Improve error message when deserializing invalid chain config

Error messages when deserializing an invalid chain config have gotten
a lot worse since #3787.

For example, when mistyping the `gas_multiplier` field as `gas_multiplie` in the per-chain config:

```
data did not match any variant of untagged enum ChainConfig for key `chains` at line 424 column 1
```

After this commit (same message as before #3787):

```
invalid CosmosSdk config: unknown field `gas_multiplie`, expected one of `id`, `rpc_addr`, `grpc_addr`, `event_source`,
`rpc_timeout`, `trusted_node`, `account_prefix`, `key_name`, `key_store_type`, `key_store_folder`, `store_prefix`,
`default_gas`, `max_gas`, `genesis_restart`, `gas_adjustment`, `gas_multiplier`, `fee_granter`, `max_msg_num`,
`max_tx_size`, `max_grpc_decoding_size`, `query_packets_chunk_size`, `clock_drift`, `max_block_time`, `trusting_period`,
`client_refresh_rate`, `ccv_consumer_chain`, `memo_prefix`, `sequential_batch_tx`, `proof_specs`, `trust_threshold`,
`gas_price`, `packet_filter`, `address_type`, `extension_options`, `compat_mode`, `clear_interval`
for key `chains` at line 424 column 1
```

For this, we now use a custom deserializer for ChainConfig instead of
relying on an untagged enum + `monostate::MustBe`.

crates/relayer-cli/src/chain_registry.rs

@@ -121,7 +121,6 @@ where
     };
 
     Ok(ChainConfig::CosmosSdk(CosmosSdkConfig {
-        r#type: Default::default(),
         id: chain_data.chain_id,
         rpc_addr: rpc_data.rpc_address,
         grpc_addr: grpc_address,

crates/relayer/Cargo.toml

@@ -70,7 +70,6 @@ strum = { version = "0.25", features = ["derive"] }
 tokio-stream = "0.1.14"
 once_cell = "1.19.0"
 tracing-subscriber = { version = "0.3.14", features = ["fmt", "env-filter", "json"] }
-monostate = "0.1.11"
 
 [dependencies.byte-unit]
 version = "4.0.19"

crates/relayer/src/chain/cosmos/config.rs

@@ -2,7 +2,6 @@ use core::time::Duration;
 use std::path::PathBuf;
 
 use byte_unit::Byte;
-use monostate::MustBe;
 use serde_derive::{Deserialize, Serialize};
 use tendermint_rpc::Url;
 
@@ -24,11 +23,6 @@ pub mod error;
 #[derive(Clone, Debug, PartialEq, Deserialize, Serialize)]
 #[serde(deny_unknown_fields)]
 pub struct CosmosSdkConfig {
-    /// The type of this chain, must be "CosmosSdk"
-    /// This is the default if not specified.
-    #[serde(default)]
-    pub r#type: MustBe!("CosmosSdk"),
-
     /// The chain's network identifier
     pub id: ChainId,
 

crates/relayer/src/config.rs

@@ -618,18 +618,8 @@ pub enum EventSourceMode {
     },
 }
 
-// NOTE:
-// To work around a limitation of serde, which does not allow
-// to specify a default variant if not tag is present,
-// every underlying chain config MUST have a field `r#type` of
-// type `monotstate::MustBe!("VariantName")`.
-//
-// For chains other than CosmosSdk, this field MUST NOT be annotated
-// with `#[serde(default)]`.
-//
-// See https://github.com/serde-rs/serde/issues/2231
-#[derive(Clone, Debug, PartialEq, Deserialize, Serialize)]
-#[serde(untagged)]
+#[derive(Clone, Debug, PartialEq, Serialize)]
+#[serde(tag = "type")]
 pub enum ChainConfig {
     CosmosSdk(CosmosSdkConfig),
 }
@@ -704,6 +694,34 @@ impl ChainConfig {
     }
 }
 
+impl<'de> Deserialize<'de> for ChainConfig {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let value = toml::Value::deserialize(deserializer)?;
+
+        let type_value = value
+            .get("type")
+            .cloned()
+            .unwrap_or_else(|| toml::Value::String("CosmosSdk".to_string()));
+
+        let type_str = type_value
+            .as_str()
+            .ok_or_else(|| serde::de::Error::custom("invalid chain type, must be a string"))?;
+
+        match type_str {
+            "CosmosSdk" => CosmosSdkConfig::deserialize(value)
+                .map(Self::CosmosSdk)
+                .map_err(|e| serde::de::Error::custom(format!("invalid CosmosSdk config: {e}"))),
+
+            chain_type => Err(serde::de::Error::custom(format!(
+                "unknown chain type: {chain_type}",
+            ))),
+        }
+    }
+}
+
 /// Attempt to load and parse the TOML config file as a `Config`.
 pub fn load(path: impl AsRef<Path>) -> Result<Config, Error> {
     let config_toml = std::fs::read_to_string(&path).map_err(Error::io)?;
@@ -779,7 +797,6 @@ mod tests {
 
     use super::{load, parse_gas_prices, store_writer};
     use crate::config::GasPrice;
-    use monostate::MustBe;
     use test_log::test;
 
     #[test]
@@ -850,8 +867,8 @@ mod tests {
         let config = load(path).expect("could not parse config");
 
         match config.chains[0] {
-            super::ChainConfig::CosmosSdk(ref config) => {
-                assert_eq!(config.r#type, MustBe!("CosmosSdk"));
+            super::ChainConfig::CosmosSdk(_) => {
+                // all good
             }
         }
     }

tools/test-framework/src/types/single/node.rs

@@ -155,7 +155,6 @@ impl FullNode {
         };
 
         Ok(config::ChainConfig::CosmosSdk(CosmosSdkConfig {
-            r#type: Default::default(),
             id: self.chain_driver.chain_id.clone(),
             rpc_addr: Url::from_str(&self.chain_driver.rpc_address())?,
             grpc_addr: Url::from_str(&self.chain_driver.grpc_address())?,