Allow lifetime elision in Pin<&(mut) Self>

[taiki-e]

This PR changes self: &(mut) S elision rules to instead visit the type of self and look for &(mut) S (where is_self_ty(S)) within it

Replaces #60944

Closes #52675

r? @eddyb
cc @cramertj @Centril @withoutboats @scottmcm

[taiki-e]

This is a breaking change as said by #60944 (comment) and #60944 (comment).

Then, as @cramertj said in #60944 (comment), a beta backport is probably preferable.

[Centril]

Overall I'm quite unsure about in what direction we should take this and I feel I have a lack of information about what the current rules are. I'd appreciate if someone would do a write-up of all the current rules wrt. lifetime elision.

Ideally, I think if we could get away with just special casing &mut? self without an explicitly written Self type then that would be conceptually simplest to understand. I would be interested in seeing a crater run for that, tho I suspect we cannot get away with it.

[taiki-e]

@Centril

Ideally, I think if we could get away with just special casing &mut? self without an explicitly written Self type then that would be conceptually simplest to understand. I would be interested in seeing a crater run for that, tho I suspect we cannot get away with it.

I also would be interested in seeing this. But what I want to do with this PR is to fix #52675. And I hope this will be beta backported to avoid the impact (see #60944 (comment)).

So, what I'm thinking is not to make big changes to the current rules with this PR, but to split that experiment into different PR (if that is not really necessary to fix #52675).

[Centril]

I also would be interested in seeing this. But what I want to do with this PR is to fix #52675. And I hope this will be beta backported to avoid the impact (see #60944 (comment)).

So, what I'm thinking is not to make big changes to the current rules with this PR, but to split that experiment into different PR (if that is not really necessary to fix #52675).

I'm confused by all of this. The current PR seems to permit more code taking us away from the simpler logic of only assigning special meaning to &self and &mut self arguments when written like so (i.e. not self: &Self). I don't see how we move towards accepting less by accepting more in the interim. Hmm... it seems like we are also rejecting some other code... could we try to not accept more code but strictly reject more code without a feature gate and then move the new logic into a feature gate?

[taiki-e]

@Centril
Oh, sorry, I think I said something weird.
Actually, my concern was not a problem, as we can have a warning period.

[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:02e4fdde:start=1558972063508831994,finish=1558972150928208358,duration=87419376364
$ 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
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
---
[00:07:55]    Compiling syntax_ext v0.0.0 (/checkout/src/libsyntax_ext)
[00:08:06] error[E0392]: parameter `F` is never used
[00:08:06]     --> src/librustc/middle/resolve_lifetime.rs:2122:36
[00:08:06]      |
[00:08:06] 2122 |             struct SelfVisitor<'a, F: FnMut(Res) -> bool> {
[00:08:06]      |                                    ^ unused type parameter
[00:08:06]      = help: consider removing `F` or using a marker such as `std::marker::PhantomData`
[00:08:06] 
[00:08:08] error: aborting due to previous error
[00:08:08] 
---
travis_time:end:10f91ec4:start=1558972661728973986,finish=1558972661733999277,duration=5025291
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:01a21a60
$ 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:021e08d7
travis_time:start:021e08d7
$ 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:0a73b77a
$ 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)

[eddyb]

I like how simple this is! But we should have a crater run, just to be sure:
@bors try

[bors]

⌛ Trying commit 914e4a3 with merge f939092...

[bors]

☀️ Try build successful - checks-travis
Build commit: f939092

[Centril]

@craterbot run mode=check-only

[craterbot]

👌 Experiment pr-61207 created and queued.
🤖 Automatically detected try build f939092
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-61207 is now running on agent aws-3-tmp.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[taiki-e]

@Centril Done!

[Centril]

@bors r+

[bors]

📌 Commit 05f67a2 has been approved by Centril