add note about why this function is unsafe

Author: folkertdev

compiler-builtins/src/lib.rs

@@ -8,6 +8,7 @@
 #![feature(linkage)]
 #![feature(naked_functions)]
 #![feature(repr_simd)]
+#![feature(rustc_attrs)]
 #![cfg_attr(f16_enabled, feature(f16))]
 #![cfg_attr(f128_enabled, feature(f128))]
 #![no_builtins]

compiler-builtins/src/probestack.rs

@@ -52,11 +52,13 @@
 // Our goal here is to touch each page between %rsp+8 and %rsp+8-%rax,
 // ensuring that if any pages are unmapped we'll make a page fault.
 //
+// This function is unsafe because it uses a custom ABI, it does not actually match `extern "C"`.
+//
 // The ABI here is that the stack frame size is located in `%rax`. Upon
 // return we're not supposed to modify `%rsp` or `%rax`.
 #[cfg(target_arch = "x86_64")]
 #[unsafe(naked)]
-#[no_mangle]
+#[rustc_std_internal_symbol]
 pub unsafe extern "C" fn __rust_probestack() {
     #[cfg(not(all(target_env = "sgx", target_vendor = "fortanix")))]
     macro_rules! ret {
@@ -141,9 +143,11 @@ pub unsafe extern "C" fn __rust_probestack() {
 // that on Unix we're expected to restore everything as it was, this
 // function basically can't tamper with anything.
 //
+// This function is unsafe because it uses a custom ABI, it does not actually match `extern "C"`.
+//
 // The ABI here is the same as x86_64, except everything is 32-bits large.
 #[unsafe(naked)]
-#[no_mangle]
+#[rustc_std_internal_symbol]
 pub unsafe extern "C" fn __rust_probestack() {
     core::arch::naked_asm!(
         "
@@ -186,14 +190,16 @@ pub unsafe extern "C" fn __rust_probestack() {
 // probestack function will also do things like _chkstk in MSVC.
 // So we need to sub %ax %sp in probestack when arch is x86.
 //
+// This function is unsafe because it uses a custom ABI, it does not actually match `extern "C"`.
+//
 // REF: Rust commit(74e80468347)
 // rust\src\llvm-project\llvm\lib\Target\X86\X86FrameLowering.cpp: 805
 // Comments in LLVM:
 //   MSVC x32's _chkstk and cygwin/mingw's _alloca adjust %esp themselves.
 //   MSVC x64's __chkstk and cygwin/mingw's ___chkstk_ms do not adjust %rsp
 //   themselves.
 #[unsafe(naked)]
-#[no_mangle]
+#[rustc_std_internal_symbol]
 pub unsafe extern "C" fn __rust_probestack() {
     core::arch::naked_asm!(
         "