components: Fix support for 0-sized flags (#4560)

This commit goes through and updates support in the various argument
passing routines to support 0-sized flags. A bit of a degenerate case
but clarified in WebAssembly/component-model#76 as intentional.

crates/component-macro/src/lib.rs

@@ -352,6 +352,7 @@ fn expand_record_for_component_type(
         #[repr(C)]
         pub struct #lower <#lower_generic_params> {
             #lower_field_declarations
+            _align: [wasmtime::ValRaw; 0],
         }
 
         unsafe impl #impl_generics wasmtime::component::ComponentType for #name #ty_generics #where_clause {
@@ -965,6 +966,10 @@ fn expand_flags(flags: &Flags) -> Result<TokenStream> {
     let count = flags.flags.len();
 
     match size {
+        FlagsSize::Size0 => {
+            ty = quote!(());
+            eq = quote!(true);
+        }
         FlagsSize::Size1 => {
             ty = quote!(u8);
 
@@ -1021,6 +1026,17 @@ fn expand_flags(flags: &Flags) -> Result<TokenStream> {
     let mut not;
 
     match size {
+        FlagsSize::Size0 => {
+            count = 0;
+            as_array = quote!([]);
+            bitor = quote!(Self {});
+            bitor_assign = quote!();
+            bitand = quote!(Self {});
+            bitand_assign = quote!();
+            bitxor = quote!(Self {});
+            bitxor_assign = quote!();
+            not = quote!(Self {});
+        }
         FlagsSize::Size1 | FlagsSize::Size2 => {
             count = 1;
             as_array = quote!([self.__inner0 as u32]);
@@ -1085,6 +1101,7 @@ fn expand_flags(flags: &Flags) -> Result<TokenStream> {
         component_names.extend(quote!(#component_name,));
 
         let fields = match size {
+            FlagsSize::Size0 => quote!(),
             FlagsSize::Size1 => {
                 let init = 1_u8 << index;
                 quote!(__inner0: #init)

crates/component-util/src/lib.rs

@@ -48,6 +48,8 @@ impl From<DiscriminantSize> for usize {
 
 /// Represents the number of bytes required to store a flags value in the component model
 pub enum FlagsSize {
+    /// There are no flags
+    Size0,
     /// Flags can fit in a u8
     Size1,
     /// Flags can fit in a u16
@@ -59,7 +61,9 @@ pub enum FlagsSize {
 impl FlagsSize {
     /// Calculate the size needed to represent a value with the specified number of flags.
     pub fn from_count(count: usize) -> FlagsSize {
-        if count <= 8 {
+        if count == 0 {
+            FlagsSize::Size0
+        } else if count <= 8 {
             FlagsSize::Size1
         } else if count <= 16 {
             FlagsSize::Size2

crates/environ/fuzz/fuzz_targets/fact-valid-module.rs

@@ -53,10 +53,8 @@ enum ValType {
     Float64,
     Char,
     Record(Vec<ValType>),
-    // FIXME(WebAssembly/component-model#75) are zero-sized flags allowed?
-    //
-    // ... otherwise go up to 65 flags to exercise up to 3 u32 values
-    Flags(UsizeInRange<1, 65>),
+    // Up to 65 flags to exercise up to 3 u32 values
+    Flags(UsizeInRange<0, 65>),
     Tuple(Vec<ValType>),
     Variant(NonZeroLenVec<ValType>),
     Union(NonZeroLenVec<ValType>),

crates/environ/src/fact/trampoline.rs

@@ -700,6 +700,7 @@ impl Compiler<'_, '_> {
         assert_eq!(src_ty.names, dst_ty.names);
         let cnt = src_ty.names.len();
         match FlagsSize::from_count(cnt) {
+            FlagsSize::Size0 => {}
             FlagsSize::Size1 => {
                 let mask = if cnt == 8 { 0xff } else { (1 << cnt) - 1 };
                 self.convert_u8_mask(src, dst, mask);

crates/wasmtime/src/component/types.rs

@@ -606,6 +606,10 @@ impl Type {
             }
 
             Type::Flags(handle) => match FlagsSize::from_count(handle.names().len()) {
+                FlagsSize::Size0 => SizeAndAlignment {
+                    size: 0,
+                    alignment: 1,
+                },
                 FlagsSize::Size1 => SizeAndAlignment {
                     size: 1,
                     alignment: 1,

crates/wasmtime/src/component/values.rs

@@ -699,6 +699,7 @@ impl Val {
                 ty: handle.clone(),
                 count: u32::try_from(handle.names().len())?,
                 value: match FlagsSize::from_count(handle.names().len()) {
+                    FlagsSize::Size0 => Box::new([]),
                     FlagsSize::Size1 => iter::once(u8::load(mem, bytes)? as u32).collect(),
                     FlagsSize::Size2 => iter::once(u16::load(mem, bytes)? as u32).collect(),
                     FlagsSize::Size4Plus(n) => (0..n)
@@ -850,6 +851,7 @@ impl Val {
 
