first draft of blog post about NLL transition

Author: nikomatsakis

posts/2019-10-28-nll-hard-errors.md

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+---
+layout: post
+title: "Completing the transition to the new borrow checker"
+author: Niko Matsakis
+---
+
+For most of 2018, we've been issue warnings about various bugs in the
+borrow checker that we plan to fix -- about two months ago, in the
+current Rust nightly, those warnings became **hard errors**. In about
+two weeks, when the nightly branches to become beta, those hard errors
+will be in the beta build, and they will eventually hit stable on
+December 19th, as part of Rust 1.40.0. **If you're testing with
+Nightly, you should be all set -- but otherwise, you may want to go
+and check to make sure your code still builds. If not, we have advice
+for fixing common problems below.**
+
+### Background: the non-lexical lifetime transition
+
+When we [released Rust 2018 in Rust 1.31][2018], it included a new
+version of the borrow checker, one that implemented ["non-lexical
+lifetimes"][nll]. This new borrow checker did a much more precise
+analysis than the original, allowing us to eliminate a lot of
+unnecessary errors and make Rust easier to use. I think most everyone
+who was using Rust 2015 can attest that this shift was a big
+improvement.
+
+### The new borrow checker also fixed a lot of bugs
+
+What is perhaps less well understood is that the new borrow checker
+implementation *also* fixed a lot of bugs. In other words, the new
+borrow checker did not just accept more programs -- **it also rejected
+some programs that never should have been accepted in the first
+place!**
+
+[2018]: https://blog.rust-lang.org/2018/12/06/Rust-1.31-and-rust-2018.html
+[nll]: https://blog.rust-lang.org/2018/12/06/Rust-1.31-and-rust-2018.html#non-lexical-lifetimes
+[MIR]: https://blog.rust-lang.org/2016/04/19/MIR.html
+
+### Until recently, those fixed bugs produced warnings, not errors
+
+As part of our commitment to stability, whenever we find bugs that
+impact existing code in a major way, we try to "phase in" those
+changes gradually. We usually begin with "Future Compatibility
+Warnings", for example, before moving those warnings to hard errors
+(sometimes a small bit at a time). Since the bug fixes to the borrow
+checker affected a lot of crates, we knew we needed a warning period
+before we could make them into hard errors.
+
+To implement this warning period, we kept two copies of the borrow
+checker around (this is a trick we use quite frequently, actually).
+The new checker ran first. If it found errors, we didn't report them
+directly: instead, we ran the old checker in order to see if the crate
+*used* to compile before. If so, we reported the errors as Future
+Compatibility Warnings, since we were changing something that used to
+compile into errors.
+
+### All good things must come to an end; and bad ones, too
+
+Over time we have been slowly transitioning those future compatibility
+warnings into errors, a bit at a time. About two months ago, we
+decided that the time had come to finish the job. So, over the course
+of two PRs, we [converted all remaining warnings to errors][a] and
+then [removed the old borrow checker implementation][b].
+
+[a]: https://github.com/rust-lang/rust/pull/63565
+[b]: https://github.com/rust-lang/rust/pull/64790
+
+### What this means for you
+
+**If you are testing your package with nightly, then you should be
+fine.** In fact, even if you build on stable, we always recommend that
+you test your builds in CI with the nightly build, so that you can
+identify upcoming issues early and report them to us.
+
+**Otherwise, you may want to check your dependencies.** When we
+decided to remove the old borrow checker, we also analyzed which
+crates would stop compiling. For anything that seemed to be widely
+used, we made sure that there were newer versions of that crate
+available that *do* compile (for the most part, this had all already
+happened during the warning period). But if you have those older
+versions in your `Cargo.lock` file, and you are only using stable
+builds, then you may find that your code no longer builds once 1.40.0
+is released -- you will have to upgrade the dependency.
+
+The most common crates that were affected are the following:
+
+* `url` version 1.7.0 -- you can upgrade to 1.7.2, though you'd be better off upgrading to 2.1.0
+* `nalgebra` version 0.16.13 -- you can upgrade to 0.16.14, though you'd be better off upgrading to 0.19.0
+
+You can find out which crates you rely upon using the [cargo-tree] command. If you find
+that you *do* rely (say) on `url` 1.7.0, you can upgrade to 1.7.2 by executing:
+
+```bash
+cargo update -p url --vers 1.7.2
+```
+
+[cargo-tree]: https://crates.io/crates/cargo-tree
+
+### Want to learn more?
+
+If you'd like to learn more about the kinds of bugs that were fixed --
+or if you are seeing errors in your code that you need to fix -- take
+a look at this [excellent blog post by Felix Klock][nllpost], which
+goes into great detail.
+
+[nllpost]: http://blog.pnkfx.org/blog/2019/06/26/breaking-news-non-lexical-lifetimes-arrives-for-everyone/