Optimize dropck

[Mark-Simulacrum]

This does two things: caches the trivial_dropck check by making it a query, and shifts around the implementation of the primary dropck itself to avoid allocating many small vectors.

[rust-highfive]

r? @eddyb

(rust_highfive has picked a reviewer for you, use r? to override)

[Mark-Simulacrum]

I spent some more time optimizing this and I have another commit locally (will wait on perf for first commit before pushing it) that further optimizes drop check in a pathological case to the point where drop check becomes much faster (i.e., completes to overflow).

However, that doesn't fully solve the problem as we then hit a hang when trying to print the types (somewhat unsurprisingly, these types are huge), I haven't quite figured out a good fix there.

This leads me to conclude that @pnkfelix's remarks on Zulip when discussing this (cc #4287) where apt -- we should try to come up with a check that detects this case more knowingly and errors out before we try to recurse down the chain of instantiations so we can provide better spans and such. I've not yet thought of a good way to do so, but am still thinking.

[rust-timer]

Finished benchmarking try commit c1c16e012812695bb3f7f84ad6c6ba53b1eeccbe, comparison URL.

[Mark-Simulacrum]

Hm, so instruction counts and such look like basically a wash -- this is somewhat expected, as we've not changed the algorithm here for most types -- however, it does show marked improvements for two test cases I examined. Both of these now complete dropck (into overflow) rather than being so slow in dropck that we don't reach overflow in reasonable time. Neither one actually completes in reasonable time due to this print which takes forever on such large types, but this PR still seems like a good thing to land (in particular, because it's not really hurting readability all that much IMO).

So I'm going to r? @pnkfelix here but feel free to reassign (we're not changing algorithm here at all so really anyone can review).

enum Perfect<T> {
    Tip(T),
    Fork(Box<Perfect<(T, T)>>),
}

fn main() {
    let _ = Perfect::Tip(Box::new(42));
}

enum Perfect<T> {
    Tip(T),
    Fork(Box<Perfect<(T, T)>>),
}

fn main() {
    let _ = Perfect::Tip(42);
}

[rust-timer]

Finished benchmarking try commit c1c16e012812695bb3f7f84ad6c6ba53b1eeccbe, comparison URL.

[pnkfelix]

I haven't fully brought all this back into my mental cache; can you say more here on why >= 1 is the right threshold?

If all you want to do is stop as soon as there is any overflow (which is how I interpret this), then fine. But the comment as written makes it sound like the threshold is a more interesting value, like 100 or something.

[pnkfelix]

(Anyway apart from that, I think these changes look fine.)

[Mark-Simulacrum]

This conditional is basically because with the new code we detect thousands (and then millions.. and so on) of overflows, eventually leading to an OOM on the Perfect enum; I initially thought maybe we should have some small number, like 10, here -- but then in thinking more, I had decided that chances are, the first overflow contains all the information you need, and there's no reason for us to spin further.

I can update this comment with that summary?

[Mark-Simulacrum]

Alright, I've updated the comment with some explanation for why we're comparing to 1 and not something larger -- I'm not sure if @pnkfelix meant #64595 (comment) as a r=me so I'll not approve this myself.

[rust-highfive]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

2019-09-27T01:53:26.6490387Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-09-27T01:53:26.6676458Z ##[command]git config gc.auto 0
2019-09-27T01:53:26.6743765Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-09-27T01:53:26.6802161Z ##[command]git config --get-all http.proxy
2019-09-27T01:53:26.6951297Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/64595/merge:refs/remotes/pull/64595/merge
---
2019-09-27T02:02:47.6411505Z     Checking rustc_traits v0.0.0 (/checkout/src/librustc_traits)
2019-09-27T02:02:47.8141259Z error[E0412]: cannot find type `Kind` in module `ty::subst`
2019-09-27T02:02:47.8141897Z    --> src/librustc_traits/dropck_outlives.rs:234:40
2019-09-27T02:02:47.8142141Z     |
2019-09-27T02:02:47.8142452Z 234 |                 .map(|t| -> ty::subst::Kind<'tcx> { t.into() }));
2019-09-27T02:02:47.8147382Z 
2019-09-27T02:02:48.3441328Z error: aborting due to previous error
2019-09-27T02:02:48.3446105Z 
2019-09-27T02:02:48.3457816Z For more information about this error, try `rustc --explain E0412`.
---
2019-09-27T02:02:59.6882533Z == clock drift check ==
2019-09-27T02:02:59.6902885Z   local time: Fri Sep 27 02:02:59 UTC 2019
2019-09-27T02:02:59.8409457Z   network time: Fri, 27 Sep 2019 02:02:59 GMT
2019-09-27T02:02:59.8410205Z == end clock drift check ==
2019-09-27T02:03:01.0456996Z ##[error]Bash exited with code '1'.
2019-09-27T02:03:01.0513182Z ##[section]Starting: Checkout
2019-09-27T02:03:01.0515208Z ==============================================================================
2019-09-27T02:03:01.0515388Z Task         : Get sources
2019-09-27T02:03:01.0515450Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[Mark-Simulacrum]

Looks like CI broke due to changes on master, rebased -- should be fixed now.

[bors]

☔ The latest upstream changes (presumably #64864) made this pull request unmergeable. Please resolve the merge conflicts.

[JohnCSimon]

Ping from triage.
@Mark-Simulacrum Can you please resolve the merge conflicts?
CC @pnkfelix
Thank you.

[JohnCSimon]

Pinging again from triage.
@Mark-Simulacrum Can you please resolve the merge conflicts?
CC @pnkfelix
Thank you.

[pnkfelix]

@bors r+

[bors]

📌 Commit 8de7fd8 has been approved by pnkfelix

[bors]

⌛ Testing commit 8de7fd8 with merge b043380...

[bors]

☀️ Test successful - checks-azure
Approved by: pnkfelix
Pushing b043380 to master...