Document opposing lints

book/src/development/adding_lints.md

@@ -30,6 +30,7 @@ because that's clearly a non-descriptive name.
   - [Documentation](#documentation)
   - [Running rustfmt](#running-rustfmt)
   - [Debugging](#debugging)
+  - [Opposing lints](#opposing-lints)
   - [PR Checklist](#pr-checklist)
   - [Adding configuration to a lint](#adding-configuration-to-a-lint)
   - [Cheat Sheet](#cheat-sheet)
@@ -612,6 +613,13 @@ output in the `stdout` part.
 
 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
 
+## Opposing lints
+
+There are several lints that deal with the same pattern but suggest different approaches. When you are creating a lint that interact with this scenario, the following tips should be encouraged to guide classification:
+
+* The only case where they should be in the same category is if that category is `restriction`. For example, `semicolon_inside_block` and `semicolon_inside_block`.
+* For all the other cases, they should be in different categories with different levels of allowance. For example, `implicit_return` (restriction, allow) and `needless_return` (style, warn).
+
 ## PR Checklist
 
 Before submitting your PR make sure you followed all the basic requirements: