1 files changed, 55 insertions, 0 deletions
diff --git a/build/docs/unified-builds.rst b/build/docs/unified-builds.rst
new file mode 100644
index 0000000000..b0e93b9e68
--- /dev/null
+++ b/build/docs/unified-builds.rst
@@ -0,0 +1,55 @@
+.. _unified-builds:
+
+==============
+Unified Builds
+==============
+
+The Firefox build system uses the technique of "unified builds" (or elsewhere
+called "`unity builds <https://en.wikipedia.org/wiki/Unity_build>`_") to
+improve compilation performance. Rather than compiling source files individually,
+groups of files in the same directory are concatenated together, then compiled once
+in a single batch.
+
+Unified builds can be configured using the ``UNIFIED_SOURCES`` variable in ``moz.build`` files.
+
+.. _unified_build_compilation_failures:
+
+Why are there unrelated compilation failures when I change files?
+=================================================================
+
+Since multiple files are concatenated together in a unified build, it's possible for a change
+in one file to cause the compilation of a seemingly unrelated file to fail.
+This is usually because source files become implicitly dependent on each other for:
+
+* ``#include`` statements
+* ``using namespace ...;`` statements
+* Other symbol imports or definitions
+
+One of the more common cases of unexpected failures are when source code files are added or
+removed, and the "chunking" is changed. There's a limit on the number of files that are combined
+together for a single compilation, so sometimes the addition of a new file will cause another one
+to be bumped into a different chunk. If that other chunk doesn't meet the implicit requirements
+of the bumped file, there will be a tough-to-debug compilation failure.
+
+Building outside of the unified environment
+===========================================
+
+As described above, unified builds can cause source files to implicitly depend on each other, which
+not only causes unexpected build failures but also can cause issues when using source-analysis tools.
+To combat this, we'll use a "non-unified" build that attempts to perform a build with as many files compiled
+individually as possible.
+
+To build in the non unified mode, set the following flag in your ``mozconfig``:
+
+``ac_add_options --disable-unified-build``
+
+Other notes:
+============
+
+* Some IDEs (such as VSCode with ``clangd``) build files in standalone mode, so they may show
+  more failures than a ``mach build``.
+* The amount of files per chunk can be adjusted in ``moz.build`` files with the
+  ``FILES_PER_UNIFIED_FILE`` variable. Note that changing the chunk size can introduce
+  compilation failures as described :ref:`above<unified_build_compilation_failures>`.
+* We are happy to accept patches that fix problematic unified build chunks (such as by adding
+  includes or namespace annotations).