Rollup merge of #139451 - madsmtm:apple-target-env-abi, r=wesleywiser

Add `target_env = "macabi"` and `target_env = "sim"`

[RFC 2992](https://github.com/rust-lang/rfcs/pull/2992) ([tracking issue](https://github.com/rust-lang/rust/issues/80970)) introduced `cfg(target_abi = ...)` with the original motivation being Mac Catalyst and Apple Simulator targets. These do not actually have a changed calling convention in the same sense that e.g. `cfg(target_abi = "eabihf")` or pointer authentication (`arm64e`) does, see https://github.com/rust-lang/rust/issues/133331.

Specifically, for Apple Simulator targets, the binary runs under the following conditions:
- Runs with the host macOS kernel (but in a mode configured for iOS/tvOS/...).
- Uses frameworks for the specific simulator version being targetted.
- System file accesses need to be made relative to the `IPHONE_SIMULATOR_ROOT` environment variable.
- Uses host GPUs directly.

And for Mac Catalyst:
- Runs with the host macOS kernel (but in a mode configured for iOS).
- Uses mostly host macOS frameworks (though with a few things changed, e.g. the [`NSImageResizingModeStretch`](https://developer.apple.com/documentation/appkit/nsimage/resizingmode-swift.enum/stretch?language=objc) enum has a different value).
- Uses host GPUs, camera and other peripherals directly.

As can be seen, these seem better suited as `target_env`s, since it really is the environment that the binary is running under that's changed (regardless of the Mac Catalyst "macabi" having "abi" in the name). So this PR adds `target_env = "sim"` and `target_env = "macabi"`, with the idea of possibly deprecating `target_abi = "sim"` and `target_abi = "macabi"` in the far future.

This affects iOS Tier 2 targets (`aarch64-apple-ios-sim`, `x86_64-apple-ios`, `aarch64-apple-ios-macabi` and `x86_64-apple-ios-macabi`), and probably needs a compiler FCP.

Fixes https://github.com/rust-lang/rust/issues/133331.
Reference PR: https://github.com/rust-lang/reference/pull/1781.
Cargo doc PR: https://github.com/rust-lang/cargo/pull/15404.

r? compiler
CC `@workingjubilee`
CC target maintainers `@deg4uss3r` `@thomcc` `@badboy` `@BlackHoleFox` `@madsmtm` `@agg23`

3 files changed, 27 insertions, 5 deletions

diff --git a/src/doc/rustc/src/platform-support/apple-ios-macabi.md b/src/doc/rustc/src/platform-support/apple-ios-macabi.md
index d4b71dbd4f4..c6f68f7a1e8 100644
--- a/src/doc/rustc/src/platform-support/apple-ios-macabi.md
+++ b/src/doc/rustc/src/platform-support/apple-ios-macabi.md
@@ -56,6 +56,17 @@ Rust programs can be built for these targets by specifying `--target`, if
 $ rustc --target aarch64-apple-ios-macabi your-code.rs
 ```
 
+The target can be differentiated from the iOS targets with the
+`target_env = "macabi"` cfg (or `target_abi = "macabi"` before Rust CURRENT_RUSTC_VERSION).
+
+```rust
+if cfg!(target_env = "macabi") {
+    // Do something only on Mac Catalyst.
+}
+```
+
+This is similar to the `TARGET_OS_MACCATALYST` define in C code.
+
 ## Testing
 
 Mac Catalyst binaries can be run directly on macOS 10.15 Catalina or newer.
diff --git a/src/doc/rustc/src/platform-support/apple-ios.md b/src/doc/rustc/src/platform-support/apple-ios.md
index 64325554ab6..586afa65226 100644
--- a/src/doc/rustc/src/platform-support/apple-ios.md
+++ b/src/doc/rustc/src/platform-support/apple-ios.md
@@ -66,6 +66,20 @@ Rust programs can be built for these targets by specifying `--target`, if
 $ rustc --target aarch64-apple-ios your-code.rs
 ```
 
+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).
+
+```rust
+if cfg!(all(target_vendor = "apple", target_env = "sim")) {
+    // Do something on the iOS/tvOS/visionOS/watchOS Simulator.
+} {
+    // Everything else, like Windows and non-Simulator iOS.
+}
+```
+
+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
diff --git a/src/tools/compiletest/src/directives/tests.rs b/src/tools/compiletest/src/directives/tests.rs
index 30d8537b51a..33a02eb29fd 100644
--- a/src/tools/compiletest/src/directives/tests.rs
+++ b/src/tools/compiletest/src/directives/tests.rs
@@ -637,6 +637,7 @@ fn matches_env() {
         ("x86_64-unknown-linux-gnu", "gnu"),
         ("x86_64-fortanix-unknown-sgx", "sgx"),
         ("arm-unknown-linux-musleabi", "musl"),
+        ("aarch64-apple-ios-macabi", "macabi"),
     ];
     for (target, env) in envs {
         let config: Config = cfg().target(target).build();
@@ -647,11 +648,7 @@ fn matches_env() {
 
 #[test]
 fn matches_abi() {
-    let abis = [
-        ("aarch64-apple-ios-macabi", "macabi"),
-        ("x86_64-unknown-linux-gnux32", "x32"),
-        ("arm-unknown-linux-gnueabi", "eabi"),
-    ];
+    let abis = [("x86_64-unknown-linux-gnux32", "x32"), ("arm-unknown-linux-gnueabi", "eabi")];
     for (target, abi) in abis {
         let config: Config = cfg().target(target).build();
         assert!(config.matches_abi(abi), "{target} {abi}");