Only compile #[used] as llvm.compiler.used for ELF targets

compiler/rustc_codegen_llvm/src/consts.rs

@@ -535,10 +535,20 @@ impl<'ll> StaticMethods for CodegenCx<'ll, '_> {
 
                 // The semantics of #[used] in Rust only require the symbol to make it into the
                 // object file. It is explicitly allowed for the linker to strip the symbol if it
-                // is dead. As such, use llvm.compiler.used instead of llvm.used.
+                // is dead, which means we are allowed use `llvm.compiler.used` instead of
+                // `llvm.used` here.
+                //
                 // Additionally, https://reviews.llvm.org/D97448 in LLVM 13 started emitting unique
                 // sections with SHF_GNU_RETAIN flag for llvm.used symbols, which may trigger bugs
-                // in some versions of the gold linker.
+                // in the handling of `.init_array` (the static constructor list) in versions of
+                // the gold linker (prior to the one released with binutils 2.36).
+                //
+                // That said, we only ever emit these when compiling for ELF targets, unless
+                // `#[used(compiler)]` is explicitly requested. This is to avoid similar breakage
+                // on other targets, in particular MachO targets have *their* static constructor
+                // lists broken if `llvm.compiler.used` is emitted rather than llvm.used. However,
+                // that check happens when assigning the `CodegenFnAttrFlags` in `rustc_typeck`,
+                // so we don't need to take care of it here.
                 self.add_compiler_used_global(g);
             }
             if attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER) {

compiler/rustc_codegen_ssa/src/traits/statics.rs

@@ -13,7 +13,9 @@ pub trait StaticMethods: BackendTypes {
     /// Same as add_used_global(), but only prevent the compiler from potentially removing an
     /// otherwise unused symbol. The linker is still permitted to drop it.
     ///
-    /// This corresponds to the semantics of the `#[used]` attribute.
+    /// This corresponds to the documented semantics of the `#[used]` attribute, although
+    /// on some targets (non-ELF), we may use `add_used_global` for `#[used]` statics
+    /// instead.
     fn add_compiler_used_global(&self, global: Self::Value);
 }
 

compiler/rustc_typeck/src/collect.rs

@@ -2850,7 +2850,37 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: DefId) -> CodegenFnAttrs {
                         )
                         .emit();
                 }
-                None => codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED,
+                None => {
+                    // Unfortunately, unconditionally using `llvm.used` causes
+                    // issues in handling `.init_array` with the gold linker,
+                    // but using `llvm.compiler.used` caused a nontrival amount
+                    // of unintentional ecosystem breakage -- particularly on
+                    // Mach-O targets.
+                    //
+                    // As a result, we emit `llvm.compiler.used` only on ELF
+                    // targets. This is somewhat ad-hoc, but actually follows
+                    // our pre-LLVM 13 behavior (prior to the ecosystem
+                    // breakage), and seems to match `clang`'s behavior as well
+                    // (both before and after LLVM 13), possibly because they
+                    // have similar compatibility concerns to us. See
+                    // https://github.com/rust-lang/rust/issues/47384#issuecomment-1019080146
+                    // and following comments for some discussion of this, as
+                    // well as the comments in `rustc_codegen_llvm` where these
+                    // flags are handled.
+                    //
+                    // Anyway, to be clear: this is still up in the air
+                    // somewhat, and is subject to change in the future (which
+                    // is a good thing, because this would ideally be a bit
+                    // more firmed up).
+                    let is_like_elf = !(tcx.sess.target.is_like_osx
+                        || tcx.sess.target.is_like_windows
+                        || tcx.sess.target.is_like_wasm);
+                    codegen_fn_attrs.flags = if is_like_elf {
+                        CodegenFnAttrFlags::USED
+                    } else {
+                        CodegenFnAttrFlags::USED_LINKER
+                    };
+                }
             }
         } else if attr.has_name(sym::cmse_nonsecure_entry) {
             if !matches!(tcx.fn_sig(did).abi(), abi::Abi::C { .. }) {

src/test/run-make-fulldeps/used-cdylib-macos/Makefile

@@ -0,0 +1,11 @@
+-include ../tools.mk
+
+# only-macos
+#
+# This checks that `#[used]` passes through to the linker on
+# darwin. This is subject to change in the future, see
+# https://github.com/rust-lang/rust/pull/93718 for discussion
+
+all:
+	$(RUSTC) -Copt-level=3 dylib_used.rs
+	nm $(TMPDIR)/libdylib_used.dylib | $(CGREP) VERY_IMPORTANT_SYMBOL

src/test/run-make-fulldeps/used-cdylib-macos/dylib_used.rs

@@ -0,0 +1,4 @@
+#![crate_type = "cdylib"]
+
+#[used]
+static VERY_IMPORTANT_SYMBOL: u32 = 12345;