Disable legacy-arith by default in consensus/types

[macladson]

Issue Addressed

Currently, consensus/types cannot build with no-default-features since we use "legacy" standard arithmetic operations.

Proposed Changes

• Remove the offending arithmetic to fix compilation.
• Rename legacy-arith to saturating-arith and disable it by default.

Additional Info

We could consider removing support for standard arithmetic entirely, although I think having an opt-in feature which makes the dangers of enabling it clear is an okay solution for now, and won't break any external users (and our own state_processing crate).

Incidentally, the ergonomics around using explicit methods such as saturating_add with integer types (which is available even without saturating-arith) is a bit awkward, since it requires casting to u64 first.

[pawanjay176]

This LGTM, but it would be good if someone else can also take a look

[michaelsproul]

Should also remove the allow lint in this file

[macladson]

There are still some more #[allow(clippy::arithmetic_side_effects)] in beacon_state.rs and in validator.rs. Those seem a bit more involved to remove so I have left them for now

[pawanjay176]

Looks like this has been resolved except the ones in validator.rs?

[macladson]

There is still the SSZ one in beacon_state.rs:

lighthouse/consensus/types/src/state/beacon_state.rs

Lines 3173 to 3174 in 8d72cc3

	#[allow(clippy::arithmetic_side_effects)]
	pub fn from_ssz_bytes(bytes: &[u8], spec: &ChainSpec) -> Result<Self, ssz::DecodeError> {

[mergify]

This pull request has merge conflicts. Could you please resolve them @macladson? 🙏

[pawanjay176]

LGTM. I'm fine with merging without handling the more involved allow lints see #8695 (comment).
Will wait for ✅ from @michaelsproul

[pawanjay176]

Gonna go ahead and merge this one to avoid it going stale

[mergify]

Merge Queue Status

Rule: default

---

• ✅ Entered queue — 2026-02-12 20:21 UTC
• ✅ Checks passed · on draft merge queue: embarking unstable (96bc561) and [#8558 + #8695] together #8811
• ✅ Merged — 2026-02-12 20:51 UTC · at 6c135938057ec9f0b4028b4c32bee5765490f596

This pull request spent 30 minutes 9 seconds in the queue, including 29 minutes 10 seconds running CI.

Required conditions to merge

• check-success=local-testnet-success
• check-success=test-suite-success