bytecodealliance · alexcrichton · Dec 19, 2025 · Dec 17, 2025
diff --git a/Cargo.toml b/Cargo.toml
@@ -19,7 +19,7 @@ edition = "2024"
 version = "0.49.0"
 license = "Apache-2.0 WITH LLVM-exception OR Apache-2.0 OR MIT"
 repository = "https://github.com/bytecodealliance/wit-bindgen"
-rust-version = "1.85.0"
+rust-version = "1.87.0"
 
 [workspace.dependencies]
 anyhow = "1.0.72"

diff --git a/crates/go/src/lib.rs b/crates/go/src/lib.rs
@@ -420,6 +420,7 @@ impl Go {
                     false,
                     imported_type,
                 );
+                generator.collect_lifters = true;
 
                 let lift_result =
                     abi::lift_from_memory(resolve, &mut generator, "src".to_string(), &ty);
@@ -432,6 +433,21 @@ impl Go {
                     "value".to_string(),
                     &ty,
                 );
+
+                let lifter_count = generator.lifter_count;
+                let (prefix, suffix) = if lifter_count > 0 {
+                    (
+                        format!("lifters := make([]func(), 0, {lifter_count})\n"),
+                        "\nreturn func() {
+        for _, lifter := range lifters {
+                lifter()
+        }
+}",
+                    )
+                } else {
+                    (String::new(), "\nreturn func() {}")
+                };
+
                 let lower = mem::take(&mut generator.src);
                 data.extend(InterfaceData::from_generator_and_code(
                     generator,
@@ -448,8 +464,12 @@ impl Go {
                     ),
                     format!("wasm_{kind}_lift_{snake}"),
                     format!(
-                        "func wasm_{kind}_lower_{snake}(pinner *runtime.Pinner, value {payload}, dst unsafe.Pointer) {{
-        {lower}
+                        "func wasm_{kind}_lower_{snake}(
+        pinner *runtime.Pinner,
+        value {payload},
+        dst unsafe.Pointer,
+) func() {{
+        {prefix}{lower}{suffix}
 }}
 "
                     ),
@@ -1014,15 +1034,33 @@ impl Go {
                 .collect::<Vec<_>>()
                 .join(", ");
 
-            let lift = if let Some(result) = func.result {
+            let lift = if let Some(ty) = func.result {
                 let result = abi::lift_from_memory(
                     resolve,
                     &mut generator,
                     IMPORT_RETURN_AREA.to_string(),
-                    &result,
+                    &ty,
                 );
                 let code = mem::take(&mut generator.src);
-                format!("{code}\nreturn {result}")
+                if let Type::Id(ty) = ty
+                    && let TypeDefKind::Tuple(tuple) = &resolve.types[ty].kind
+                {
+                    let count = tuple.types.len();
+                    let tuple = generator.locals.tmp("tuple");
+
+                    let results = (0..count)
+                        .map(|index| format!("{tuple}.F{index}"))
+                        .collect::<Vec<_>>()
+                        .join(", ");
+
+                    format!(
+                        "{code}
+{tuple} := {result}
+return {results}"
+                    )
+                } else {
+                    format!("{code}\nreturn {result}")
+                }
             } else {
                 String::new()
             };
@@ -1403,6 +1441,8 @@ struct FunctionGenerator<'a> {
     need_unsafe: bool,
     need_pinner: bool,
     need_math: bool,
+    collect_lifters: bool,
+    lifter_count: u32,
     return_area_size: ArchitectureSize,
     return_area_align: Alignment,
     imports: BTreeSet<String>,
@@ -1441,6 +1481,8 @@ impl<'a> FunctionGenerator<'a> {
             need_unsafe: false,
             need_pinner: false,
             need_math: false,
+            collect_lifters: false,
+            lifter_count: 0,
             return_area_size: ArchitectureSize::default(),
             return_area_align: Alignment::default(),
             imports: BTreeSet::new(),
@@ -1744,13 +1786,18 @@ for index := 0; index < int({length}); index++ {{
                         && let TypeDefKind::Tuple(tuple) = &resolve.types[ty].kind
                     {
                         let count = tuple.types.len();
+                        let tuple = self.locals.tmp("tuple");
 
