Apple: Rebuild when deployment target changes

Rebuild when the `*_DEPLOYMENT_TARGET` variables change when building
Apple targets.

This is done by:
1. Adding it as a tracked variable to `Options` to make sure it clears
   the incremental cache.
2. Emitting the variable in `--emit=dep-info` (`.d`) files, so that
   Cargo can pick up changes to it, and correctly trigger a rebuild.

compiler/rustc_interface/src/passes.rs

@@ -391,8 +391,7 @@ fn escape_dep_filename(filename: &str) -> String {
 
 // Makefile comments only need escaping newlines and `\`.
 // The result can be unescaped by anything that can unescape `escape_default` and friends.
-fn escape_dep_env(symbol: Symbol) -> String {
-    let s = symbol.as_str();
+fn escape_dep_env(s: &str) -> String {
     let mut escaped = String::with_capacity(s.len());
     for c in s.chars() {
         match c {
@@ -492,16 +491,22 @@ fn write_out_deps(tcx: TyCtxt<'_>, outputs: &OutputFilenames, out_filenames: &[P
 
             // Emit special comments with information about accessed environment variables.
             let env_depinfo = sess.psess.env_depinfo.borrow();
+
+            // We will soon sort, so the initial order does not matter.
+            #[allow(rustc::potential_query_instability)]
+            let mut env_depinfo: Vec<_> = env_depinfo
+                .iter()
+                .map(|(k, v)| (escape_dep_env(k.as_str()), v.map(|v| escape_dep_env(v.as_str()))))
+                .chain(tcx.sess.target.is_like_osx.then(|| {
+                    // On Apple targets, we also depend on the deployment target environment variable.
+                    let name = rustc_target::spec::apple_deployment_target_env(&tcx.sess.target.os);
+                    (name.into(), std::env::var(name).ok().map(|var| escape_dep_env(&var)))
+                }))
+                .collect();
+            env_depinfo.sort_unstable();
             if !env_depinfo.is_empty() {
-                // We will soon sort, so the initial order does not matter.
-                #[allow(rustc::potential_query_instability)]
-                let mut envs: Vec<_> = env_depinfo
-                    .iter()
-                    .map(|(k, v)| (escape_dep_env(*k), v.map(escape_dep_env)))
-                    .collect();
-                envs.sort_unstable();
                 writeln!(file)?;
-                for (k, v) in envs {
+                for (k, v) in env_depinfo {
                     write!(file, "# env-dep:{k}")?;
                     if let Some(v) = v {
                         write!(file, "={v}")?;

compiler/rustc_session/src/config.rs

@@ -1164,6 +1164,7 @@ impl Default for Options {
             color: ColorConfig::Auto,
             logical_env: FxIndexMap::default(),
             verbose: false,
+            apple_deployment_target_env: None,
         }
     }
 }
@@ -2710,6 +2711,7 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
         color,
         logical_env,
         verbose,
+        apple_deployment_target_env: None,
     }
 }
 

compiler/rustc_session/src/options.rs

@@ -222,6 +222,15 @@ top_level_options!(
         color: ColorConfig [UNTRACKED],
 
         verbose: bool [TRACKED_NO_CRATE_HASH],
+
+        /// The raw value of the `*_DEPLOYMENT_TARGET` environment variable
+        /// for the selected target OS.
+        ///
+        /// The exact environment variable to use depends on the target that
+        /// the user has chosen, and we do not want to re-compile if an
+        /// unrelated deployment target environment variable changed, so we
+        /// defer the initialization of this to `build_session`.
+        apple_deployment_target_env: Option<String> [TRACKED],
     }
 );
 

compiler/rustc_session/src/session.rs

@@ -1006,7 +1006,7 @@ fn default_emitter(
 #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 pub fn build_session(
     early_dcx: EarlyDiagCtxt,
-    sopts: config::Options,
+    mut sopts: config::Options,
     io: CompilerIO,
     bundle: Option<Lrc<rustc_errors::FluentBundle>>,
     registry: rustc_errors::registry::Registry,
@@ -1110,6 +1110,13 @@ pub fn build_session(
 
     let asm_arch = if target.allow_asm { InlineAsmArch::from_str(&target.arch).ok() } else { None };
 
+    // Configure the deployment target for change-tracking, now that target
+    // details are available.
+    if target.is_like_osx {
+        let name = rustc_target::spec::apple_deployment_target_env(&target.os);
+        sopts.apple_deployment_target_env = std::env::var(name).ok();
+    }
+
     let sess = Session {
         target,
         host,

compiler/rustc_target/src/spec/base/apple/mod.rs

@@ -251,6 +251,19 @@ pub fn platform(target: &Target) -> Option<u32> {
     })
 }
 
+/// Name of the environment variable used to fetch the deployment target on
+/// the given OS.
+pub fn deployment_target_env(os: &str) -> &'static str {
+    match os {
+        "macos" => "MACOSX_DEPLOYMENT_TARGET",
+        "ios" => "IPHONEOS_DEPLOYMENT_TARGET",
+        "watchos" => "WATCHOS_DEPLOYMENT_TARGET",
+        "tvos" => "TVOS_DEPLOYMENT_TARGET",
+        "visionos" => "XROS_DEPLOYMENT_TARGET",
+        _ => unreachable!("tried to get deployment target env var for non-Apple platform"),
+    }
+}
+
 /// Hack for calling `deployment_target` outside of this module.
 pub fn deployment_target_for_target(target: &Target) -> (u16, u8, u8) {
     let arch = if target.llvm_target.starts_with("arm64e") {
@@ -300,17 +313,13 @@ fn deployment_target(os: &str, arch: Arch, abi: TargetAbi) -> (u16, u8, u8) {
         _ => os_min,
     };
 
-    // The environment variable used to fetch the deployment target.
-    let env_var = match os {
-        "macos" => "MACOSX_DEPLOYMENT_TARGET",
-        "ios" => "IPHONEOS_DEPLOYMENT_TARGET",
-        "watchos" => "WATCHOS_DEPLOYMENT_TARGET",
-        "tvos" => "TVOS_DEPLOYMENT_TARGET",
-        "visionos" => "XROS_DEPLOYMENT_TARGET",
-        _ => unreachable!("tried to get deployment target env var for non-Apple platform"),
-    };
-
-    if let Ok(deployment_target) = env::var(env_var) {
+    // NOTE: We access the deployment target environment variable here, which
+    // makes the variable an **implicit** input which affects compilation.
+    //
+    // We make sure to rebuild when the variable changes, both by busting the
+    // incremental cache, and by telling Cargo that it is a dependency.
+    // Search for usages of `deployment_target_env` to see how.
+    if let Ok(deployment_target) = env::var(deployment_target_env(os)) {
         match parse_version(&deployment_target) {
             // It is common that the deployment target is set too low, e.g. on
             // macOS Aarch64 to also target older x86_64, the user may set a

compiler/rustc_target/src/spec/mod.rs

@@ -60,6 +60,7 @@ pub mod crt_objects;
 
 mod base;
 pub use base::apple::{
+    deployment_target_env as apple_deployment_target_env,
     deployment_target_for_target as current_apple_deployment_target,
     platform as current_apple_platform, sdk_version as current_apple_sdk_version,
 };