src/core/build_steps/test.rs]).
 
-> **TODO**
->
-> - Is there any support for using an iOS emulator?
+To run on the iOS/tvOS/watchOS/visionOS simulator, we can similarly treat it as
+a "remote" machine. A curious detail here is that the network is shared between
+the simulator instance and the host macOS, so we can use the local loopback
+address `127.0.0.1`. Something like the following should work:
+
+```sh
+# Build the test server for the iOS simulator:
+./x build src/tools/remote-test-server --target aarch64-apple-ios-sim
+
+# If you already have a simulator instance open, copy the device UUID from:
+xcrun simctl list devices booted
+UDID=01234567-89AB-CDEF-0123-456789ABCDEF
+
+# Alternatively, create and boot a new simulator instance:
+xcrun simctl list runtimes
+xcrun simctl list devicetypes
+UDID=$(xcrun simctl create $CHOSEN_DEVICE_TYPE $CHOSEN_RUNTIME)
+xcrun simctl boot $UDID
+# See https://nshipster.com/simctl/ for details.
+
+# Spawn the runner on port 12345:
+xcrun simctl spawn $UDID ./build/host/stage2-tools/aarch64-apple-ios-sim/release/remote-test-server -v --bind 127.0.0.1:12345
+
+# In a new terminal, run tests via the runner:
+export TEST_DEVICE_ADDR="127.0.0.1:12345"
+./x test --host='' --target aarch64-apple-ios-sim --skip tests/debuginfo
+# FIXME(madsmtm): Allow debuginfo tests to work (maybe needs `.dSYM` folder to be copied to the target?).
+```
 
 [armhf-gnu]: https://github.com/rust-lang/rust/tree/master/src/ci/docker/host-x86_64/armhf-gnu/Dockerfile
 [QEMU]: https://www.qemu.org/
diff --git a/src/doc/rustc/src/platform-support/apple-ios.md b/src/doc/rustc/src/platform-support/apple-ios.md
index 586afa65226..3ac14704754 100644
--- a/src/doc/rustc/src/platform-support/apple-ios.md
+++ b/src/doc/rustc/src/platform-support/apple-ios.md
@@ -66,6 +66,11 @@ Rust programs can be built for these targets by specifying `--target`, if
 $ rustc --target aarch64-apple-ios your-code.rs
 ```
 
+Or if using Cargo and `-Zbuild-std`:
+```console
+$ cargo +nightly build -Zbuild-std --target armv7s-apple-ios
+```
+
 The simulator variants can be differentiated from the variants running
 on-device with the `target_env = "sim"` cfg (or `target_abi = "sim"` before
 Rust CURRENT_RUSTC_VERSION).
@@ -73,7 +78,7 @@ Rust CURRENT_RUSTC_VERSION).
 ```rust
 if cfg!(all(target_vendor = "apple", target_env = "sim")) {
     // Do something on the iOS/tvOS/visionOS/watchOS Simulator.
-} {
+} else {
     // Everything else, like Windows and non-Simulator iOS.
 }
 ```
@@ -82,8 +87,15 @@ This is similar to the `TARGET_OS_SIMULATOR` define in C code.
 
 ## Testing
 
-There is no support for running the Rust or standard library testsuite at the
-moment. Testing has mostly been done manually with builds of static libraries
-embedded into applications called from Xcode or a simulator.
+Running and testing your code naturally requires either an actual device
+running iOS, or the equivalent Xcode simulator environment. There exists
+several tools in the ecosystem for running a Cargo project on one of these.
+One of these tools is [`cargo-dinghy`]. [madsmtm/objc2#459] contains a more
+exhaustive list.
+
+See also [testing on emulators in the `rustc-dev-guide`][test-sim] for
+instructions on running the standard library's test suite.
 
-It hopefully will be possible to improve this in the future.
+[`cargo-dinghy`]: https://github.com/sonos/dinghy
+[madsmtm/objc2#459]: https://github.com/madsmtm/objc2/issues/459
+[test-sim]: https://rustc-dev-guide.rust-lang.org/tests/running.html#testing-on-emulators
diff --git a/src/doc/rustc/src/platform-support/apple-tvos.md b/src/doc/rustc/src/platform-support/apple-tvos.md
index 193d6466612..a952d8e230d 100644
--- a/src/doc/rustc/src/platform-support/apple-tvos.md
+++ b/src/doc/rustc/src/platform-support/apple-tvos.md
@@ -65,17 +65,8 @@ Using the unstable `-Zbuild-std` with a nightly Cargo may also work.
 
 ## Building Rust programs
 
-Rust programs can be built for these targets by specifying `--target`, if
-`rustc` has been built with support for them. For example:
-
-```console
-$ rustc --target aarch64-apple-tvos your-code.rs
-```
+See [the instructions for iOS](./apple-ios.md#building-rust-programs).
 
 ## Testing
 
-There is no support for running the Rust or standard library testsuite at the
-moment. Testing has mostly been done manually with builds of static libraries
-embedded into applications called from Xcode or a simulator.
-
-It hopefully will be possible to improve this in the future.
+See [the instructions for iOS](./apple-ios.md#testing).
diff --git a/src/doc/rustc/src/platform-support/apple-visionos.md b/src/doc/rustc/src/platform-support/apple-visionos.md
index ed96912da7a..2ac069248ee 100644
--- a/src/doc/rustc/src/platform-support/apple-visionos.md
+++ b/src/doc/rustc/src/platform-support/apple-visionos.md
@@ -46,20 +46,11 @@ be fixed in [#124560](https://github.com/rust-lang/rust/pull/124560).
 
 ## Building Rust programs
 
-Rust programs can be built for these targets by specifying `--target`, if
-`rustc` has been built with support for them. For example:
-
-```console
-$ rustc --target aarch64-apple-visionos-sim your-code.rs
-```
+See [the instructions for iOS](./apple-ios.md#building-rust-programs).
 
 ## Testing
 
-There is no support for running the Rust or standard library testsuite at the
-moment. Testing has mostly been done manually with builds of static libraries
-embedded into applications called from Xcode or a simulator.
-
-It hopefully will be possible to improve this in the future.
+See [the instructions for iOS](./apple-ios.md#testing).
 
 ## Cross-compilation toolchains and C code
 
diff --git a/src/doc/rustc/src/platform-support/apple-watchos.md b/src/doc/rustc/src/platform-support/apple-watchos.md
index 6ac09d0d1e5..c1a00961425 100644
--- a/src/doc/rustc/src/platform-support/apple-watchos.md
+++ b/src/doc/rustc/src/platform-support/apple-watchos.md
@@ -50,17 +50,8 @@ Using the unstable `-Zbuild-std` with a nightly Cargo may also work.
 
 ## Building Rust programs
 
-Rust programs can be built for these targets by specifying `--target`, if
-`rustc` has been built with support for them. For example:
-
-```console
-$ rustc --target aarch64-apple-watchos-sim your-code.rs
-```
+See [the instructions for iOS](./apple-ios.md#building-rust-programs).
 
 ## Testing
 
-There is no support for running the Rust or standard library testsuite at the
-moment. Testing has mostly been done manually with builds of static libraries
-embedded into applications called from Xcode or a simulator.
-
-It hopefully will be possible to improve this in the future.
+See [the instructions for iOS](./apple-ios.md#testing).
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