Lift TypeFoldable over Result and remove the Relate hack in combine/nll

[arielb1]

No functional changes intended - just a cleanup.

r? @nikomatsakis

[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed on Travis (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.

travis_time:end:11f30564:start=1549751305892799430,finish=1549751306856293444,duration=963494014
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-6.0
---

[00:03:51] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:03:51] tidy error: /checkout/src/librustc/traits/query/normalize.rs:194: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/traits/project.rs:398: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/traits/structural_impls.rs:751: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/traits/structural_impls.rs:761: TODO is deprecated; use FIXME
[00:03:51] tidy error: /checkout/src/librustc/traits/structural_impls.rs:885: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/traits/structural_impls.rs:901: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/traits/structural_impls.rs:942: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/traits/structural_impls.rs:963: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/infer/combine.rs:359: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/subst.rs:132: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/subst.rs:435: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/fold.rs:46: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/fold.rs:47: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/fold.rs:203: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/fold.rs:433: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/fold.rs:495: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/fold.rs:758: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/structural_impls.rs:599: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/structural_impls.rs:824: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/structural_impls.rs:971: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/ty/structural_impls.rs:1176: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/mir/mod.rs:2627: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/mir/mod.rs:3118: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/mir/mod.rs:3285: TODO is deprecated; use FIXME
[00:03:51] tidy error: /checkout/src/librustc/mir/mod.rs:3286: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/mir/mod.rs:3303: line longer than 100 chars
[00:03:51] tidy error: /checkout/src/librustc/mir/mod.rs:3392: line longer than 100 chars
[00:03:52] some tidy checks failed
[00:03:52] 
[00:03:52] 
[00:03:52] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:03:52] 
[00:03:52] 
[00:03:52] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:03:52] Build completed unsuccessfully in 0:00:44
[00:03:52] Build completed unsuccessfully in 0:00:44
[00:03:52] make: *** [tidy] Error 1
[00:03:52] Makefile:68: recipe for target 'tidy' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0ea87945
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Sat Feb  9 22:32:30 UTC 2019
---
travis_time:end:07946775:start=1549751551610346988,finish=1549751551614784069,duration=4437081
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:06031398
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:0006d81b
travis_time:start:0006d81b
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:248c4f54
$ dmesg | grep -i kill

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)

[bors]

☔️ The latest upstream changes (presumably #58389) made this pull request unmergeable. Please resolve the merge conflicts.

[arielb1]

I'll check that this doesn't hurt perf

@bors try

[bors]

⌛️ Trying commit 7abc7ed with merge 258c318...

[nikomatsakis]

r=me

[nikomatsakis]

r=me

[nikomatsakis]

Nit: blank line in the middle of a /// comment? Seems unusual

[nikomatsakis]

This seems ok, but also error-prone. i.e., if somebody forgets to make use_variances be true, we'll have a problem. It might be worth thinking if we can turn that into a dynamic assertion or something. But I don't obviously see how to do that with this setup as in this PR -- I was imagining that if the variance was tracked in a field, then accessing it could assert that use_variances is true or something...

[arielb1]

I could make variances an Option, but t hen the users would need to handle that. I think that forgetting to use use_variances would cause fairly visible errors, however, so I don't think this is a large risk.

[nikomatsakis]

So, in the branches that I was working on, I made the folder track the ambient variance iirc. I don't however remember exactly how I did this =)

[nikomatsakis]

It's not a thing that Rust handles particularly well

[bors]

☀️ Test successful - checks-travis
State: approved= try=True

[arielb1]

@rust-timer build 258c318

[rust-timer]

Insufficient permissions to issue commands to rust-timer.

[arielb1]

Could someone with perfbot permissions give me a rust-timer build?

[oli-obk]

@rust-timer build 258c318

[rust-timer]

Success: Queued 258c318 with parent e544947, comparison URL.

[arielb1]

I'm poking with the fairly hot TypeFoldable, so I want to see whether I have any weird perf impacts.

[arielb1]

@bors try

[bors]

⌛️ Trying commit 090f21e with merge c5e0b61...

[arielb1]

Bad github, deleting my test results like that, now I need another try

[rust-timer]

Finished benchmarking try commit 258c318

[arielb1]

Serde is very red in this PR (24%!). Need to investigate why.

[bors]

☀️ Test successful - checks-travis
State: approved= try=True

[Mark-Simulacrum]

@rust-timer build c5e0b61

[rust-timer]

Success: Queued c5e0b61 with parent eac0908, comparison URL.

[rust-timer]

Finished benchmarking try commit c5e0b61