codegen for root level error (#930)

* code gen for root error

* cargo fmt

* polkadot.rs regenerated

* use pallet name and decode with metadata

* remove pallet by name fn

* test that we can decode a ModuleError via as_root_error

* nits

---------

Co-authored-by: James Wilson <james@jsdw.me>

subxt/src/error/dispatch_error.rs

@@ -10,6 +10,9 @@ use core::fmt::Debug;
 use scale_decode::visitor::DecodeAsTypeResult;
 use std::borrow::Cow;
 
+use super::Error;
+use crate::error::RootError;
+
 /// An error dispatching a transaction.
 #[derive(Debug, thiserror::Error, PartialEq, Eq)]
 #[non_exhaustive]
@@ -133,12 +136,13 @@ impl PartialEq for ModuleError {
         self.raw == other.raw
     }
 }
+
 impl Eq for ModuleError {}
 
 impl std::fmt::Display for ModuleError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let Ok(details) = self.details() else {
-            return f.write_str("Unknown pallet error (pallet and error details cannot be retrieved)")
+            return f.write_str("Unknown pallet error (pallet and error details cannot be retrieved)");
         };
 
         let pallet = details.pallet();
@@ -159,6 +163,12 @@ impl ModuleError {
     pub fn raw(&self) -> RawModuleError {
         self.raw
     }
+
+    /// Attempts to decode the ModuleError into a value implementing the trait `RootError`
+    /// where the actual type of value is the generated top level enum `Error`.
+    pub fn as_root_error<E: RootError>(&self) -> Result<E, Error> {
+        E::root_error(&self.raw.error, self.details()?.pallet(), &self.metadata)
+    }
 }
 
 /// The error details about a module error that has occurred.

subxt/src/error/mod.rs

@@ -14,7 +14,7 @@ pub use dispatch_error::{
 };
 
 // Re-expose the errors we use from other crates here:
-pub use crate::metadata::{InvalidMetadataError, MetadataError};
+pub use crate::metadata::{InvalidMetadataError, Metadata, MetadataError};
 pub use scale_decode::Error as DecodeError;
 pub use scale_encode::Error as EncodeError;
 
@@ -162,3 +162,15 @@ pub enum StorageAddressError {
         fields: usize,
     },
 }
+
+/// This trait is implemented on the statically generated root ModuleError type
+#[doc(hidden)]
+pub trait RootError: Sized {
+    /// Given details of the pallet error we want to decode
+    fn root_error(
+        // typically a [u8; 4] encodes the error of a pallet
+        pallet_bytes: &[u8],
+        pallet_name: &str,
+        metadata: &Metadata,
+    ) -> Result<Self, Error>;
+}