Apple: Improve comments for -arch linker argument

Author: madsmtm

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

@@ -39,6 +39,25 @@ impl Arch {
 }
 }
 
+/// The architecture name to forward to the linker.
+fnld_arch(self) -> &'staticstr{
+// Supported architecture names can be found in the source:
+// https://github.com/apple-oss-distributions/ld64/blob/ld64-951.9/src/abstraction/MachOFileAbstraction.hpp#L578-L648
+matchself{
+Armv7k => "armv7k",
+Armv7s => "armv7s",
+Arm64 => "arm64",
+Arm64e => "arm64e",
+Arm64_32 => "arm64_32",
+// ld64 doesn't understand i686, so fall back to i386 instead
+//
+// Same story when linking with cc, since that ends up invoking ld64.
+I386 | I686 => "i386",
+X86_64 => "x86_64",
+X86_64h => "x86_64h",
+}
+}
+
 pub(crate)fntarget_arch(self) -> Cow<'static,str>{
 Cow::Borrowed(matchself{
 Armv7k | Armv7s => "arm",
@@ -116,20 +135,29 @@ fn pre_link_args(os: &'static str, arch: Arch, abi: TargetAbi) -> LinkArgs {
 };
 let sdk_version = min_version.clone();
 
-letmut args = TargetOptions::link_args(
-LinkerFlavor::Darwin(Cc::No,Lld::No),
-&["-arch", arch.target_name(),"-platform_version"],
-);
+// From the man page for ld64 (`man ld`):
+// > The linker accepts universal (multiple-architecture) input files,
+// > but always creates a "thin" (single-architecture), standard Mach-O
+// > output file. The architecture for the output file is specified using
+// > the -arch option.
+//
+// The linker has heuristics to determine the desired architecture, but to
+// be safe, and to avoid a warning, we set the architecture explicitly.
+letmut args =
+TargetOptions::link_args(LinkerFlavor::Darwin(Cc::No,Lld::No),&["-arch", arch.ld_arch()]);
+
 add_link_args_iter(
 &mut args,
 LinkerFlavor::Darwin(Cc::No,Lld::No),
-[platform_name, min_version, sdk_version].into_iter(),
+["-platform_version".into(),platform_name, min_version, sdk_version].into_iter(),
 );
 if abi != TargetAbi::MacCatalyst{
+// CC forwards the `-arch` to the linker, so we use the same value
+// here intentionally.
 add_link_args(
 &mut args,
 LinkerFlavor::Darwin(Cc::Yes,Lld::No),
-&["-arch", arch.target_name()],
+&["-arch", arch.ld_arch()],
 );
 }else{
 add_link_args_iter(

compiler/rustc_target/src/spec/targets/i686_apple_darwin.rs

@@ -2,8 +2,7 @@ use crate::spec::base::apple::{macos_llvm_target, opts, Arch, TargetAbi};
 usecrate::spec::{Cc,FramePointer,LinkerFlavor,Lld,Target,TargetOptions};
 
 pub(crate)fntarget() -> Target{
-// ld64 only understands i386 and not i686
-let arch = Arch::I386;
+let arch = Arch::I686;
 letmut base = opts("macos", arch,TargetAbi::Normal);
  base.max_atomic_width = Some(64);
  base.add_pre_link_args(LinkerFlavor::Darwin(Cc::Yes,Lld::No),&["-m32"]);
@@ -13,9 +12,7 @@ pub(crate) fn target() -> Target {
 // Clang automatically chooses a more specific target based on
 // MACOSX_DEPLOYMENT_TARGET. To enable cross-language LTO to work
 // correctly, we do too.
-//
-// While ld64 doesn't understand i686, LLVM does.
-llvm_target:macos_llvm_target(Arch::I686).into(),
+llvm_target:macos_llvm_target(arch).into(),
 metadata:crate::spec::TargetMetadata{
 description:Some("32-bit macOS (10.12+, Sierra+)".into()),
 tier:Some(3),