Add Never type to avoid ibc_packet_receive errors

CHANGELOG.md

@@ -25,11 +25,16 @@ and this project adheres to
 - cosmwasm-std: Implement `AsRef<[u8]>` for `Binary` and `HexBinary` ([#1550]).
 - cosmwasm-std: Allow constructing `SupplyResponse` via a `Default`
   implementation ([#1552], [#1560]).
+- cosmwasm-std: Add `Never` type which cannot be instantiated. This can be used
+  as the error type for `ibc_packet_receive` or `ibc_packet_ack` to gain
+  confidence that the implementations never errors and the transaction does not
+  get reverted. ([#1513])
 
 [#1436]: https://github.com/CosmWasm/cosmwasm/issues/1436
 [#1437]: https://github.com/CosmWasm/cosmwasm/issues/1437
 [#1481]: https://github.com/CosmWasm/cosmwasm/pull/1481
 [#1478]: https://github.com/CosmWasm/cosmwasm/pull/1478
+[#1513]: https://github.com/CosmWasm/cosmwasm/pull/1513
 [#1533]: https://github.com/CosmWasm/cosmwasm/pull/1533
 [#1550]: https://github.com/CosmWasm/cosmwasm/issues/1550
 [#1552]: https://github.com/CosmWasm/cosmwasm/pull/1552

