Rollup merge of #146521 - folkertdev:document-va-arg-safe, r=workingjubilee

document `core::ffi::VaArgSafe`

tracking issue: https://github.com/rust-lang/rust/issues/44930

A modification of https://github.com/rust-lang/rust/pull/146454, keeping just the documentation changes, but not unsealing the trait.

Although conceptually we'd want to unseal the trait, there are many edge cases to supporting arbitrary types. We'd need to exhaustively test that all targets/calling conventions support all types that rust might generate (or generate proper error messages for unsupported cases). At present, many of the `va_arg` implementations assume that the argument is a scalar, and has an alignment of at most 8. That is totally  sufficient for an MVP (accepting all of the "standard" C types), but clearly does not cover all rust types.

This PR also adds some various other tests for edge cases of c-variadic:

- the `#[inline]` attribute in its various forms. At present, LLVM is unable to inline c-variadic functions, but the attribute should still be accepted. `#[rustc_force_inline]` already rejects c-variadic functions.
- naked functions should accept and work with a C variable argument list. In the future we'd like to allow more ABIs with naked functions (basically, any ABI for which we accept defining foreign c-variadic functions), but for now only  `"C"` and `"C-unwind` are supported
- guaranteed tail calls: c-variadic functions cannot be tail-called. That was already rejected, but there was not test for it.

r? `@workingjubilee`

4 files changed, 109 insertions, 0 deletions

diff --git a/tests/codegen-llvm/cffi/c-variadic-inline.rs b/tests/codegen-llvm/cffi/c-variadic-inline.rs
new file mode 100644
index 00000000000..369b7e571ca
--- /dev/null
+++ b/tests/codegen-llvm/cffi/c-variadic-inline.rs
@@ -0,0 +1,47 @@
+//@ compile-flags: -C opt-level=3
+#![feature(c_variadic)]
+
+// Test that the inline attributes are accepted on C-variadic functions.
+//
+// Currently LLVM is unable to inline C-variadic functions, but that is valid because despite
+// the name even `#[inline(always)]` is just a hint.
+
+#[inline(always)]
+unsafe extern "C" fn inline_always(mut ap: ...) -> u32 {
+    ap.arg::<u32>()
+}
+
+#[inline]
+unsafe extern "C" fn inline(mut ap: ...) -> u32 {
+    ap.arg::<u32>()
+}
+
+#[inline(never)]
+unsafe extern "C" fn inline_never(mut ap: ...) -> u32 {
+    ap.arg::<u32>()
+}
+
+#[cold]
+unsafe extern "C" fn cold(mut ap: ...) -> u32 {
+    ap.arg::<u32>()
+}
+
+#[unsafe(no_mangle)]
+#[inline(never)]
+fn helper() {
+    // CHECK-LABEL: helper
+    // CHECK-LABEL: call c_variadic_inline::inline_always
+    // CHECK-LABEL: call c_variadic_inline::inline
+    // CHECK-LABEL: call c_variadic_inline::inline_never
+    // CHECK-LABEL: call c_variadic_inline::cold
+    unsafe {
+        inline_always(1);
+        inline(2);
+        inline_never(3);
+        cold(4);
+    }
+}
+
+fn main() {
+    helper()
+}
diff --git a/tests/ui/c-variadic/naked.rs b/tests/ui/c-variadic/naked.rs
new file mode 100644
index 00000000000..46b59395485
--- /dev/null
+++ b/tests/ui/c-variadic/naked.rs
@@ -0,0 +1,40 @@
+//@ run-pass
+//@ only-x86_64
+//@ only-linux
+#![feature(c_variadic)]
+
+#[repr(C)]
+#[derive(Debug, PartialEq)]
+struct Data(i32, f64);
+
+#[unsafe(naked)]
+unsafe extern "C" fn c_variadic(_: ...) -> Data {
+    // This assembly was generated with GCC, because clang/LLVM is unable to
+    // optimize out the spilling of all registers to the stack.
+    core::arch::naked_asm!(
+        "        sub     rsp, 96",
+        "        mov     QWORD PTR [rsp-88], rdi",
+        "        test    al, al",
+        "        je      .L7",
+        "        movaps  XMMWORD PTR [rsp-40], xmm0",
+        ".L7:",
+        "        lea     rax, [rsp+104]",
+        "        mov     rcx, QWORD PTR [rsp-40]",
+        "        mov     DWORD PTR [rsp-112], 0",
+        "        mov     QWORD PTR [rsp-104], rax",
+        "        lea     rax, [rsp-88]",
+        "        mov     QWORD PTR [rsp-96], rax",
+        "        movq    xmm0, rcx",
+        "        mov     eax, DWORD PTR [rsp-88]",
+        "        mov     DWORD PTR [rsp-108], 48",
+        "        add     rsp, 96",
+        "        ret",
+    )
+}
+
+fn main() {
+    unsafe {
+        assert_eq!(c_variadic(1, 2.0), Data(1, 2.0));
+        assert_eq!(c_variadic(123, 4.56), Data(123, 4.56));
+    }
+}
diff --git a/tests/ui/explicit-tail-calls/c-variadic.rs b/tests/ui/explicit-tail-calls/c-variadic.rs
new file mode 100644
index 00000000000..e6eebe4228e
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/c-variadic.rs
@@ -0,0 +1,14 @@
+#![expect(incomplete_features)]
+#![feature(c_variadic, explicit_tail_calls)]
+#![allow(unused)]
+
+unsafe extern "C" fn foo(mut ap: ...) -> u32 {
+    ap.arg::<u32>()
+}
+
+extern "C" fn bar() -> u32 {
+    unsafe { become foo(1, 2, 3) }
+    //~^ ERROR c-variadic functions can't be tail-called
+}
+
+fn main() {}
diff --git a/tests/ui/explicit-tail-calls/c-variadic.stderr b/tests/ui/explicit-tail-calls/c-variadic.stderr
new file mode 100644
index 00000000000..5293339d218
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/c-variadic.stderr
@@ -0,0 +1,8 @@
+error: c-variadic functions can't be tail-called
+  --> $DIR/c-variadic.rs:10:14
+   |
+LL |     unsafe { become foo(1, 2, 3) }
+   |              ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+