diff --git a/compiler/rustc_const_eval/src/interpret/terminator.rs b/compiler/rustc_const_eval/src/interpret/terminator.rs
index b2ebcceceb328..33c808649562c 100644
--- a/compiler/rustc_const_eval/src/interpret/terminator.rs
+++ b/compiler/rustc_const_eval/src/interpret/terminator.rs
@@ -255,6 +255,16 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         caller_abi: &ArgAbi<'tcx, Ty<'tcx>>,
         callee_abi: &ArgAbi<'tcx, Ty<'tcx>>,
     ) -> bool {
+        let primitive_abi_compat = |a1: abi::Primitive, a2: abi::Primitive| -> bool {
+            match (a1, a2) {
+                // For integers, ignore the sign.
+                (abi::Primitive::Int(int_ty1, _sign1), abi::Primitive::Int(int_ty2, _sign2)) => {
+                    int_ty1 == int_ty2
+                }
+                // For everything else we require full equality.
+                _ => a1 == a2,
+            }
+        };
         // Heuristic for type comparison.
         let layout_compat = || {
             if caller_abi.layout.ty == callee_abi.layout.ty {
@@ -267,28 +277,24 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 // then who knows what happens.
                 return false;
             }
-            if caller_abi.layout.size != callee_abi.layout.size
-                || caller_abi.layout.align.abi != callee_abi.layout.align.abi
-            {
-                // This cannot go well...
-                return false;
-            }
-            // The rest *should* be okay, but we are extra conservative.
+            // This is tricky. Some ABIs split aggregates up into multiple registers etc, so we have
+            // to be super careful here. For the scalar ABIs we conveniently already have all the
+            // newtypes unwrapped etc, so in those cases we can just compare the scalar components.
+            // Everything else we just reject for now.
             match (caller_abi.layout.abi, callee_abi.layout.abi) {
-                // Different valid ranges are okay (once we enforce validity,
-                // that will take care to make it UB to leave the range, just
-                // like for transmute).
+                // Different valid ranges are okay (the validity check will complain if this leads
+                // to invalid transmutes).
                 (abi::Abi::Scalar(caller), abi::Abi::Scalar(callee)) => {
-                    caller.primitive() == callee.primitive()
+                    primitive_abi_compat(caller.primitive(), callee.primitive())
                 }
                 (
                     abi::Abi::ScalarPair(caller1, caller2),
                     abi::Abi::ScalarPair(callee1, callee2),
                 ) => {
-                    caller1.primitive() == callee1.primitive()
-                        && caller2.primitive() == callee2.primitive()
+                    primitive_abi_compat(caller1.primitive(), callee1.primitive())
+                        && primitive_abi_compat(caller2.primitive(), callee2.primitive())
                 }
-                // Be conservative
+                // Be conservative.
                 _ => false,
             }
         };
@@ -309,7 +315,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             return true;
         };
         let mode_compat = || match (&caller_abi.mode, &callee_abi.mode) {
-            (PassMode::Ignore, PassMode::Ignore) => true,
+            (PassMode::Ignore, PassMode::Ignore) => true, // can still be reached for the return type
             (PassMode::Direct(a1), PassMode::Direct(a2)) => arg_attr_compat(a1, a2),
             (PassMode::Pair(a1, b1), PassMode::Pair(a2, b2)) => {
                 arg_attr_compat(a1, a2) && arg_attr_compat(b1, b2)
@@ -326,7 +332,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             _ => false,
         };
 
+        // We have to check both. `layout_compat` is needed to reject e.g. `i32` vs `f32`,
+        // which is not reflected in `PassMode`. `mode_compat` is needed to reject `u8` vs `bool`,
+        // which have the same `abi::Primitive` but different `arg_ext`.
         if layout_compat() && mode_compat() {
+            // Something went very wrong if our checks don't even imply that the layout is the same.
+            assert!(
+                caller_abi.layout.size == callee_abi.layout.size
+                    && caller_abi.layout.align.abi == callee_abi.layout.align.abi
+            );
             return true;
         }
         trace!(
diff --git a/src/tools/miri/tests/pass/function_calls/abi_compat.rs b/src/tools/miri/tests/pass/function_calls/abi_compat.rs
new file mode 100644
index 0000000000000..67b87b46bd9bc
--- /dev/null
+++ b/src/tools/miri/tests/pass/function_calls/abi_compat.rs
@@ -0,0 +1,27 @@
+use std::num;
+use std::mem;
+
+fn test_abi_compat<T, U>(t: T, u: U) {
+    fn id<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 f: fn(U) -> U = id;
+    let f: fn(T) -> T = unsafe { std::mem::transmute(f) };
+    drop(f(t));
+}
+
+fn main() {
+    test_abi_compat(0u32, 'x');
+    test_abi_compat(&0u32, &([true; 4], [0u32; 0]));
+    test_abi_compat(0u32, mem::MaybeUninit::new(0u32));
+    test_abi_compat(42u32, num::NonZeroU32::new(1).unwrap());
+    test_abi_compat(0u32, Some(num::NonZeroU32::new(1).unwrap()));
+    test_abi_compat(0u32, 0i32);
+    // Note that `bool` and `u8` are *not* compatible!
+    // One of them has `arg_ext: Zext`, the other does not.
+}