Bail when doing wit_component::decode on a component (#1473)

* Bail when doing wit-component::decode on a component

Signed-off-by: Ryan Levick <ryan.levick@fermyon.com>

* Add test for component-new-existing-component

Signed-off-by: Ryan Levick <ryan.levick@fermyon.com>

---------

Signed-off-by: Ryan Levick <ryan.levick@fermyon.com>

Author: rylev

crates/wit-component/src/metadata.rs

@@ -50,7 +50,7 @@ use wasm_encoder::{
     ComponentBuilder, ComponentExportKind, ComponentType, ComponentTypeRef, CustomSection,
 };
 use wasm_metadata::Producers;
-use wasmparser::{BinaryReader, Parser, Payload};
+use wasmparser::{BinaryReader, Encoding, Parser, Payload};
 use wit_parser::{Package, PackageName, Resolve, World, WorldId, WorldItem};
 
 const CURRENT_VERSION: u8 = 0x04;
@@ -127,6 +127,8 @@ pub struct ModuleMetadata {
 /// The extraction here provides the metadata necessary to continue the process
 /// later on.
 ///
+/// This will return an error if `wasm` is not a valid WebAssembly module.
+///
 /// Note that a "stripped" binary where `component-type` sections are removed
 /// is returned as well to embed within a component.
 pub fn decode(wasm: &[u8]) -> Result<(Vec<u8>, Bindgen)> {
@@ -142,6 +144,9 @@ pub fn decode(wasm: &[u8]) -> Result<(Vec<u8>, Bindgen)> {
                 ret.merge(data)
                     .with_context(|| format!("updating metadata for section {}", cs.name()))?;
             }
+            wasmparser::Payload::Version { encoding, .. } if encoding != Encoding::Module => {
+                bail!("decoding a component is not supported")
+            }
             _ => {
                 if let Some((id, range)) = payload.as_section() {
                     new_module.section(&wasm_encoder::RawSection {

tests/cli/component-new-existing-component.wat

@@ -0,0 +1,2 @@
+;; FAIL: component new % -o component.wasm
+(component)

tests/cli/component-new-existing-component.wat.stderr

@@ -0,0 +1 @@
+error: decoding a component is not supported