Add post about using regression labels

Author: camelid

posts/inside-rust/2020-10-14-regression-labels.md

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+---
+layout: post
+title: "Marking issues as regressions"
+description: "Now anyone can mark issues as regressions!"
+author: Camelid
+team: the release team <https://www.rust-lang.org/governance/teams/operations#release>
+---
+
+If you've contributed to Rust before, you have likely used a command like this
+to [set labels][rustbot-labeling-docs]:
+
+> **@rustbot** modify labels: A-typesystem C-enhancement
+
+Setting labels on issues and pull requests helps us keep track of and filter
+issues. There are many kinds of labels you can set to mark an issue
+as pertinent to a particular team, related to a part of the compiler, or
+what kind of bug it is (a hang, a crash, etc.).
+
+We also have labels that help us keep track of regressions so that we can
+prioritize them and fix them quickly if they are severe. We have three
+regression labels, each representing when the regression occurred:
+
+* `regression-from-stable-to-nightly`
+* `regression-from-stable-to-beta`
+* `regression-from-stable-to-stable`
+
+We recently had a case where a [regression was not caught][internals-thread]
+before a release because the issue was not marked with a regression label.
+So we have now [added the ability][regression-label-pr] for *anyone* to set
+regression labels on issues! This is all you have to do to mark an issue as a
+regression and it will automatically be prioritized:
+
+> **@rustbot** modify labels: regression-from-stable-to-{release-channel}
+
+Just replace `{release-channel}` with one of `nightly`, `beta`, or `stable`,
+depending on when the regression occurred, and you're good to go!
+
+Alternatively, if you are reporting a new regression, you can use the regression
+[issue template]. It will guide you through the process of reporting a
+regression and providing information that will help us fix the issue.
+
+Finally, if you have an issue that is *not* a regression, but is still something
+that is important to be fixed, you can request prioritization with:
+
+> **@rustbot** prioritize
+
+We really appreciate it if you mark all regressions with an appropriate label
+so we can track them and fix them as soon as possible!
+
+[rustbot-labeling-docs]: https://rustc-dev-guide.rust-lang.org/rustbot.html#issue-relabeling
+[internals-thread]: https://internals.rust-lang.org/t/1-46-is-unusable-for-me-solved/13161/10
+[regression-label-pr]: https://github.com/rust-lang/rust/pull/77555
+[issue template]: https://github.com/rust-lang/rust/issues/new/choose