Less duplication in activate

[Eh2406]

One of the "background context" things I re-explained in #6919 (comment) is why we have checks in RemainingCandidates::next and in flag_activated. "Why the duplication? Because cloning a BacktrackFrame is more expensive than doing the work twice." But thanks to a lot of work, and im.rs, the clone is now not so bad.

This PR attempts to go all in on removing this duplication. Anything to simplify the resolver is good! But, this removes or hamstrings several important optimizations that make NOP builds fast. Specifically #5132 will kick in far less often. Alex, can you look at the times after/before this PR on some larger projects? You have seen this be slow with perf record ./x.py build before, but servo or cargo may be easy and interesting.

Note that Master is at "Fast but duplicated" and this PR may be at "Slow but clean", the best solution may be somewhere in between.

[rust-highfive]

r? @nrc

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[Eh2406]

This is waiting on someone with a fairly powerful and idle computer doing a comparison of time cargo generate-lockfile with a build of master vers a build with this PR on some large project like servo or like rustc. I don't have access to the compute and @alexcrichton appears not to have the time. But anyone is open to step in!

[ehuss]

Did some quick measurements on my laptop (2.6GHz i7), average of 10 runs, with --offline:

Project	master	6967
2000 crate stress test	2.127s	2.671s
rust-lang/rust	0.246s	0.270s
servo	0.620s	0.641s
cargo	0.106s	0.112s

[Eh2406]

Thanks for doing the work! Adding a % column,

Project	master	6967	%
2000 crate stress test	2.127s	2.671s	25%
rust-lang/rust	0.246s	0.270s	9%
servo	0.620s	0.641s	3%
cargo	0.106s	0.112s	5%

That is a pretty significant cost. :-(

[Eh2406]

@ehuss Can you share the 2000 crate stress test I'd love to experiment with that. If I don't find anything in that to change things, this should probably be changed to just adding a comment explaining the connection between RemainingCandidates::next and in flag_activated.

[ehuss]

Here you go: https://gist.github.com/ehuss/757952b83a239b37cb8811f90e45c6d6