naked functions: emit .private_extern on macos

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

@@ -181,7 +181,8 @@ fn prefix_and_suffix<'tcx>(
                 }
             }
             Linkage::Internal => {
-                // write nothing
+                // LTO can fail when internal linkage is used.
+                emit_fatal("naked functions may not have internal linkage")
             }
             Linkage::Common => emit_fatal("Functions may not have common linkage"),
             Linkage::AvailableExternally => {

compiler/rustc_monomorphize/src/partitioning.rs

@@ -580,6 +580,16 @@ fn internalize_symbols<'tcx>(
                 }
             }
 
+            // When LTO inlines the caller of a naked function, it will attempt but fail to make the
+            // naked function symbol visible. To ensure that LTO works correctly, do not default
+            // naked functions to internal linkage and default visibility.
+            if let MonoItem::Fn(instance) = item {
+                let flags = cx.tcx.codegen_instance_attrs(instance.def).flags;
+                if flags.contains(CodegenFnAttrFlags::NAKED) {
+                    continue;
+                }
+            }
+
             // If we got here, we did not find any uses from other CGUs, so
             // it's fine to make this monomorphization internal.
             data.linkage = Linkage::Internal;

tests/assembly-llvm/naked-functions/aix.rs

@@ -9,7 +9,7 @@
 //@[aix] needs-llvm-components: powerpc
 
 #![crate_type = "lib"]
-#![feature(no_core, asm_experimental_arch, f128, linkage, fn_align)]
+#![feature(no_core, asm_experimental_arch)]
 #![no_core]
 
 // tests that naked functions work for the `powerpc64-ibm-aix` target.

tests/assembly-llvm/naked-functions/hidden.rs

@@ -0,0 +1,41 @@
+//@ revisions: macos-x86 macos-aarch64 linux-x86
+//@ add-core-stubs
+//@ assembly-output: emit-asm
+//
+//@[macos-aarch64] compile-flags: --target aarch64-apple-darwin
+//@[macos-aarch64] needs-llvm-components: aarch64
+//
+//@[macos-x86] compile-flags: --target x86_64-apple-darwin
+//@[macos-x86] needs-llvm-components: x86
+//
+//@[linux-x86] compile-flags: --target x86_64-unknown-linux-gnu
+//@[linux-x86] needs-llvm-components: x86
+
+#![crate_type = "lib"]
+#![feature(no_core, asm_experimental_arch)]
+#![no_core]
+
+// Tests that naked functions that are not externally linked (e.g. via `no_mangle`)
+// are marked as `Visibility::Hidden` and emit `.private_extern` or `.hidden`.
+//
+// Without this directive, LTO may fail because the symbol is not visible.
+// See also https://github.com/rust-lang/rust/issues/148307.
+
+extern crate minicore;
+use minicore::*;
+
+// CHECK: .p2align 2
+// macos-x86,macos-aarch64: .private_extern
+// linux-x86: .globl
+// linux-x86: .hidden
+// CHECK: ret
+#[unsafe(naked)]
+extern "C" fn ret() {
+    naked_asm!("ret")
+}
+
+// CHECK-LABEL: entry
+#[no_mangle]
+pub fn entry() {
+    ret()
+}