ACP: `impl From<usize> for u{64, 128}`

Proposal

Problem statement

One point that's been mostly left out of the proposed integer conversion APIs (#811) has been usizes - which is unfortunate, because interconverting between them and fixed-size integer types is a relatively common task. Right now, we're left with only .try_into().unwrap() and the proposed checked cast.

Motivating examples or use cases

It's somewhat common for code to implement the same try-into + unwrap semantics as a helper function; heck, we even do it in Miri. Alternatively, not implementing such a helper can make code harder to parse and audit for possible panics due to the noise from many unwraps that can never trigger.

Solution sketch

Add the following impls, which we would (once we have 128-bit-usize targets) gate on the target and put under an allow-by-default lint:

impl From<usize> for u64 { ... }
impl From<usize> for u128 { ... }

Alternatives

The main obvious alternative is to not do this at all and force people to spell out the unwrap. Alternatively, we could also only offer the From<usize> for u128 impl given that 128-bit-address systems are beginning to exist in some limited capacities (e.g. a draft spec for 128-bit risc-v exists).

Links and related work

• Relevant Zulip thread
• The team meeting where we bikeshedded the integer conversions
• Current integer conversion proposals

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

Start by reading the linked current integer conversion proposals, Zulip discussion, and libs-team meeting notes to understand the unresolved target-gating and lint questions. Done means the libs-api team has reviewed and approved the API direction; implementation would follow in a subsequent change.