                         let results = (0..count)
-                            .map(|index| format!("({result}).F{index}"))
+                            .map(|index| format!("{tuple}.F{index}"))
                             .collect::<Vec<_>>()
                             .join(", ");
 
-                        uwriteln!(self.src, "return {results}");
+                        uwriteln!(
+                            self.src,
+                            "{tuple} := {result}
+return {results}"
+                        );
                     } else {
                         uwriteln!(self.src, "return {result}");
                     }
@@ -2275,7 +2322,25 @@ default:
             | Instruction::HandleLower {
                 handle: Handle::Own(_),
                 ..
-            } => results.push(format!("({}).TakeHandle()", operands[0])),
+            } => {
+                let op = &operands[0];
+                if self.collect_lifters {
+                    self.lifter_count += 1;
+                    let resource = self.locals.tmp("resource");
+                    let handle = self.locals.tmp("handle");
+                    uwriteln!(
+                        self.src,
+                        "{resource} := {op}
+{handle} := {resource}.TakeHandle()
+lifters = append(lifters, func() {{
+        {resource}.SetHandle({handle})
+}})"
+                    );
+                    results.push(handle)
+                } else {
+                    results.push(format!("({op}).TakeHandle()"))
+                }
+            }
             Instruction::HandleLower {
                 handle: Handle::Borrow(_),
                 ..
@@ -2469,6 +2534,10 @@ func (self *{camel}) TakeHandle() int32 {{
         return self.handle.Take()
 }}
 
+func (self *{camel}) SetHandle(handle int32) {{
+        self.handle.Set(handle)
+}}
+
 func (self *{camel}) Handle() int32 {{
         return self.handle.Use()
 }}
@@ -2525,6 +2594,12 @@ func (self *{camel}) TakeHandle() int32 {{
 	return self.handle
 }}
 
+func (self *{camel}) SetHandle(handle int32) {{
+        if self.handle != handle {{
+                panic("invalid handle")
+        }}
+}}
+
 func (self *{camel}) Drop() {{
 	handle := self.handle
 	if self.handle != 0 {{
@@ -3011,12 +3086,12 @@ fn func_declaration(resolve: &Resolve, func: &Function) -> (String, bool) {
 }
 
 fn maybe_gofmt<'a>(format: Format, code: &'a [u8]) -> Cow<'a, [u8]> {
-    return thread::scope(|s| {
+    thread::scope(|s| {
         if let Format::True = format
             && let Ok((reader, mut writer)) = io::pipe()
         {
             s.spawn(move || {
-                _ = writer.write_all(&code);
+                _ = writer.write_all(code);
             });
 
             if let Ok(output) = Command::new("gofmt").stdin(reader).output()
@@ -3027,5 +3102,5 @@ fn maybe_gofmt<'a>(format: Format, code: &'a [u8]) -> Cow<'a, [u8]> {
         }
 
         Cow::Borrowed(code)
-    });
+    })
 }
diff --git a/crates/go/src/wit_future.go b/crates/go/src/wit_future.go
@@ -17,7 +17,7 @@ type FutureVtable[T any] struct {
 	DropReadable func(handle int32)
 	DropWritable func(handle int32)
 	Lift         func(src unsafe.Pointer) T
-	Lower        func(pinner *runtime.Pinner, value T, dst unsafe.Pointer)
+	Lower        func(pinner *runtime.Pinner, value T, dst unsafe.Pointer) func()
 }
 
 type FutureReader[T any] struct {
@@ -63,6 +63,10 @@ func (self *FutureReader[T]) TakeHandle() int32 {
 	return self.handle.Take()
 }
 
+func (self *FutureReader[T]) SetHandle(handle int32) {
+	self.handle.Set(handle)
+}
+
 func MakeFutureReader[T any](vtable *FutureVtable[T], handleValue int32) *FutureReader[T] {
 	handle := wit_runtime.MakeHandle(handleValue)
 	value := &FutureReader[T]{vtable, handle}
@@ -87,24 +91,26 @@ func (self *FutureWriter[T]) Write(item T) bool {
 	pinner := runtime.Pinner{}
 	defer pinner.Unpin()
 
