Add instructions for doing mass renames easily (#6865)

book/src/SUMMARY.md

@@ -36,5 +36,6 @@
     - [Network Architecture](dev/diagrams/zebra-network.md)
   - [Continuous Integration](dev/continuous-integration.md)
   - [Continuous Delivery](dev/continuous-delivery.md)
-  - [zebra-checkpoints](dev/zebra-checkpoints.md)
+  - [Generating Zebra Checkpoints](dev/zebra-checkpoints.md)
+  - [Doing Mass Renames](dev/mass-renames.md)
 - [API Reference](api.md)

book/src/dev/mass-renames.md

@@ -0,0 +1,113 @@
+# Doing Mass Renames in Zebra Code
+
+Sometimes we want to rename a Rust type or function, or change a log message.
+
+But our types and functions are also used in our documentation,
+so the compiler can sometimes miss when their names are changed.
+
+Our log messages are also used in our integration tests,
+so changing them can lead to unexpected test failures or hangs.
+
+## Universal Renames with `sed`
+
+You can use `sed` to rename all the instances of a name in Zebra's code, documentation, and tests:
+```sh
+git ls-tree --full-tree -r --name-only HEAD | \
+xargs sed -i 's/OldName/NewName/g'
+```
+
+Or excluding specific paths:
+```sh
+git ls-tree --full-tree -r --name-only HEAD | \
+grep -v 'path-to-skip' | \
+xargs sed -i 's/OldName/NewName/g'
+```
+
+`sed` also supports regular expressions to replace a pattern with another pattern.
+
+Here's how to make a PR with these replacements:
+1. Run the `sed` commands
+2. Run `cargo fmt --all` after doing all the replacements
+3. Put the commands in the commit message and pull request, so the reviewer can check them
+
+Here's how to review that PR:
+1. Check out two copies of the repository, one with the PR, and one without:
+```sh
+cd zebra
+git fetch --all
+# clear the checkout so we can use main elsewhere
+git checkout main^
+# Use the base branch or commit for the PR, which is usually main
+git worktree add ../zebra-sed main
+git worktree add ../zebra-pr origin/pr-branch-name
+```
+
+2. Run the scripts on the repository without the PR:
+```sh
+cd ../zebra-sed
+# run the scripts in the PR or commit message
+git ls-tree --full-tree -r --name-only HEAD | \
+xargs sed -i 's/OldName/NewName/g'
+cargo fmt --all
+```
+
+3. Automatically check that they match
+```sh
+cd ..
+git diff zebra-sed zebra-pr
+```
+
+If there are no differences, then the PR can be approved.
+
+If there are differences, then post them as a review in the PR,
+and ask the author to re-run the script on the latest `main`.
+
+## Interactive Renames with `fastmod`
+
+You can use `fastmod` to rename some instances, but skip others:
+```sh
+fastmod --fixed-strings "OldName" "NewName" [paths to change]
+```
+
+`fastmod` also supports regular expressions to replace a pattern with another pattern.
+
+Here's how to make a PR with these replacements:
+1. Run the `fastmod` commands, choosing which instances to replace
+2. Run `cargo fmt --all` after doing all the replacements
+3. Put the commands in the commit message and pull request, so the reviewer can check them
+4. If there are a lot of renames:
+   - use `sed` on any directories or files that are always renamed, and put them in the first PR,
+   - do a cleanup using `fastmod` in the next PR.
+
+Here's how to review that PR:
+1. Manually review each replacement (there's no shortcut)
+
+## Using `rustdoc` links to detect name changes
+
+When you're referencing a type or function in a doc comment,
+use a `rustdoc` link to refer to it.
+
+This makes the documentation easier to navigate,
+and our `rustdoc` lint will detect any typos or name changes.
+
+```rust
+//! This is what `rustdoc` links look like:
+//! - [`u32`] type or trait
+//! - [`drop()`] function
+//! - [`Clone::clone()`] method
+//! - [`Option::None`] enum variant
+//! - [`Option::Some(_)`](Option::Some) enum variant with data
+//! - [`HashMap`](std::collections::HashMap) fully-qualified path
+//! - [`BTreeSet<String>`](std::collections::BTreeSet) fully-qualified path with generics
+```
+
+If a type isn't imported in the module or Rust prelude,
+then it needs a fully-qualified path in the docs, or an unused import:
+```rust
+// For rustdoc
+#[allow(unused_imports)]
+use std::collections::LinkedList;
+
+//! Link to [`LinkedList`].
+struct Type;
+```