swarm/src/protocols_handler: Make ProtocolsHandlerEvent depend on `ProtocolsHandler only

[mxinden]

Today ProtocolsHandlerEvent depends on 4 trait parameters.

rust-libp2p/swarm/src/protocols_handler.rs

Lines 311 to 324 in b6c82a4

	/// Event produced by a handler.
	#[derive(Debug, Copy, Clone, PartialEq, Eq)]
	pub enum ProtocolsHandlerEvent<TConnectionUpgrade, TOutboundOpenInfo, TCustom, TErr> {
	/// Request a new outbound substream to be opened with the remote.
	OutboundSubstreamRequest {
	/// The protocol(s) to apply on the substream.
	protocol: SubstreamProtocol<TConnectionUpgrade, TOutboundOpenInfo>,
	},
	/// Close the connection for the given reason.
	Close(TErr),
	/// Other event.
	Custom(TCustom),

I think it is worth exploring simplifications, e.g. having ProtocolsHandlerEvent depend on a ProtocolsHandler implementation only:

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum ProtocolsHandlerEvent<THandler: ProtocolsHandler> {
    /// Request a new outbound substream to be opened with the remote.
    OutboundSubstreamRequest {
        /// The protocol(s) to apply on the substream.
        protocol: SubstreamProtocol<THandler::OutboundProtocol, THandler::OutboundOpenInfo>,
    },

    /// Close the connection for the given reason.
    Close(THandler::Errror),

    /// Other event.
    Custom(THandler::OutEvent),
}

Note that this might clash with the various ProtocolsHandlerEvent::map_xxx methods, as they assume being able to map individual trait parameters.

Proposed by @elenaf9 in #2059 (comment).

[drHuangMHT]

Update: ProtocolHandler has been renamed to ConnectionHandler.
Just attempted to do this but the compiler denied an inobvious equality: associated types of ToggleConnectionHandler<TInner> and associated types of TInner. This is expected, because the wrapper and inner are different after all. But this makes it impossible to remove all generic parameters of ConnectionHandlerEvent without removing ToggleConnectionHandler.

[thomaseizinger]

Three of these trait parameters will go away eventually through other refactorings:

• Close: swarm: Remove ConnectionHandler::Error #3591
• TOutboundOpenInfo: swarm: Deprecate ConnectionHandler::{In,Out}boundOpenInfo #3268
• TConnectionUpgrade: Simplify ConnectionHandler trait by removing as many associated types as possible #2863

[thomaseizinger]

Closing this as a result.