mention tidy

Author: nikomatsakis

src/conventions.md

@@ -1,9 +1,9 @@
 This file offers some tips on the coding conventions for rustc.  This
-chapter covers [formatting](#f), [coding for correctness](#cc),
+chapter covers [formatting](#formatting), [coding for correctness](#cc),
 [using crates from crates.io](#cio), and some tips on
 [structuring your PR for easy review](#er).
 
-<a name=f>
+<a name=formatting>
 
 # Formatting and the tidy script
 

src/tests/running.md

@@ -13,6 +13,13 @@ a very long time, and anyway bors / travis will do it for you. (Often,
 I will run this command in the background after opening a PR that I
 think is done, but rarely otherwise. -nmatsakis)
 
+## Tidy
+
+When you run the full suite of tests via `./x.py test`, the first
+thing that executes is a "tidy suite" that checks for long lines and
+other formatting conventions. There is more information in the
+[section on coding conventions](../conventions.html#formatting).
+
 ## Running a subset of the test suites
 
 When working on a specific PR, you will usually want to run a smaller