From e749a38886ab868669029cf564c34bf9cad0d1fc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E8=AE=B8=E6=9D=B0=E5=8F=8B=20Jieyou=20Xu=20=28Joe=29?=
 <39484203+jieyouxu@users.noreply.github.com>
Date: Mon, 20 Jan 2025 21:53:59 +0800
Subject: [PATCH] docs: update contributing docs for submodule/subtree changes

---
 CONTRIBUTING.md | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index eeff563d8ecdf..a5ddff595f5d6 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -12,6 +12,15 @@ Documentation for contributing to the compiler or tooling is located in the [Gui
 Development][rustc-dev-guide], commonly known as the [rustc-dev-guide]. Documentation for the
 standard library in the [Standard library developers Guide][std-dev-guide], commonly known as the [std-dev-guide].
 
+## Making changes to subtrees and submodules
+
+For submodules, changes need to be made against the repository corresponding the
+submodule, and not the main `rust-lang/rust` repository.
+
+For subtrees, prefer sending a PR against the subtree's repository if it does
+not need to be made against the main `rust-lang/rust` repostory (e.g. a
+rustc-dev-guide change that does not accompany a compiler change).
+
 ## About the [rustc-dev-guide]
 
 The [rustc-dev-guide] is meant to help document how rustc –the Rust compiler– works,