Remove migrate borrowck mode

[jackh726]

Closes #58781
Closes #43234

Stabilization proposal

This PR proposes the stabilization of #![feature(nll)] and the removal of -Z borrowck. Current borrow checking behavior of item bodies is currently done by first infering regions lexically and reporting any errors during HIR type checking. If there are any errors, then MIR borrowck (NLL) never occurs. If there aren't any errors, then MIR borrowck happens and any errors there would be reported. This PR removes the lexical region check of item bodies entirely and only uses MIR borrowck. Because MIR borrowck could never not be run for a compiled program, this should not break any programs. It does, however, change diagnostics significantly and allows a slightly larger set of programs to compile.

Tracking issue: #43234
RFC: https://github.com/rust-lang/rfcs/blob/master/text/2094-nll.md
Version: 1.63 (2022-06-30 => beta, 2022-08-11 => stable).

Motivation

Over time, the Rust borrow checker has become "smarter" and thus allowed more programs to compile. There have been three different implementations: AST borrowck, MIR borrowck, and polonius (well, in progress). Additionally, there is the "lexical region resolver", which (roughly) solves the constraints generated through HIR typeck. It is not a full borrow checker, but does emit some errors.

The AST borrowck was the original implementation of the borrow checker and was part of the initially stabilized Rust 1.0. In mid 2017, work began to implement the current MIR borrow checker and that effort ompleted by the end of 2017, for the most part. During 2018, efforts were made to migrate away from the AST borrow checker to the MIR borrow checker - eventually culminating into "migrate" mode - where HIR typeck with lexical region resolving following by MIR borrow checking - being active by default in the 2018 edition.

In early 2019, migrate mode was turned on by default in the 2015 edition as well, but with MIR borrowck errors emitted as warnings. By late 2019, these warnings were upgraded to full errors. This was followed by the complete removal of the AST borrow checker.

In the period since, various errors emitted by the MIR borrow checker have been improved to the point that they are mostly the same or better than those emitted by the lexical region resolver.

While there do remain some degradations in errors (tracked under the NLL-diagnostics tag, those are sufficiently small and rare enough that increased flexibility of MIR borrow check-only is now a worthwhile tradeoff.

What is stabilized

As said previously, this does not fundamentally change the landscape of accepted programs. However, there are a few cases where programs can compile under feature(nll), but not otherwise.

There are two notable patterns that are "fixed" by this stabilization. First, the scoped_threads feature, which is a continutation of a pre-1.0 API, can sometimes emit a weird lifetime error without NLL. Second, actually seen in the standard library. In the Extend impl for HashMap, there is an implied bound of K: 'a that is available with NLL on but not without - this is utilized in the impl.

As mentioned before, there are a large number of diagnostic differences. Most of them are better, but some are worse. None are serious or happen often enough to need to block this PR. The biggest change is the loss of error code for a number of lifetime errors in favor of more general "lifetime may not live long enough" error. While this may seem bad, the former error codes were just attempts to somewhat-arbitrarily bin together lifetime errors of the same type; however, on paper, they end up being roughly the same with roughly the same kinds of solutions.

What isn't stabilized

This PR does not completely remove the lexical region resolver. In the future, it may be possible to remove that (while still keeping HIR typeck) or to remove it together with HIR typeck.

Tests

Many test outputs get updated by this PR. However, there are number of tests specifically geared towards NLL under src/test/ui/nll

History

• On 2017-07-14, tracking issue opened
• On 2017-07-20, initial empty MIR pass added
• On 2017-08-29, RFC opened
• On 2017-11-16, Integrate MIR type-checker with NLL
• On 2017-12-20, NLL feature complete
• On 2018-07-07, Don't run AST borrowck on mir mode
• On 2018-07-27, Add migrate mode
• On 2019-04-22, Enable migrate mode on 2015 edition
• On 2019-08-26, Don't downgrade errors on 2015 edition
• On 2019-08-27, Remove AST borrowck

[bors]

☔ The latest upstream changes (presumably #95552) made this pull request unmergeable. Please resolve the merge conflicts.

