Disallow stream<char> in component-model-async (#2437)

Accounts for WebAssembly/component-model#607

Author: alexcrichton

crates/wasmparser/src/validator/component.rs

@@ -3965,10 +3965,25 @@ impl ComponentState {
                         "`stream` requires the component model async feature"
                     )
                 }
-                Ok(ComponentDefinedType::Stream(
-                    ty.map(|ty| self.create_component_val_type(ty, offset))
-                        .transpose()?,
-                ))
+                let ty = ty
+                    .map(|ty| self.create_component_val_type(ty, offset))
+                    .transpose()?;
+                let prim = match ty {
+                    Some(ComponentValType::Primitive(p)) => Some(p),
+                    Some(ComponentValType::Type(id)) => match types[id] {
+                        ComponentDefinedType::Primitive(p) => Some(p),
+                        _ => None,
+                    },
+                    None => None,
+                };
+                if prim == Some(crate::PrimitiveValType::Char) {
+                    bail!(
+                        offset,
+                        "`stream<char>` is not valid at this time, use `stream<u8>` \
+                         with a defined by encoding instead for now"
+                    )
+                }
+                Ok(ComponentDefinedType::Stream(ty))
             }
         }
     }

tests/cli/component-model/async/streams.wast

@@ -211,3 +211,17 @@
   )
   "type mismatch for export `stream.drop-writable` of module instantiation argument ``"
 )
+
+;; stream<char> isn't valid yet
+(assert_invalid
+  (component (type (stream char)))
+  "`stream<char>` is not valid at this time, use `stream<u8>` with a defined by encoding instead for now"
+)
+(assert_invalid
+  (component (type $t char) (type (stream $t)))
+  "`stream<char>` is not valid at this time, use `stream<u8>` with a defined by encoding instead for now"
+)
+(assert_invalid
+  (component (type (func (param "x" (stream char)))))
+  "`stream<char>` is not valid at this time, use `stream<u8>` with a defined by encoding instead for now"
+)

tests/snapshots/cli/component-model/async/streams.wast.json

@@ -111,6 +111,27 @@
       "filename": "streams.16.wasm",
       "module_type": "binary",
       "text": "type mismatch for export `stream.drop-writable` of module instantiation argument ``"
+    },
+    {
+      "type": "assert_invalid",
+      "line": 217,
+      "filename": "streams.17.wasm",
+      "module_type": "binary",
+      "text": "`stream<char>` is not valid at this time, use `stream<u8>` with a defined by encoding instead for now"
+    },
+    {
+      "type": "assert_invalid",
+      "line": 221,
+      "filename": "streams.18.wasm",
+      "module_type": "binary",
+      "text": "`stream<char>` is not valid at this time, use `stream<u8>` with a defined by encoding instead for now"
+    },
+    {
+      "type": "assert_invalid",
+      "line": 225,
+      "filename": "streams.19.wasm",
+      "module_type": "binary",
+      "text": "`stream<char>` is not valid at this time, use `stream<u8>` with a defined by encoding instead for now"
     }
   ]
 }