introduce errors with info (#1834)

polkadot/erasure-coding/src/lib.rs

@@ -30,6 +30,7 @@ use primitives::v0::{self, Hash as H256, BlakeTwo256, HashT};
 use primitives::v1;
 use sp_core::Blake2Hasher;
 use trie::{EMPTY_PREFIX, MemoryDB, Trie, TrieMut, trie_types::{TrieDBMut, TrieDB}};
+use thiserror::Error;
 
 use self::wrapped_shard::WrappedShard;
 
@@ -39,35 +40,43 @@ mod wrapped_shard;
 const MAX_VALIDATORS: usize = <galois_16::Field as reed_solomon::Field>::ORDER;
 
 /// Errors in erasure coding.
-#[derive(Debug, Clone, PartialEq, derive_more::Display)]
+#[derive(Debug, Clone, PartialEq, Error)]
 pub enum Error {
 	/// Returned when there are too many validators.
+	#[error("There are too many validators")]
 	TooManyValidators,
 	/// Cannot encode something for no validators
+	#[error("Validator set is empty")]
 	EmptyValidators,
 	/// Cannot reconstruct: wrong number of validators.
+	#[error("Validator count mismatches between encoding and decoding")]
 	WrongValidatorCount,
 	/// Not enough chunks present.
+	#[error("Not enough chunks to reconstruct message")]
 	NotEnoughChunks,
 	/// Too many chunks present.
+	#[error("Too many chunks present")]
 	TooManyChunks,
 	/// Chunks not of uniform length or the chunks are empty.
+	#[error("Chunks are not unform, mismatch in length or are zero sized")]
 	NonUniformChunks,
 	/// An uneven byte-length of a shard is not valid for GF(2^16) encoding.
+	#[error("Uneven length is not valid for field GF(2^16)")]
 	UnevenLength,
 	/// Chunk index out of bounds.
-	#[display(fmt = "Chunk is out of bounds: {} {}", _0, _1)]
-	ChunkIndexOutOfBounds(usize, usize),
+	#[error("Chunk is out of bounds: {chunk_index} not included in 0..{n_validators}")]
+	ChunkIndexOutOfBounds{ chunk_index: usize, n_validators: usize },
 	/// Bad payload in reconstructed bytes.
+	#[error("Reconstructed payload invalid")]
 	BadPayload,
 	/// Invalid branch proof.
+	#[error("Invalid branch proof")]
 	InvalidBranchProof,
 	/// Branch out of bounds.
+	#[error("Branch is out of bounds")]
 	BranchOutOfBounds,
 }
 
-impl std::error::Error for Error { }
-
 #[derive(Debug, PartialEq)]
 struct CodeParams {
 	data_shards: usize,
@@ -206,7 +215,7 @@ fn reconstruct<'a, I: 'a, T: Decode>(n_validators: usize, chunks: I) -> Result<T
 	let mut shard_len = None;
 	for (chunk_data, chunk_idx) in chunks.into_iter().take(n_validators) {
 		if chunk_idx >= n_validators {
-			return Err(Error::ChunkIndexOutOfBounds(chunk_idx, n_validators));
+			return Err(Error::ChunkIndexOutOfBounds{ chunk_index: chunk_idx, n_validators });
 		}
 
 		let shard_len = shard_len.get_or_insert_with(|| chunk_data.len());