few comments.

wasmtime-api/src/instance.rs

@@ -110,7 +110,8 @@ impl Instance {
             let export = mutable.lookup(name).expect("export");
             if let wasmtime_runtime::Export::Function { signature, .. } = &export {
                 // HACK ensure all handles, instantiated outside Store, present in
-                // the store's SignatureRegistry.
+                // the store's SignatureRegistry, e.g. WASI instances that are
+                // imported into this store using the from_handle() method.
                 use crate::runtime::SignatureRegistry;
                 let _ = store.borrow_mut().register_cranelift_signature(signature);
             }

wasmtime-api/src/runtime.rs

@@ -79,6 +79,9 @@ impl Engine {
 
 // Store
 
+/// The SignatureRegistry allow register new cranelift signature,
+/// and also quickly retrive the signature back using registration
+/// index.
 pub(crate) trait SignatureRegistry {
     fn register_cranelift_signature(
         &mut self,