Propose "unsafe fields" as a 2025H1 Project Goal

[jswrenn]

Relevant Links:

• RFC 3458
• Living Design Doc
• Zulip Topic

[nikomatsakis]

This looks good, I'm pruning out some of the "team asks" because I think you don't need them actually.

[joshtriplett]

What does this mean, in practice?

[jswrenn]

For example: Using offset_of has no impact on the safety invariants of its given field. Requiring unsafe { ... } to compute the offset of an unsafe field would be pure noise in an unsafe code review, and, as such, we should make an effort to not require unsafe { ... } in this case.

Unlike the first and second tenet, this tenet is a soft requirement. We shouldn't ignore it entirely in our design work, but we don't actually have to implement any of these safe carve-outs initially since it's forwards-compatible to make an unsafe operation safe (so long as doing so doesn't violate existing safety conditions).