+	var lifter func()
 	var buffer unsafe.Pointer
 	if self.vtable.Lower == nil {
 		buffer = unsafe.Pointer(unsafe.SliceData([]T{item}))
 		pinner.Pin(buffer)
 	} else {
 		buffer = wit_runtime.Allocate(&pinner, uintptr(self.vtable.Size), uintptr(self.vtable.Align))
-		self.vtable.Lower(&pinner, item, buffer)
+		lifter = self.vtable.Lower(&pinner, item, buffer)
 	}
 
 	code, _ := wit_async.FutureOrStreamWait(self.vtable.Write(handle, buffer), handle)
 
-	// TODO: restore handles to any unwritten resources, streams, or futures
-
 	switch code {
 	case wit_async.RETURN_CODE_COMPLETED:
 		return true
 
 	case wit_async.RETURN_CODE_DROPPED:
+		if lifter != nil {
+			lifter()
+		}
 		return false
 
 	default:

diff --git a/crates/go/src/wit_runtime.go b/crates/go/src/wit_runtime.go
@@ -26,6 +26,16 @@ func (self *Handle) Take() int32 {
 	return value
 }
 
+func (self *Handle) Set(value int32) {
+	if value == 0 {
+		panic("nil handle")
+	}
+	if self.value != 0 {
+		panic("handle already set")
+	}
+	self.value = value
+}
+
 func (self *Handle) TakeOrNil() int32 {
 	value := self.value
 	self.value = 0

diff --git a/crates/go/src/wit_stream.go b/crates/go/src/wit_stream.go
@@ -17,7 +17,7 @@ type StreamVtable[T any] struct {
 	DropReadable func(handle int32)
 	DropWritable func(handle int32)
 	Lift         func(src unsafe.Pointer) T
-	Lower        func(pinner *runtime.Pinner, value T, dst unsafe.Pointer)
+	Lower        func(pinner *runtime.Pinner, value T, dst unsafe.Pointer) func()
 }
 
 type StreamReader[T any] struct {
@@ -78,6 +78,10 @@ func (self *StreamReader[T]) TakeHandle() int32 {
 	return self.handle.Take()
 }
 
+func (self *StreamReader[T]) SetHandle(handle int32) {
+	self.handle.Set(handle)
+}
+
 func MakeStreamReader[T any](vtable *StreamVtable[T], handleValue int32) *StreamReader[T] {
 	handle := wit_runtime.MakeHandle(handleValue)
 	value := &StreamReader[T]{vtable, handle, false}
@@ -112,24 +116,33 @@ func (self *StreamWriter[T]) Write(items []T) uint32 {
 
 	writeCount := uint32(len(items))
 
+	var lifters []func()
 	var buffer unsafe.Pointer
 	if self.vtable.Lower == nil {
 		buffer = unsafe.Pointer(unsafe.SliceData(items))
 		pinner.Pin(buffer)
 	} else {
+		lifters = make([]func(), 0, writeCount)
 		buffer = wit_runtime.Allocate(
 			&pinner,
 			uintptr(self.vtable.Size*writeCount),
 			uintptr(self.vtable.Align),
 		)
 		for index, item := range items {
-			self.vtable.Lower(&pinner, item, unsafe.Add(buffer, index*int(self.vtable.Size)))
+			lifters = append(
+				lifters,
+				self.vtable.Lower(&pinner, item, unsafe.Add(buffer, index*int(self.vtable.Size))),
+			)
 		}
 	}
 
 	code, count := wit_async.FutureOrStreamWait(self.vtable.Write(handle, buffer, writeCount), handle)
 
-	// TODO: restore handles to any unwritten resources, streams, or futures
+	if lifters != nil && count < writeCount {
+		for _, lifter := range lifters[count:] {
+			lifter()
+		}
+	}
 
 	if code == wit_async.RETURN_CODE_DROPPED {
 		self.readerDropped = true

diff --git a/tests/runtime-async/async/incomplete-writes/leaf.go b/tests/runtime-async/async/incomplete-writes/leaf.go
@@ -0,0 +1,19 @@
+package export_my_test_leaf_interface
+
+import "runtime"
+
+type LeafThing struct {
+	pinner runtime.Pinner
+	handle int32
+	value  string
+}
+
+func (self *LeafThing) Get() string {
+	return self.value
+}
+
+func (self *LeafThing) OnDrop() {}
+
+func MakeLeafThing(value string) *LeafThing {
+	return &LeafThing{runtime.Pinner{}, 0, value}
+}