[jackh726]

@bors try

[bors]

⌛ Trying commit 3a8985296e0cf16b2a428371561897ec678f359c with merge eb43a7af9abeacf703345e49daba222ddc0623e9...

[bors]

☀️ Try build successful - checks-actions
Build commit: eb43a7af9abeacf703345e49daba222ddc0623e9 (eb43a7af9abeacf703345e49daba222ddc0623e9)

[jackh726]

@cratebot check

[compiler-errors]

assuming jack meant

@craterbot check

[craterbot]

👌 Experiment pr-95565 created and queued.
🤖 Automatically detected try build eb43a7af9abeacf703345e49daba222ddc0623e9
🔍 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-95565 is now running

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

[craterbot]

🎉 Experiment pr-95565 is completed!
📊 454 regressed and 33 fixed (223389 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the blacklist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[jackh726]

I'm going to nominate for lang team. Particularly, I'd like to see the following discussed:

1. There are a number of nontrivial breakage found with crater. This is likely covered by NLL: complete transition from migrate to full NLL #57895. Are we willing to accept this breakage, or do we somehow need to engineer a way to get around it in older editions?
2. General temperature on stabilizing NLL.

(I'm going to try to add a couple more points this weekend to discuss).

[m-ou-se]

🎉 Experiment pr-95565 is completed!
📊 454 regressed and 33 fixed (223389 total)
📰 Open the full report.

160 of the 179 root regressions that craterbot found are cases where rustc previously already warned the user the code will stop working in the future:

warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future

The other 19 are:

_{(Found using $ for c in $(jq -r '.crates[] | select(.res == "regressed") | select(.runs[1].res | startswith("build-fail:depends") | not).runs[0].log | "https://crater-reports.s3.amazonaws.com/pr-95565/" + . + "/log.txt"' results.json); do curl -s $c | grep -q 'may become a hard error in the future' || echo $c; done)}

• 12 "cannot borrow as mutable because it is also borrowed as immutable":
  • reg/ffsend-0.2.74 before after
    • Explicitly supressed the warning using #[allow(mutable_borrow_reservation_conflict)] [source]
  • reg/notedown_ast-0.16.2 before after
    • Explicitly supressed the warning using #[allow(mutable_borrow_reservation_conflict)] [source]
  • reg/rx-editor-0.3.0 before after
    • Explicitly supressed the warning using #[allow(mutable_borrow_reservation_conflict)] [source]
  • reg/voltdb-client-rust-0.1.11 before after
    • Explicitly supressed the warning using #[allow(mutable_borrow_reservation_conflict)] [source]
  • reg/wire4-0.0.2 before after
    • Explicitly supressed the warning using #[allow(mutable_borrow_reservation_conflict)] [source]
  • gh/Belovedbb.Crlox before after
    • Explicitly supressed the warning using #[allow(mutable_borrow_reservation_conflict)] [source]
  • gh/GCYYfun.OSTEP_RUST before after
    • Explicitly supressed the warning using #[allow(mutable_borrow_reservation_conflict)] // 危险 [source]
  • gh/gjconklin1.advent2021 before after
    • Explicitly supressed the warning using #[allow(mutable_borrow_reservation_conflict)] [source]
  • gh/jumang4423.nylang before after
    • Explicitly supressed the warning using #[allow(mutable_borrow_reservation_conflict)] [source]
  • gh/laopo001.leetcode-rust before after
    • Explicitly supressed the warning using #[allow(mutable_borrow_reservation_conflict)] [source]
  • gh/mwillsey.ruler before after
    • Explicitly supressed the warning using #[allow(mutable_borrow_reservation_conflict)] [source]
  • gh/qinxiaoguang.rs-lc before after
    • Supressed the warning using #![allow(warnings)] [source]

7 spurious or unrelated:

• gh/Ayrx.binja-rs-hello-world before after
• gh/ababup1192.elm-init before after
• gh/etke.bff before after
• gh/lcdr.lu_packets before after
• gh/stuartZhang.scaffold-wizard before after
• reg/bitrust-0.1.0 before after
• reg/rttrust-0.1.3 before after