             Val::Flags(Flags { count, value, .. }) => {
                 match FlagsSize::from_count(*count as usize) {
+                    FlagsSize::Size0 => {}
                     FlagsSize::Size1 => u8::try_from(value[0]).unwrap().store(mem, offset)?,
                     FlagsSize::Size2 => u16::try_from(value[0]).unwrap().store(mem, offset)?,
                     FlagsSize::Size4Plus(_) => {
@@ -1018,6 +1020,7 @@ fn lower_list<T>(
 /// Note that this will always return at least 1, even if the `count` parameter is zero.
 pub(crate) fn u32_count_for_flag_count(count: usize) -> usize {
     match FlagsSize::from_count(count) {
+        FlagsSize::Size0 => 0,
         FlagsSize::Size1 | FlagsSize::Size2 => 1,
         FlagsSize::Size4Plus(n) => n,
     }

tests/all/component_model.rs

@@ -210,7 +210,9 @@ fn make_echo_component(type_definition: &str, type_size: u32) -> String {
 }
 
 fn make_echo_component_with_params(type_definition: &str, params: &[Param]) -> String {
-    let func = if params.len() == 1 || params.len() > 16 {
+    let func = if params.len() == 0 {
+        format!("(func (export \"echo\"))")
+    } else if params.len() == 1 || params.len() > 16 {
         let primitive = if params.len() == 1 {
             params[0].0.primitive()
         } else {

tests/all/component_model/macros.rs

@@ -426,6 +426,22 @@ fn enum_derive() -> Result<()> {
 
 #[test]
 fn flags() -> Result<()> {
+    let engine = super::engine();
+    let mut store = Store::new(&engine, ());
+
+    // Edge case of 0 flags
+    wasmtime::component::flags! {
+        Flags0 {}
+    }
+    assert_eq!(Flags0::default(), Flags0::default());
+
+    let component = Component::new(&engine, make_echo_component(r#"(flags)"#, 0))?;
+    let instance = Linker::new(&engine).instantiate(&mut store, &component)?;
+    let func = instance.get_typed_func::<(Flags0,), Flags0, _>(&mut store, "echo")?;
+    let output = func.call_and_post_return(&mut store, (Flags0::default(),))?;
+    assert_eq!(output, Flags0::default());
+
+    // Simple 8-bit flags
     wasmtime::component::flags! {
         Foo {
             #[component(name = "foo-bar-baz")]
@@ -442,9 +458,6 @@ fn flags() -> Result<()> {
     assert_eq!(Foo::default(), Foo::A ^ Foo::A);
     assert_eq!(Foo::B | Foo::C, !Foo::A);
 
-    let engine = super::engine();
-    let mut store = Store::new(&engine, ());
-
     // Happy path: component type matches flag count and names
 
     let component = Component::new(

tests/misc_testsuite/component-model/fused.wast

@@ -1228,6 +1228,7 @@
 
 ;; test that flags get their upper bits all masked off
 (component
+  (type $f0 (flags))
   (type $f1 (flags "f1"))
   (type $f8 (flags "f1" "f2" "f3" "f4" "f5" "f6" "f7" "f8"))
   (type $f9 (flags "f1" "f2" "f3" "f4" "f5" "f6" "f7" "f8" "f9"))
@@ -1277,6 +1278,7 @@
 
   (component $c1
     (core module $m
+      (func (export "f0"))
       (func (export "f1") (param i32)
         (if (i32.ne (local.get 0) (i32.const 0x1)) (unreachable))
       )
@@ -1310,6 +1312,7 @@
       )
     )
     (core instance $m (instantiate $m))
+    (func (export "f0") (param $f0) (canon lift (core func $m "f0")))
     (func (export "f1") (param $f1) (canon lift (core func $m "f1")))
     (func (export "f8") (param $f8) (canon lift (core func $m "f8")))
     (func (export "f9") (param $f9) (canon lift (core func $m "f9")))
@@ -1324,6 +1327,7 @@
 
   (component $c2
     (import "" (instance $i
+      (export "f0" (func (param $f0)))
       (export "f1" (func (param $f1)))
       (export "f8" (func (param $f8)))
       (export "f9" (func (param $f9)))
@@ -1334,6 +1338,7 @@
       (export "f64" (func (param $f64)))
       (export "f65" (func (param $f65)))
     ))
+    (core func $f0 (canon lower (func $i "f0")))
     (core func $f1 (canon lower (func $i "f1")))
     (core func $f8 (canon lower (func $i "f8")))
     (core func $f9 (canon lower (func $i "f9")))
@@ -1345,6 +1350,7 @@
     (core func $f65 (canon lower (func $i "f65")))
 
     (core module $m
+      (import "" "f0" (func $f0))
       (import "" "f1" (func $f1 (param i32)))
       (import "" "f8" (func $f8 (param i32)))
       (import "" "f9" (func $f9 (param i32)))
@@ -1356,6 +1362,7 @@
       (import "" "f65" (func $f65 (param i32 i32 i32)))
 
       (func $start
+        (call $f0)
         (call $f1 (i32.const 0xffffff01))
         (call $f8 (i32.const 0xffffff11))
         (call $f9 (i32.const 0xffffff11))
@@ -1371,6 +1378,7 @@
     )
     (core instance $m (instantiate $m
       (with "" (instance
+        (export "f0" (func $f0))
         (export "f1" (func $f1))
         (export "f8" (func $f8))
         (export "f9" (func $f9))