do not add prolog for variadic naked functions

fixes #99858

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -2,6 +2,7 @@
 use crate::traits::*;
 use rustc_index::bit_set::BitSet;
 use rustc_index::IndexVec;
+use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir;
 use rustc_middle::mir::traversal;
 use rustc_middle::mir::UnwindTerminateReason;
@@ -290,6 +291,12 @@ fn arg_local_refs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 
     let mut num_untupled = None;
 
+    let codegen_fn_attrs = bx.tcx().codegen_fn_attrs(fx.instance.def_id());
+    let naked = codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED);
+    if naked {
+        return vec![];
+    }
+
     let args = mir
         .args_iter()
         .enumerate()

tests/codegen/cffi/c-variadic-naked.rs

@@ -0,0 +1,19 @@
+//@ needs-asm-support
+//@ only-x86_64
+
+// tests that `va_start` is not injected into naked functions
+
+#![crate_type = "lib"]
+#![feature(c_variadic)]
+#![feature(naked_functions)]
+#![no_std]
+
+#[naked]
+pub unsafe extern "C" fn c_variadic(_: usize, _: ...) {
+    // CHECK-NOT: va_start
+    // CHECK-NOT: alloca
+    core::arch::asm! {
+        "ret",
+        options(noreturn),
+    }
+}

tests/codegen/naked-fn/naked-functions.rs

@@ -19,7 +19,7 @@
 }
 
 // CHECK: Function Attrs: naked
-// CHECK-NEXT: define{{.*}}i{{[0-9]+}} @naked_with_args_and_return(i64 %a, i64 %b)
+// CHECK-NEXT: define{{.*}}i{{[0-9]+}} @naked_with_args_and_return(i64 %0, i64 %1)
 #[no_mangle]
 #[naked]
 pub unsafe extern "C" fn naked_with_args_and_return(a: isize, b: isize) -> isize {