Tidy up Rust RoomVersion structs

[erikjohnston]

This is in prep for using the room versions more from Rust.

Main changes:

• Change it so each room version is defined as a delta to the last one. This is a cosmetic change that makes it easier to ensure the room version definitions are correct (as they're defined as deltas from previous versions).
• Move constants to RoomVersion constants, like RoomVersion::V1, for convenience.
• Change visibility of various attributes.

[Copilot]

Pull request overview

This PR refactors Synapse’s Rust-side room version definitions (RoomVersion) to make them easier to maintain and more convenient to consume from Rust/PyO3, ahead of increased Rust usage of room versions.

Changes:

• Reworks room version definitions so each version is expressed as a delta from the previous version via associated RoomVersion::{V1..} constants.
• Promotes various version constants to pub and updates mapping accessors to use Clone instead of Copy.
• Adds Display for RoomVersion and a FromStr parser returning references to known room versions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
rust/src/room_versions.rs	Refactors RoomVersion constants into associated consts, adds parsing/formatting traits, and updates mapping access patterns.
changelog.d/19766.misc	Adds a changelog entry for the Rust room version refactor.

Comments suppressed due to low confidence (2)

rust/src/room_versions.rs:448

• items() uses self.versions.read().unwrap(), which can panic on a poisoned lock and bring down the Python interpreter. Please handle the poisoning case and return a PyRuntimeError, consistent with add_room_version/__len__.

        let versions = self.versions.read().unwrap();
        Ok(versions.iter().map(|v| (v.identifier, v.clone())).collect())
    }

rust/src/room_versions.rs:466

• get() uses self.versions.read().unwrap(), which can panic if the lock is poisoned (crashing the process). Consider using read().map_err(|_| PyRuntimeError::new_err(...))? here as well for consistency and safety.

        let versions = self.versions.read().unwrap();
        if let Some(version) = versions.iter().find(|v| v.identifier == key) {
            return Ok(Some(version.clone().into_bound_py_any(py)?));
        }

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[MadLittleMods]

MSC4242V12 supposed to be in this list? 🤷 I guess it wasn't there before

Overall, I haven't gone through everything with a fine tooth comb to spot this kind of thing if it happened elsewhere but this stood out to me.

[erikjohnston]

Yeah, we only add it in to that mapping if the experimental config is set:

synapse/synapse/config/experimental.py

Lines 479 to 483 in ff0420a

	# MSC4242: State DAGs
	self.msc4242_enabled: bool = experimental.get("msc4242_enabled", False)
	if self.msc4242_enabled:
	# Enable the room version
	KNOWN_ROOM_VERSIONS.add_room_version(RoomVersions.MSC4242v12)