IBC.md

@@ -264,7 +264,9 @@ pub fn ibc_packet_receive(
     deps: DepsMut,
     env: Env,
     msg: IbcPacketReceiveMsg,
-) -> StdResult<IbcReceiveResponse> { }
+) -> Result<IbcReceiveResponse, Never> {
+    // ...
+}
 ```
 
 This is a very special entry point as it has a unique workflow. (Please see the
@@ -350,27 +352,33 @@ produced 3 suggestions on how to handle errors and rollbacks _inside
    [main dispatch loop in `ibc-reflect`](https://github.com/CosmWasm/cosmwasm/blob/cd784cd1148ee395574f3e564f102d0d7b5adcc3/contracts/ibc-reflect/src/contract.rs#L217-L248):
 
    ```rust
-    (|| {
-        // which local channel did this packet come on
-        let caller = packet.dest.channel_id;
-        let msg: PacketMsg = from_slice(&packet.data)?;
-        match msg {
-            PacketMsg::Dispatch { msgs } => receive_dispatch(deps, caller, msgs),
-            PacketMsg::WhoAmI {} => receive_who_am_i(deps, caller),
-            PacketMsg::Balances {} => receive_balances(deps, caller),
-        }
-    })()
-    .or_else(|e| {
-        // we try to capture all app-level errors and convert them into
-        // acknowledgement packets that contain an error code.
-        let acknowledgement = encode_ibc_error(format!("invalid packet: {}", e));
-        Ok(IbcReceiveResponse {
-            acknowledgement,
-            submessages: vec![],
-            messages: vec![],
-            attributes: vec![],
-        })
-    })
+   pub fn ibc_packet_receive(
+       deps: DepsMut,
+       _env: Env,
+       msg: IbcPacketReceiveMsg,
+   ) -> Result<IbcReceiveResponse, Never> {
+       (|| {
+           // which local channel did this packet come on
+           let caller = packet.dest.channel_id;
+           let msg: PacketMsg = from_slice(&packet.data)?;
+           match msg {
+               PacketMsg::Dispatch { msgs } => receive_dispatch(deps, caller, msgs),
+               PacketMsg::WhoAmI {} => receive_who_am_i(deps, caller),
+               PacketMsg::Balances {} => receive_balances(deps, caller),
+           }
+       })()
+       .or_else(|e| {
+           // we try to capture all app-level errors and convert them into
+           // acknowledgement packets that contain an error code.
+           let acknowledgement = encode_ibc_error(format!("invalid packet: {}", e));
+           Ok(IbcReceiveResponse {
+               acknowledgement,
+               submessages: vec![],
+               messages: vec![],
+               attributes: vec![],
+           })
+       })
+   }
    ```
 
 2. If we modify state with an external call, we need to wrap it in a

contracts/ibc-reflect-send/src/ibc.rs

@@ -1,7 +1,7 @@
 use cosmwasm_std::{
     entry_point, from_slice, to_binary, DepsMut, Env, IbcBasicResponse, IbcChannelCloseMsg,
     IbcChannelConnectMsg, IbcChannelOpenMsg, IbcMsg, IbcOrder, IbcPacketAckMsg,
-    IbcPacketReceiveMsg, IbcPacketTimeoutMsg, IbcReceiveResponse, StdError, StdResult,
+    IbcPacketReceiveMsg, IbcPacketTimeoutMsg, IbcReceiveResponse, Never, StdError, StdResult,
 };
 
 use crate::ibc_msg::{
@@ -95,7 +95,7 @@ pub fn ibc_packet_receive(
     _deps: DepsMut,
     _env: Env,
     _packet: IbcPacketReceiveMsg,
-) -> StdResult<IbcReceiveResponse> {
+) -> Result<IbcReceiveResponse, Never> {
     Ok(IbcReceiveResponse::new()
         .set_ack(b"{}")
         .add_attribute("action", "ibc_packet_ack"))

contracts/ibc-reflect/src/contract.rs

@@ -2,7 +2,7 @@ use cosmwasm_std::{
     entry_point, from_slice, to_binary, wasm_execute, BankMsg, Binary, CosmosMsg, Deps, DepsMut,
     Empty, Env, Event, Ibc3ChannelOpenResponse, IbcBasicResponse, IbcChannelCloseMsg,
     IbcChannelConnectMsg, IbcChannelOpenMsg, IbcChannelOpenResponse, IbcOrder, IbcPacketAckMsg,
-    IbcPacketReceiveMsg, IbcPacketTimeoutMsg, IbcReceiveResponse, MessageInfo, Order,
+    IbcPacketReceiveMsg, IbcPacketTimeoutMsg, IbcReceiveResponse, MessageInfo, Never, Order,
     QueryResponse, Reply, Response, StdError, StdResult, SubMsg, SubMsgResponse, SubMsgResult,
     WasmMsg,
 };
@@ -233,7 +233,7 @@ pub fn ibc_packet_receive(
     deps: DepsMut,
     _env: Env,
     msg: IbcPacketReceiveMsg,
-) -> StdResult<IbcReceiveResponse> {
+) -> Result<IbcReceiveResponse, Never> {
     // put this in a closure so we can convert all error responses into acknowledgements
     (|| {
         let packet = msg.packet;

packages/std/src/lib.rs

@@ -15,6 +15,7 @@ mod import_helpers;
 #[cfg(feature = "iterator")]
 mod iterator;
 mod math;
+mod never;
 mod panic;
 mod query;
 mod results;
@@ -48,6 +49,7 @@ pub use crate::math::{
     Decimal, Decimal256, Decimal256RangeExceeded, DecimalRangeExceeded, Fraction, Isqrt, Uint128,
     Uint256, Uint512, Uint64,
 };
+pub use crate::never::Never;
 #[cfg(feature = "cosmwasm_1_1")]
 pub use crate::query::SupplyResponse;
 pub use crate::query::{

packages/std/src/never.rs

@@ -0,0 +1,27 @@
+/// Never can never be instantiated. This can be used in places
+/// where we want to ensure that no error is returned, such as
+/// the `ibc_packet_receive` entry point.
+///
+/// In contrast to `Empty`, this does not have a JSON schema
+/// and cannot be used for message and query types.
+///
+/// Once the ! type is stable, this is not needed anymore.
+/// See <https://github.com/rust-lang/rust/issues/35121>.
+pub enum Never {}
+
+// The Debug implementation is needed to allow the use of `Result::unwrap`.
+impl core::fmt::Debug for Never {
+    fn fmt(&self, _f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        // Unreachable because no instance of Never can exist
+        match *self {}
+    }
+}
+
+// The Display implementation is needed to fulfill the ToString requirement of
+// entry point errors: `Result<IbcReceiveResponse<C>, E>` with `E: ToString`.
+impl core::fmt::Display for Never {
+    fn fmt(&self, _f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        // Unreachable because no instance of Never can exist
+        match *self {}
+    }
+}