only set noalias on Box with the global allocator

tests/codegen/function-arguments.rs

@@ -2,6 +2,7 @@
 #![crate_type = "lib"]
 #![feature(dyn_star)]
 #![feature(generic_nonzero)]
+#![feature(allocator_api)]
 
 use std::mem::MaybeUninit;
 use std::num::NonZero;
@@ -182,6 +183,15 @@ pub fn _box(x: Box<i32>) -> Box<i32> {
   x
 }
 
+// With a custom allocator, it should *not* have `noalias`. (See
+// <https://github.com/rust-lang/miri/issues/3341> for why.) The second argument is the allocator,
+// which is a reference here that still carries `noalias` as usual.
+// CHECK: @_box_custom(ptr noundef nonnull align 4 %x.0, ptr noalias noundef nonnull readonly align 1 %x.1)
+#[no_mangle]
+pub fn _box_custom(x: Box<i32, &std::alloc::Global>) {
+  drop(x)
+}
+
 // CHECK: noundef nonnull align 4 ptr @notunpin_box(ptr noundef nonnull align 4 %x)
 #[no_mangle]
 pub fn notunpin_box(x: Box<NotUnpin>) -> Box<NotUnpin> {