Pass deployment target to cc linker with -m*-version-min=

compiler/rustc_codegen_ssa/src/back/apple.rs

@@ -56,6 +56,11 @@ pub fn pretty_version(version: OSVersion) -> impl Display {
     })
 }
 
+fn full_version(version: OSVersion) -> impl Display {
+    let (major, minor, patch) = version;
+    from_fn(move |f| write!(f, "{major}.{minor}.{patch}"))
+}
+
 /// Minimum operating system versions currently supported by `rustc`.
 fn os_minimum_deployment_target(os: &str) -> OSVersion {
     // When bumping a version in here, remember to update the platform-support docs too.
@@ -155,7 +160,7 @@ pub(super) fn add_version_to_llvm_target(
     let environment = components.next();
     assert_eq!(components.next(), None, "too many LLVM triple components");
 
-    let (major, minor, patch) = deployment_target;
+    let version = full_version(deployment_target);
 
     assert!(
         !os.contains(|c: char| c.is_ascii_digit()),
@@ -164,8 +169,47 @@ pub(super) fn add_version_to_llvm_target(
 
     if let Some(env) = environment {
         // Insert version into OS, before environment
-        format!("{arch}-{vendor}-{os}{major}.{minor}.{patch}-{env}")
+        format!("{arch}-{vendor}-{os}{version}-{env}")
     } else {
-        format!("{arch}-{vendor}-{os}{major}.{minor}.{patch}")
+        format!("{arch}-{vendor}-{os}{version}")
+    }
+}
+
+/// The flag needed to specify the OS and deployment target to a C compiler.
+///
+/// There are many aliases for these, and `-mtargetos=` is preferred on Clang
+/// nowadays, but for compatibility with older Clang and other tooling, we try
+/// use the earliest supported name here.
+pub(super) fn cc_os_version_min_flag(os: &str, abi: &str, deployment_target: OSVersion) -> String {
+    let version = full_version(deployment_target);
+    // NOTE: GCC does not support `-miphoneos-version-min=` etc. (because it
+    // does not support iOS in general), but we specify them there anyhow in
+    // case GCC adds support for these in the future, and to force a
+    // compilation error if GCC compiler is not configured for Darwin:
+    // https://gcc.gnu.org/onlinedocs/gcc/Darwin-Options.html
+    //
+    // See also:
+    // https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mmacos-version-min
+    // https://clang.llvm.org/docs/AttributeReference.html#availability
+    // https://gcc.gnu.org/onlinedocs/gcc/Darwin-Options.html#index-mmacosx-version-min
+    //
+    // Note: These are intentionally passed as a single argument, Clang
+    // doesn't seem to like it otherwise.
+    match (os, abi) {
+        ("macos", "") => format!("-mmacosx-version-min={version}"),
+        ("ios", "") => format!("-miphoneos-version-min={version}"),
+        ("ios", "sim") => format!("-mios-simulator-version-min={version}"),
+        // Mac Catalyst came after the introduction of `-mtargetos=`, so no
+        // other flag exists for that.
+        ("ios", "macabi") => format!("-mtargetos=ios{version}-macabi"),
+        ("tvos", "") => format!("-mappletvos-version-min={version}"),
+        ("tvos", "sim") => format!("-mappletvsimulator-version-min={version}"),
+        ("watchos", "") => format!("-mwatchos-version-min={version}"),
+        ("watchos", "sim") => format!("-mwatchsimulator-version-min={version}"),
+        // `-mxros-version-min=` does not exist, use `-mtargetos=`.
+        // https://github.com/llvm/llvm-project/issues/88271
+        ("visionos", "") => format!("-mtargetos=xros{version}"),
+        ("visionos", "sim") => format!("-mtargetos=xros{version}-simulator"),
+        _ => unreachable!("tried to get cc version flag for non-Apple platform"),
     }
 }

compiler/rustc_codegen_ssa/src/back/apple/tests.rs

@@ -1,4 +1,4 @@
-use super::{add_version_to_llvm_target, parse_version};
+use super::{add_version_to_llvm_target, cc_os_version_min_flag, parse_version};
 
