Rollup merge of #133041 - madsmtm:print-deployment-target-env-var, r=davidtwco

Print name of env var in `--print=deployment-target`

The deployment target environment variable is OS-specific, and if you're in a place where you're asking `rustc` for the deployment target, you're likely to also wanna know the name of the environment variable. I myself wanted this for some code I'm working on in bootstrap, for example.

Behaviour before this PR:
```console
$ rustc --print=deployment-target --target=aarch64-apple-darwin
deployment_target=11.0
$ rustc --print=deployment-target --target=aarch64-apple-visionos
deployment_target=1.0
```

Behaviour after this PR:
```console
$ rustc --print=deployment-target --target=aarch64-apple-darwin
MACOSX_DEPLOYMENT_TARGET=11.0
$ rustc --print=deployment-target --target=aarch64-apple-visionos
XROS_DEPLOYMENT_TARGET=1.0
```

My _belief_ is that this option is extremely rarely used in general, and a GitHub search for "rustc print deployment-target" seems to confirm this, it revealed only the following actual pieces of code using this:
- b292ef6934/src/build_context.rs (L1199-L1220)
- daab9244b0/src/lib.rs (L3422-L3426)

`maturin` does `.split('=').last()`, so it will continue to work after this change, but `cc v1.0.84` did `.strip_prefix("deployment_target=")` since [this PR](https://github.com/rust-lang/cc-rs/pull/848), so it would break. That's _probably_ fine though, it was broken in a lot of scenarios anyway, and [got](https://github.com/rust-lang/cc-rs/pull/901) [reverted](https://github.com/rust-lang/cc-rs/pull/943) in `v1.0.85`.

So while this is _technically_ a breaking change, I really doubt that anyone is going to observe it, so it's probably fine.

``@BlackHoleFox`` wdyt?

``@rustbot`` label O-apple
r? compiler

compiler/rustc_codegen_ssa/src/back/apple.rs

@@ -97,7 +97,7 @@ fn minimum_deployment_target(target: &Target) -> OSVersion {
 }
 
 /// Name of the environment variable used to fetch the deployment target on the given OS.
-fn deployment_target_env_var(os: &str) -> &'static str {
+pub fn deployment_target_env_var(os: &str) -> &'static str {
     match os {
         "macos" => "MACOSX_DEPLOYMENT_TARGET",
         "ios" => "IPHONEOS_DEPLOYMENT_TARGET",

compiler/rustc_driver_impl/src/lib.rs

@@ -867,8 +867,9 @@ fn print_crate_info(
             DeploymentTarget => {
                 if sess.target.is_like_osx {
                     println_info!(
-                        "deployment_target={}",
-                        apple::pretty_version(apple::deployment_target(sess))
+                        "{}={}",
+                        apple::deployment_target_env_var(&sess.target.os),
+                        apple::pretty_version(apple::deployment_target(sess)),
                     )
                 } else {
                     #[allow(rustc::diagnostic_outside_of_impl)]

tests/run-make/apple-deployment-target/rmake.rs

@@ -24,21 +24,31 @@ fn minos(file: &str, version: &str) {
 
 fn main() {
     // These versions should generally be higher than the default versions
-    let (env_var, example_version, higher_example_version) = match apple_os() {
-        "macos" => ("MACOSX_DEPLOYMENT_TARGET", "12.0", "13.0"),
+    let (example_version, higher_example_version) = match apple_os() {
+        "macos" => ("12.0", "13.0"),
         // armv7s-apple-ios and i386-apple-ios only supports iOS 10.0
-        "ios" if target() == "armv7s-apple-ios" || target() == "i386-apple-ios" => {
-            ("IPHONEOS_DEPLOYMENT_TARGET", "10.0", "10.0")
-        }
-        "ios" => ("IPHONEOS_DEPLOYMENT_TARGET", "15.0", "16.0"),
-        "watchos" => ("WATCHOS_DEPLOYMENT_TARGET", "7.0", "9.0"),
-        "tvos" => ("TVOS_DEPLOYMENT_TARGET", "14.0", "15.0"),
-        "visionos" => ("XROS_DEPLOYMENT_TARGET", "1.1", "1.2"),
+        "ios" if target() == "armv7s-apple-ios" || target() == "i386-apple-ios" => ("10.0", "10.0"),
+        "ios" => ("15.0", "16.0"),
+        "watchos" => ("7.0", "9.0"),
+        "tvos" => ("14.0", "15.0"),
+        "visionos" => ("1.1", "1.2"),
         _ => unreachable!(),
     };
-    let default_version =
-        rustc().target(target()).env_remove(env_var).print("deployment-target").run().stdout_utf8();
-    let default_version = default_version.strip_prefix("deployment_target=").unwrap().trim();
+
+    // Remove env vars to get `rustc`'s default
+    let output = rustc()
+        .target(target())
+        .env_remove("MACOSX_DEPLOYMENT_TARGET")
+        .env_remove("IPHONEOS_DEPLOYMENT_TARGET")
+        .env_remove("WATCHOS_DEPLOYMENT_TARGET")
+        .env_remove("TVOS_DEPLOYMENT_TARGET")
+        .env_remove("XROS_DEPLOYMENT_TARGET")
+        .print("deployment-target")
+        .run()
+        .stdout_utf8();
+    let (env_var, default_version) = output.split_once('=').unwrap();
+    let env_var = env_var.trim();
+    let default_version = default_version.trim();
 
     // Test that version makes it to the object file.
     run_in_tmpdir(|| {

tests/run-make/apple-sdk-version/rmake.rs

@@ -26,8 +26,7 @@ fn main() {
     // Fetch rustc's inferred deployment target.
     let current_deployment_target =
         rustc().target(target()).print("deployment-target").run().stdout_utf8();
-    let current_deployment_target =
-        current_deployment_target.strip_prefix("deployment_target=").unwrap().trim();
+    let current_deployment_target = current_deployment_target.split('=').last().unwrap().trim();
 
     // Fetch current SDK version via. xcrun.
     //

tests/ui/print-request/macos-target.rs

@@ -1,5 +1,6 @@
 //@ only-apple
 //@ compile-flags: --print deployment-target
+//@ normalize-stdout-test: "\w*_DEPLOYMENT_TARGET" -> "$$OS_DEPLOYMENT_TARGET"
 //@ normalize-stdout-test: "\d+\." -> "$$CURRENT_MAJOR_VERSION."
 //@ normalize-stdout-test: "\d+" -> "$$CURRENT_MINOR_VERSION"
 //@ check-pass

tests/ui/print-request/macos-target.stdout

@@ -1 +1 @@
-deployment_target=$CURRENT_MAJOR_VERSION.$CURRENT_MINOR_VERSION
+$OS_DEPLOYMENT_TARGET=$CURRENT_MAJOR_VERSION.$CURRENT_MINOR_VERSION