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.

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(