Improve memoization and refactor NLL type check

[nikomatsakis]

I have a big branch that is refactoring NLL type check with the goal of introducing canonicalization-based memoization for all of the operations it does. This PR contains an initial prefix of that branch which, I believe, stands alone. It does introduce a few smaller optimizations of its own:

• Skip operations that are trivially a no-op
• Cache the results of the dropck-outlives computations done by liveness
• Skip resetting unifications if nothing changed

r? @pnkfelix

[nikomatsakis]

@bors try -- might be nice to do a perf run while we wait -- but I wouldn't block on it if @pnkfelix gets around to doing a review sooner

[bors]

⌛ Trying commit 2e25bed with merge 757cd05...

[Mark-Simulacrum]

Perf queued.

[bors]

💥 Test timed out

[kennytm]

Perf result is great for those that can be compiled. The timings for futures and serde are missing though.

http://perf.rust-lang.org/compare.html?start=61d88318aa66669fba061e9af529365172d63cd0&end=757cd050fc1ef84d7235d6f4d9228189eed878cc&stat=instructions%3Au

[Mark-Simulacrum]

Compilation failures for serde/futures:

Serde

thread 'main' panicked at 'region_obligations not empty: [
    (
        NodeId(
            18904
        ),
        RegionObligation(sub_region='_#16r, sup_type=M)
    ),
    (
        NodeId(
            18904
        ),
        RegionObligation(sub_region='_#17r, sup_type=M)
    ),
    (
        NodeId(
            18904
        ),
        RegionObligation(sub_region='_#18r, sup_type=M)
    ),
    (
        NodeId(
            18904
        ),
        RegionObligation(sub_region='_#19r, sup_type=M)
    ),
    (
        NodeId(
            18904
        ),
        RegionObligation(sub_region='_#20r, sup_type=M)
    ),
    (
        NodeId(
            18904
        ),
        RegionObligation(sub_region='_#21r, sup_type=M)
    ),
    (
        NodeId(
            18904
        ),
        RegionObligation(sub_region='_#22r, sup_type=M)
    ),
    (
        NodeId(
            18904
        ),
        RegionObligation(sub_region='_#23r, sup_type=M)
    ),
    (
        NodeId(
            18904
        ),
        RegionObligation(sub_region='_#24r, sup_type=M)
    )
]', librustc/infer/mod.rs:1057:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.28.0-nightly (757cd050f 2018-06-09) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z borrowck=mir -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

thread 'main' panicked at 'assertion failed: cmd.status().expect("failed to spawn").success()', collector/src/bin/rustc-fake.rs:33:17

futures

thread 'main' panicked at 'region_obligations not empty: [
    (
        NodeId(
            3805
        ),
        RegionObligation(sub_region=ReStatic, sup_type=U)
    ),
    (
        NodeId(
            3805
        ),
        RegionObligation(sub_region=ReStatic, sup_type=U)
    )
]', librustc/infer/mod.rs:1057:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.28.0-nightly (757cd050f 2018-06-09) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z borrowck=mir -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

[nikomatsakis]

Interesting. Thanks!

[pnkfelix]

(FYI I plan to start my review on Tuesday June 12th)

[nikomatsakis]

Minimized failure:

struct Map<A, F> where A: Future {
    a: A,
    f: F,
}

trait Future {
    type Item;
    type Error;

    fn tailcall(&mut self) -> Option<Box<Future<Item=Self::Item, Error=Self::Error>>>;
}

impl<A, F, U> Future for Map<A, F>
where
    A: Future,
    F: FnOnce(A::Item) -> U + Send + 'static,
    U: Send + 'static,
{
    type Item = U;
    type Error = A::Error;

    fn tailcall(&mut self)
                -> Option<Box<Future<Item=Self::Item, Error=Self::Error>>> {
        None
    }
}

fn main() {}

I found the bug, it is a pre-existing weirdness in the NLL checker that I have to think about the best way to fix. =)

[pnkfelix]

just from skimming 956e2f8 I'll just say it would be nice if the rustfmt runs were isolated to their own commits...)

((I am now experimenting with "ignore whitespace changes" to try to deal with this...))

(((oh, apparently there are commits that are factored out rustfmt runs. maybe 956e2f8 is just the result of IDE auto-indent...)))

[pnkfelix]

The PR seems fine from what I saw. (Obviously I must have missed something since there is the aforementioned bug that niko is working on.)

[pnkfelix]

@bors r+

[bors]

📌 Commit 2e25bed has been approved by pnkfelix

[bors]

⌛ Testing commit 2e25bed with merge b36917b...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: pnkfelix
Pushing b36917b to master...

[nnethercote]

This gave some big perf wins for NLL perf, but also lots of smaller improvements for non-NLL builds:
http://perf.rust-lang.org/compare.html?start=862703e05e275d77b0b594bb5d1a26a6340933f2&end=b36917b33104dc0af4d26d53899b7cd064a40b22&stat=instructions:u

Nice work.

cc @rust-lang/wg-compiler-performance