Explain our use of mypyc in the FAQ (#3002)

I realized we don't have a FAQ entry about this, let's change that so
compiled: yes/no doesn't surprise as many people :)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

diff --git a/docs/faq.md b/docs/faq.md
index a5919a39af5c589c43be2a0651da43d4685f4986..b2fe42de2822973f4893e8debe408b44382efe86 100644 (file)
--- a/docs/faq.md
+++ b/docs/faq.md
@@ -113,3 +113,22 @@ _Black_ is an autoformatter, not a Python linter or interpreter. Detecting all s
 errors is not a goal. It can format all code accepted by CPython (if you find an example
 where that doesn't hold, please report a bug!), but it may also format some code that
 CPython doesn't accept.
+
+## What is `compiled: yes/no` all about in the version output?
+
+While _Black_ is indeed a pure Python project, we use [mypyc] to compile _Black_ into a
+C Python extension, usually doubling performance. These compiled wheels are available
+for 64-bit versions of Windows, Linux (via the manylinux standard), and macOS across all
+supported CPython versions.
+
+Platforms including musl-based and/or ARM Linux distributions, and ARM Windows are
+currently **not** supported. These platforms will fall back to the slower pure Python
+wheel available on PyPI.
+
+If you are experiencing exceptionally weird issues or even segfaults, you can try
+passing `--no-binary black` to your pip install invocation. This flag excludes all
+wheels (including the pure Python wheel), so this command will use the [sdist].
+
+[mypyc]: https://mypyc.readthedocs.io/en/latest/
+[sdist]:
+  https://packaging.python.org/en/latest/glossary/#term-Source-Distribution-or-sdist