Add docs to ConvertTransaction and NoTransactionConverter (#977)

* Add docs to ConvertTransaction and NoTransactionConverter

* Update lib.rs

primitives/rpc/src/lib.rs

@@ -252,17 +252,20 @@ sp_api::decl_runtime_apis! {
 	}
 }
 
+/// Fallback transaction converter when the `ConvertTransactionRuntimeApi` is not available. For almost all
+/// non-legacy cases, you can instantiate this type as `NoTransactionConverter`.
 pub trait ConvertTransaction<E> {
 	fn convert_transaction(&self, transaction: ethereum::TransactionV2) -> E;
 }
 
+/// No fallback transaction converter is available.
 // `NoTransactionConverter` is a non-instantiable type (an enum with no variants),
 // so we are guaranteed at compile time that `NoTransactionConverter` can never be instantiated.
 pub enum NoTransactionConverter {}
 impl<E> ConvertTransaction<E> for NoTransactionConverter {
 	// `convert_transaction` is a method taking `&self` as a parameter, so it can only be called via an instance of type Self,
 	// so we are guaranteed at compile time that this method can never be called.
 	fn convert_transaction(&self, _transaction: ethereum::TransactionV2) -> E {
-		unreachable!()
+		match *self {}
 	}
 }

shell.nix

@@ -5,7 +5,7 @@ let
       rev = "78e723925daf5c9e8d0a1837ec27059e61649cb6";
     });
   nixpkgs = import <nixpkgs> { overlays = [ mozillaOverlay ]; };
-  rust-nightly = with nixpkgs; ((rustChannelOf { date = "2022-11-15"; channel = "nightly"; }).rust.override {
+  rust-nightly = with nixpkgs; ((rustChannelOf { date = "2023-07-23"; channel = "nightly"; }).rust.override {
     extensions = [ "rust-src" ];
     targets = [ "wasm32-unknown-unknown" ];
   });