add cleanup crew (#556)

* add cleanup crew

* update the text about carog-bisect-rustc etc

* Update src/ice-breaker/cleanup-crew.md

Co-Authored-By: lqd <remy.rakic+github@gmail.com>

* finish the 3rd section

* fix typo

* clarify when you need to find PR range

* add link to zulip stream

Co-authored-by: lqd <remy.rakic+github@gmail.com>

src/SUMMARY.md

@@ -30,6 +30,7 @@
         - [`LintStore`](./diagnostics/lintstore.md)
         - [Diagnostic Codes](./diagnostics/diagnostic-codes.md)
     - [ICE-breaker teams](ice-breaker/about.md)
+        - ["Cleanup Crew" ICE-breakers](ice-breaker/cleanup-crew.md)
         - [LLVM ICE-breakers](ice-breaker/llvm.md)
 - [Part 2: How rustc works](./part-2-intro.md)
     - [High-level overview of the compiler source](./high-level-overview.md)

src/ice-breaker/cleanup-crew.md

@@ -0,0 +1,114 @@
+# Cleanup Crew
+
+**Github Label:** [ICEBreaker-Cleanup-Crew]
+
+[ICEBreaker-Cleanup-Crew]: https://github.com/rust-lang/rust/labels/ICEBreaker-Cleanup-Crew
+
+The "Cleanup Crew" are focused on improving bug reports. Specifically,
+the goal is to try to ensure that every bug report has all the
+information that will be needed for someone to fix it:
+
+* a minimal, standalone example that shows the problem
+* links to duplicates or related bugs
+* if the bug is a regression (something that used to work, but no longer does),
+  then a bisection to the PR or nightly that caused the regression
+
+This kind of cleanup is invaluable in getting bugs fixed. Better
+still, it can be done by anybody who knows Rust, without any
+particularly deep knowledge of the compiler.
+
+Let's look a bit at the workflow for doing "cleanup crew" actions.
+
+## Finding a minimal, standalone example
+
+Here the ultimate goal is to produce an example that reproduces the same
+problem but without relying on any external crates. Such a test ought to contain
+as little code as possible, as well. This will make it much easier to isolate the problem.
+
+However, even if the "ultimate minimal test" cannot be achieved, it's
+still useful to post incremental minimizations. For example, if you
+can eliminate some of the external dependencies, that is helpful, and
+so forth. 
+
+It's particularly useful to reduce to an example that works
+in the [Rust playground](https://play.rust-lang.org/), rather than
+requiring people to checkout a cargo build.
+
+There are many resources for how to produce minimized test cases. Here
+are a few:
+
+* The [rust-reduce](https://github.com/jethrogb/rust-reduce) tool can try to reduce
+  code automatically.
+  * The [C-reduce](https://embed.cs.utah.edu/creduce/) tool also works
+    on Rust code, though it requires that you start from a single
+    file. (XXX link to some post explaining how to do it?)
+* pnkfelix's [Rust Bug Minimization Patterns] blog post
+  * This post focuses on "heavy bore" techniques, where you are
+    starting with a large, complex cargo project that you wish to
+    narrow down to something standalone.
+
+[Rust Bug Minimization Patterns]: http://blog.pnkfx.org/blog/2019/11/18/rust-bug-minimization-patterns/
+
+## Links to duplicate or related bugs
+
+If you are on the "Cleanup Crew", you will sometimes see multiple bug
+reports that seem very similar. You can link one to the other just by
+mentioning the other bug number in a Github comment. Sometimes it is
+useful to close duplicate bugs. But if you do so, you should always
+copy any test case from the bug you are closing to the other bug that
+remains open, as sometimes duplicate-looking bugs will expose
+different facets of the same problem.
+
+## Bisecting regressions
+
+For regressions (something that used to work, but no longer does), it
+is super useful if we can figure out precisely when the code stopped
+working.  The gold standard is to be able to identify the precise
+**PR** that broke the code, so we can ping the author, but even
+narrowing it down to a nightly build is helpful, especially as that
+then gives us a range of PRs. (One other challenge is that we
+sometimes land "rollup" PRs, which combine multiple PRs into one.)
+
+### cargo-bisect-rustc
+
+To help in figuring out the cause of a regression we have a tool
+called [cargo-bisect-rustc]. It will automatically download and test
+various builds of rustc. For recent regressions, it is even able to
+use the builds from our CI to track down the regression to a specific
+PR; for older regressions, it will simply identify a nightly.
+
+To learn to use [cargo-bisect-rustc], check out [this blog
+post][learn], which gives a quick introduction to how it works. You
+can also ask questions at the Zulip stream
+[`#t-compiler/cargo-bisect-rustc`][zcbr], or help in improving the tool.
+
+[cargo-bisect-rustc]: https://github.com/rust-lang/cargo-bisect-rustc/
+[learn]: https://blog.rust-lang.org/inside-rust/2019/12/18/bisecting-rust-compiler.html
+[zcbr]: https://rust-lang.zulipchat.com/#narrow/stream/217417-t-compiler.2Fcargo-bisect-rustc
+
+### identifying the range of PRs in a nightly
+
+If the regression occurred more than 90 days ago, then
+cargo-bisect-rustc will not able to identify the particular PR that
+caused the regression, just the nightly build. In that case, we can
+identify the set of PRs that this corresponds to by using the git
+history. 
+
+The command `rustc +nightly -vV` will cause rustc to output a number
+of useful bits of version info, including the `commit-hash`. Given the
+commit-hash of two nightly versions, you can find all of PRs that have
+landed in between by taking the following steps:
+
+1. Go to an update checkout of the [rust-lang/rust] repository
+2. Execute the command `git log --author=bors --format=oneline SHA1..SHA2`
+  * This will list out all of the commits by bors, which is our merge bot
+  * Each commit corresponds to one PR, and information about the PR should be in the description
+3. Copy and paste that information into the bug report
+
+Often, just eye-balling the PR descriptions (which are included in the
+commit messages) will give you a good idea which one likely caused the
+problem. But if you're unsure feel free to just ping the compiler team
+(`@rust-lang/compiler`) or else to ping the authors of the PR
+themselves.
+
+[rust-lang/rust]: https://github.com/rust-lang/rust/