diff --git a/tests/pass/function_calls/abi_compat.rs b/tests/pass/function_calls/abi_compat.rs
index 30ab2d0588..01f4be70af 100644
--- a/tests/pass/function_calls/abi_compat.rs
+++ b/tests/pass/function_calls/abi_compat.rs
@@ -3,50 +3,83 @@ use std::mem;
 use std::num;
 use std::simd;
 
-fn test_abi_compat<T, U>(t: T, u: U) {
+#[derive(Copy, Clone)]
+struct Zst;
+
+fn test_abi_compat<T: Copy, U: Copy>(t: T, u: U) {
     fn id<T>(x: T) -> T {
         x
     }
+    extern "C" fn id_c<T>(x: T) -> T {
+        x
+    }
 
     // This checks ABI compatibility both for arguments and return values,
     // in both directions.
     let f: fn(T) -> T = id;
     let f: fn(U) -> U = unsafe { std::mem::transmute(f) };
-    drop(f(u));
-
+    let _val = f(u);
     let f: fn(U) -> U = id;
     let f: fn(T) -> T = unsafe { std::mem::transmute(f) };
-    drop(f(t));
+    let _val = f(t);
+
+    // And then we do the same for `extern "C"`.
+    let f: extern "C" fn(T) -> T = id_c;
+    let f: extern "C" fn(U) -> U = unsafe { std::mem::transmute(f) };
+    let _val = f(u);
+    let f: extern "C" fn(U) -> U = id_c;
+    let f: extern "C" fn(T) -> T = unsafe { std::mem::transmute(f) };
+    let _val = f(t);
 }
 
 /// Ensure that `T` is compatible with various repr(transparent) wrappers around `T`.
 fn test_abi_newtype<T: Copy>(t: T) {
     #[repr(transparent)]
+    #[derive(Copy, Clone)]
     struct Wrapper1<T>(T);
     #[repr(transparent)]
+    #[derive(Copy, Clone)]
     struct Wrapper2<T>(T, ());
     #[repr(transparent)]
+    #[derive(Copy, Clone)]
     struct Wrapper2a<T>((), T);
     #[repr(transparent)]
-    struct Wrapper3<T>(T, [u8; 0]);
+    #[derive(Copy, Clone)]
+    struct Wrapper3<T>(Zst, T, [u8; 0]);
 
     test_abi_compat(t, Wrapper1(t));
     test_abi_compat(t, Wrapper2(t, ()));
     test_abi_compat(t, Wrapper2a((), t));
-    test_abi_compat(t, Wrapper3(t, []));
+    test_abi_compat(t, Wrapper3(Zst, t, []));
+    test_abi_compat(t, mem::MaybeUninit::new(t)); // MaybeUninit is `repr(transparent)`
 }
 
 fn main() {
+    // Here we check:
+    // - unsigned vs signed integer is allowed
+    // - u32/i32 vs char is allowed
+    // - u32 vs NonZeroU32/Option<NonZeroU32> is allowed
+    // - reference vs raw pointer is allowed
+    // - references to things of the same size and alignment are allowed
+    // These are very basic tests that should work on all ABIs. However it is not clear that any of
+    // these would be stably guaranteed. Code that relies on this is equivalent to code that relies
+    // on the layout of `repr(Rust)` types. They are also fragile: the same mismatches in the fields
+    // of a struct (even with `repr(C)`) will not always be accepted by Miri.
+    test_abi_compat(0u32, 0i32);
+    test_abi_compat(simd::u32x8::splat(1), simd::i32x8::splat(1));
     test_abi_compat(0u32, 'x');
-    test_abi_compat(&0u32, &([true; 4], [0u32; 0]));
-    test_abi_compat(0u32, mem::MaybeUninit::new(0u32));
+    test_abi_compat(0i32, 'x');
     test_abi_compat(42u32, num::NonZeroU32::new(1).unwrap());
     test_abi_compat(0u32, Some(num::NonZeroU32::new(1).unwrap()));
-    test_abi_compat(0u32, 0i32);
-    test_abi_compat(simd::u32x8::splat(1), simd::i32x8::splat(1));
+    test_abi_compat(&0u32, &0u32 as *const u32);
+    test_abi_compat(&0u32, &([true; 4], [0u32; 0]));
     // Note that `bool` and `u8` are *not* compatible, at least on x86-64!
     // One of them has `arg_ext: Zext`, the other does not.
 
+    // These must work for *any* type, since we guarantee that `repr(transparent)` is ABI-compatible
+    // with the wrapped field.
+    test_abi_newtype(());
+    // FIXME: this still fails! test_abi_newtype(Zst);
     test_abi_newtype(0u32);
     test_abi_newtype(0f32);
     test_abi_newtype((0u32, 1u32, 2u32));