Rollup merge of #125672 - Lokathor:update-miri-result-ffi, r=RalfJung

Add more ABI test cases to miri (RFC 3391)

Part of https://github.com/rust-lang/rust/issues/110503

cc `@RalfJung`

2 files changed, 36 insertions, 11 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/terminator.rs b/compiler/rustc_const_eval/src/interpret/terminator.rs
index 0649bb5617c..cbfe25ca8df 100644
--- a/compiler/rustc_const_eval/src/interpret/terminator.rs
+++ b/compiler/rustc_const_eval/src/interpret/terminator.rs
@@ -294,17 +294,30 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
     /// Unwrap types that are guaranteed a null-pointer-optimization
     fn unfold_npo(&self, layout: TyAndLayout<'tcx>) -> InterpResult<'tcx, TyAndLayout<'tcx>> {
-        // Check if this is `Option` wrapping some type.
-        let inner = match layout.ty.kind() {
-            ty::Adt(def, args) if self.tcx.is_diagnostic_item(sym::Option, def.did()) => {
-                args[0].as_type().unwrap()
-            }
-            _ => {
-                // Not an `Option`.
-                return Ok(layout);
+        // Check if this is `Option` wrapping some type or if this is `Result` wrapping a 1-ZST and
+        // another type.
+        let ty::Adt(def, args) = layout.ty.kind() else {
+            // Not an ADT, so definitely no NPO.
+            return Ok(layout);
+        };
+        let inner = if self.tcx.is_diagnostic_item(sym::Option, def.did()) {
+            // The wrapped type is the only arg.
+            self.layout_of(args[0].as_type().unwrap())?
+        } else if self.tcx.is_diagnostic_item(sym::Result, def.did()) {
+            // We want to extract which (if any) of the args is not a 1-ZST.
+            let lhs = self.layout_of(args[0].as_type().unwrap())?;
+            let rhs = self.layout_of(args[1].as_type().unwrap())?;
+            if lhs.is_1zst() {
+                rhs
+            } else if rhs.is_1zst() {
+                lhs
+            } else {
+                return Ok(layout); // no NPO
             }
+        } else {
+            return Ok(layout); // no NPO
         };
-        let inner = self.layout_of(inner)?;
+
         // Check if the inner type is one of the NPO-guaranteed ones.
         // For that we first unpeel transparent *structs* (but not unions).
         let is_npo = |def: AdtDef<'tcx>| {
diff --git a/src/tools/miri/tests/pass/function_calls/abi_compat.rs b/src/tools/miri/tests/pass/function_calls/abi_compat.rs
index 136660a305a..0cfcd532ff4 100644
--- a/src/tools/miri/tests/pass/function_calls/abi_compat.rs
+++ b/src/tools/miri/tests/pass/function_calls/abi_compat.rs
@@ -83,12 +83,24 @@ fn main() {
     test_abi_compat(main as fn(), id::<i32> as fn(i32) -> i32);
     // - 1-ZST
     test_abi_compat((), [0u8; 0]);
-    // - Guaranteed null-pointer-optimizations (RFC 3391).
+    // - Guaranteed Option<X> null-pointer-optimizations (RFC 3391).
     test_abi_compat(&0u32 as *const u32, Some(&0u32));
     test_abi_compat(main as fn(), Some(main as fn()));
     test_abi_compat(0u32, Some(num::NonZero::new(1u32).unwrap()));
     test_abi_compat(&0u32 as *const u32, Some(Wrapper(&0u32)));
-    test_abi_compat(0u32, Some(Wrapper(num::NonZero::new(1u32).unwrap())));
+    test_abi_compat(0u32, Some(Wrapper(num::NonZeroU32::new(1u32).unwrap())));
+    // - Guaranteed Result<X, ZST1> does the same as Option<X> (RFC 3391)
+    test_abi_compat(&0u32 as *const u32, Result::<_, ()>::Ok(&0u32));
+    test_abi_compat(main as fn(), Result::<_, ()>::Ok(main as fn()));
+    test_abi_compat(0u32, Result::<_, ()>::Ok(num::NonZeroU32::new(1).unwrap()));
+    test_abi_compat(&0u32 as *const u32, Result::<_, ()>::Ok(Wrapper(&0u32)));
+    test_abi_compat(0u32, Result::<_, ()>::Ok(Wrapper(num::NonZeroU32::new(1).unwrap())));
+    // - Guaranteed Result<ZST1, X> also does the same as Option<X> (RFC 3391)
+    test_abi_compat(&0u32 as *const u32, Result::<(), _>::Err(&0u32));
+    test_abi_compat(main as fn(), Result::<(), _>::Err(main as fn()));
+    test_abi_compat(0u32, Result::<(), _>::Err(num::NonZeroU32::new(1).unwrap()));
+    test_abi_compat(&0u32 as *const u32, Result::<(), _>::Err(Wrapper(&0u32)));
+    test_abi_compat(0u32, Result::<(), _>::Err(Wrapper(num::NonZeroU32::new(1).unwrap())));
 
     // These must work for *any* type, since we guarantee that `repr(transparent)` is ABI-compatible
     // with the wrapped field.