add repr to the allowlist for naked functions, and test that it works

compiler/rustc_passes/src/check_attr.rs

@@ -516,6 +516,7 @@ fn check_naked(
             sym::no_mangle,
             sym::naked,
             sym::instruction_set,
+            sym::repr,
             // code generation
             sym::cold,
             sym::target_feature,

tests/codegen/naked-fn/aligned.rs

@@ -0,0 +1,20 @@
+//@ compile-flags: -C no-prepopulate-passes -Copt-level=0
+//@ needs-asm-support
+//@ ignore-arm no "ret" mnemonic
+
+#![crate_type = "lib"]
+#![feature(naked_functions, fn_align)]
+use std::arch::asm;
+
+// CHECK: Function Attrs: naked
+// CHECK-NEXT: define{{.*}}void @naked_empty()
+// CHECK: align 16
+#[repr(align(16))]
+#[no_mangle]
+#[naked]
+pub unsafe extern "C" fn naked_empty() {
+    // CHECK-NEXT: start:
+    // CHECK-NEXT: call void asm
+    // CHECK-NEXT: unreachable
+    asm!("ret", options(noreturn));
+}