 #[test]
 fn test_add_version_to_llvm_target() {
@@ -19,3 +19,13 @@ fn test_parse_version() {
     assert_eq!(parse_version("10.12.6"), Ok((10, 12, 6)));
     assert_eq!(parse_version("9999.99.99"), Ok((9999, 99, 99)));
 }
+
+#[test]
+fn test_cc_os_version_min_flag() {
+    assert_eq!(cc_os_version_min_flag("macos", "", (10, 14, 1)), "-mmacosx-version-min=10.14.1");
+    assert_eq!(cc_os_version_min_flag("ios", "macabi", (13, 1, 0)), "-mtargetos=ios13.1.0-macabi");
+    assert_eq!(
+        cc_os_version_min_flag("visionos", "sim", (1, 0, 0)),
+        "-mtargetos=xros1.0.0-simulator"
+    );
+}

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -3163,39 +3163,43 @@ fn add_apple_link_args(cmd: &mut dyn Linker, sess: &Session, flavor: LinkerFlavo
     } else {
         // cc == Cc::Yes
         //
-        // We'd _like_ to use `-target` everywhere, since that can uniquely
+        // We'd _like_ to pass `--target` everywhere, since that can uniquely
         // communicate all the required details except for the SDK version
-        // (which is read by Clang itself from the SDKROOT), but that doesn't
-        // work on GCC, and since we don't know whether the `cc` compiler is
-        // Clang, GCC, or something else, we fall back to other options that
-        // also work on GCC when compiling for macOS.
+        // (which is read by Clang itself from the SDK root). But that has a
+        // few problems:
+        // - It doesn't work with GCC.
+        // - It doesn't work with Zig CC:
+        //   <https://github.com/ziglang/zig/issues/4911>.
+        // - It worked poorly with Clang when using config files before:
+        //   <https://github.com/llvm/llvm-project/pull/111387>
+        // - It worked poorly with certain versions of the `llvm` package in
+        //   Homebrew that expected the older flags:
+        //   <https://github.com/Homebrew/homebrew-core/issues/197532>
         //
-        // Targets other than macOS are ill-supported by GCC (it doesn't even
-        // support e.g. `-miphoneos-version-min`), so in those cases we can
-        // fairly safely use `-target`. See also the following, where it is
-        // made explicit that the recommendation by LLVM developers is to use
-        // `-target`: <https://github.com/llvm/llvm-project/issues/88271>
-        if target_os == "macos" {
-            // `-arch` communicates the architecture.
-            //
-            // CC forwards the `-arch` to the linker, so we use the same value
-            // here intentionally.
-            cmd.cc_args(&["-arch", ld64_arch]);
-
-            // The presence of `-mmacosx-version-min` makes CC default to
-            // macOS, and it sets the deployment target.
-            let (major, minor, patch) = apple::deployment_target(sess);
-            // Intentionally pass this as a single argument, Clang doesn't
-            // seem to like it otherwise.
-            cmd.cc_arg(&format!("-mmacosx-version-min={major}.{minor}.{patch}"));
-
-            // macOS has no environment, so with these two, we've told CC the
-            // four desired parameters.
-            //
-            // We avoid `-m32`/`-m64`, as this is already encoded by `-arch`.
-        } else {
-            cmd.cc_args(&["-target", &versioned_llvm_target(sess)]);
-        }
+        // So instead, we fall back to to using a combination of `-arch` with
+        // `-mtargetos=` (and similar).
+
+        // cc usually forwards the `-arch` to the linker, so we use the same
+        // value here intentionally.
+        //
+        // NOTE: We avoid `-m32`/`-m64` as this is already encoded by `-arch`.
+        cmd.cc_args(&["-arch", ld64_arch]);
+
+        let version = apple::deployment_target(sess);
+
+        // The presence of the `-mmacosx-version-min=`, `-mtargetos=` etc.
+        // flag both allows Clang to figure out the desired target OS,
+        // environment / ABI, and the deployment target. See the logic in:
+        // <https://github.com/llvm/llvm-project/blob/llvmorg-19.1.7/clang/lib/Driver/ToolChains/Darwin.cpp#L2235-L2320>
+        //
+        // NOTE: These all (correctly) take precedence over the equivalent
+        // `*_DEPLOYMENT_TARGET` environment variables (which we read
+        // ourselves to figure out the desired deployment target), so the
+        // current state of the environment should not have an effect.
+        cmd.cc_arg(&apple::cc_os_version_min_flag(target_os, target_abi, version));
+
+        // With these two flags + the SDK root, we've told cc the five desired
+        // parameters.
     }
 }