Add UnwindTable::into_current_row

[koute]

This will make it possible to drop the UnwindTable and return a reference to UnwindTableRow whose lifetime will be bound only to the lifetime of the UninitializedUnwindContext. This is still not perfect though since the UnwindTable constructor originally accepts the unwind context through a &mut, so you still won't be able to e.g. search through multiple sections and return a reference to UnwindTableRow for only one of them due to the limitations of Rust's current borrow checker without laundering the lifetime through unsafe, but it's better than what we have now.

(Some background on this change: I've been refactoring this horrible method from my profiler, and what I'd like to do is basically to look through multiple sections, and return the UnwindTableRow when I get a hit. I could just clone it, but it's over 6 kilobytes so if the compiler won't be able to elide that copy the performance will tank.)

An alternative to this would be to not store the UnwindContext inside of UnwindTable and just pass the unwind context manually each time we iterate over the rows instead of setting it once in the constructor. That would obviously be a more intrusive change than this one though, so I'm not sure if we actually want to go that way? (Especially since the new Polonius-based borrow checker fixes the issue I've described in the first paragraph.)

[philipc]

An alternative to this would be to not store the UnwindContext inside of UnwindTable and just pass the unwind context manually each time we iterate over the rows instead of setting it once in the constructor.

Does this even avoid the lifetime problem? I experimented to get code like this in unwind_info_for_address:

        while let Some(row) = table.next_row(ctx)? {
            if row.contains(address) {
                return Ok(row);
            }
        }

but that gave a lifetime error.

I think the ideal API would be for next_row to return a reference with the context lifetime, but I'm not sure that's possible. Otherwise I think what you've done in this PR is fine. It would be good to change unwind_info_for_address to use this too though, since currently it is cloning the row.

[koute]

Does this even avoid the lifetime problem? I experimented to get code like this in unwind_info_for_address:

Currently it doesn't, mostly due to the borrow checker limitations that won't be fixed until Polonius lands. (If you turn on -Zpolonius it will compile.) But by splitting it like that you could call ctx.row() directly after advancing the row since the ctx wouldn't be stored inside of the UnwindTable anymore.

I think the ideal API would be for next_row to return a reference with the context lifetime, but I'm not sure that's possible.

Yeah, I don't think that's currently possible.

Otherwise I think what you've done in this PR is fine. It would be good to change unwind_info_for_address to use this too though, since currently it is cloning the row.

Sure, I'll change it.

[koute]

Done.

[philipc]

Thanks!