wasmtime: Initial, partial support for externref

This is enough to get an `externref -> externref` identity function
passing.

However, `externref`s that are dropped by compiled Wasm code are (safely)
leaked. Follow up work will leverage cranelift's stack maps to resolve this
issue.

crates/jit/src/compiler.rs

@@ -149,8 +149,10 @@ impl Compiler {
         let mut cx = FunctionBuilderContext::new();
         let mut trampolines = HashMap::new();
         let mut trampoline_relocations = HashMap::new();
-        for sig in translation.module.local.signatures.values() {
-            let index = self.signatures.register(sig);
+        for (wasm_func_ty, native_sig) in translation.module.local.signatures.values() {
+            let index = self
+                .signatures
+                .register(wasm_func_ty.clone(), native_sig.clone());
             if trampolines.contains_key(&index) {
                 continue;
             }
@@ -158,7 +160,7 @@ impl Compiler {
                 &*self.isa,
                 &mut self.code_memory,
                 &mut cx,
-                sig,
+                native_sig,
                 std::mem::size_of::<u128>(),
             )?;
             trampolines.insert(index, trampoline);
@@ -167,7 +169,7 @@ impl Compiler {
             // show up be sure to log it in case anyone's listening and there's
             // an accidental bug.
             if relocations.len() > 0 {
-                log::info!("relocations found in trampoline for {:?}", sig);
+                log::info!("relocations found in trampoline for {:?}", native_sig);
                 trampoline_relocations.insert(index, relocations);
             }
         }

crates/jit/src/imports.rs

@@ -31,8 +31,8 @@ pub fn resolve_imports(
 
         match (import, &export) {
             (EntityIndex::Function(func_index), Some(Export::Function(f))) => {
-                let import_signature = module.local.func_signature(*func_index);
-                let signature = signatures.lookup(f.signature).unwrap();
+                let import_signature = module.local.native_func_signature(*func_index);
+                let signature = signatures.lookup_native(f.signature).unwrap();
                 if signature != *import_signature {
                     // TODO: If the difference is in the calling convention,
                     // we could emit a wrapper function to fix it up.

crates/jit/src/instantiate.rs

@@ -93,7 +93,7 @@ impl<'data> RawCompiledModule<'data> {
                 .local
                 .signatures
                 .values()
-                .map(|sig| signature_registry.register(sig))
+                .map(|(wasm, native)| signature_registry.register(wasm.clone(), native.clone()))
                 .collect::<PrimaryMap<_, _>>()
         };