add some more conventions

src/conventions.md

@@ -1,17 +1,24 @@
+This file offers some tips on the coding conventions for rustc.  This
+chapter covers [formatting](#f), [coding for correctness](#cc),
+[using crates from crates.io](#cio), and some tips on
+[structuring your PR for easy review](#er).
+
+<a name=f>
+
+# Formatting and the tidy script
+
 rustc is slowly moving towards the [Rust standard coding style][fmt];
-at the moment, however, it follows a rather more *chaotic*
-style. There are a few things that are always true.
+at the moment, however, it follows a rather more *chaotic* style.  We
+do have some mandatory formatting conventions, which are automatically
+enforced by a script we affectionately[^not_true] call the "tidy"
+script.  The tidy script runs automatically when you do `./x.py test`.
 
+[^not_true]: Secretly, I hate tidy. -nmatsakis
 [fmt]: https://github.com/rust-lang-nursery/fmt-rfcs
 
-# The tidy script
-
-Running `./x.py test` begins with a "tidy" step. This tidy step checks
-that your source files meet some basic conventions.
-
 <a name=copyright>
 
-## Copyright notice
+### Copyright notice
 
 All files must begin with the following copyright notice:
 
@@ -50,14 +57,91 @@ the copyright notice) like so:
 
 Prefer 4-space indent.
 
-# Warnings and lints
+<a name=cc>
 
-In general, Rust crates 
+# Coding for correctness
 
-# Policy on using crates from crates.io
+Beyond formatting, there are a few other tips that are worth
+following. 
+
+## Prefer exhaustive matches
+
+Using `_` in a match is convenient, but it means that when new
+variants are added to the enum, they may not get handled correctly.
+Ask yourself: if a new variant were added to this enum, what's the
+chance that it would want to use the `_` code, versus having some
+other treatment?  Unless the answer is "low", then prefer an
+exhaustive match. (The same advice applies to `if let` and `while
+let`, which are effectively tests for a single variant.)
+
+## Use "TODO" comments for things you don't want to forget
+
+As a useful tool to yourself, you can insert a `// TODO` comment
+for something that you want to get back to before you land your PR:
+
+```rust,ignore
+fn do_something() {
+    if something_else {
+        unimplemented!(): // TODO write this
+    }
+}
+```
+
+The tidy script will report an error for a `// TODO` comment, so this
+code would not be able to land until the TODO is fixed (or removed).
+
+This can also be useful in a PR as a way to signal from one commit that you are
+leaving a bug that a later commit will fix:
+
+```rust,ignore
+if foo {
+    return true; // TODO wrong, but will be fixed in a later commit
+}
+```
+
+<a name=cio>
+
+# Using crates from crates.io
 
 It is allowed to use crates from crates.io, though external
 dependencies should not be added gratuitously. All such crates must
 have a suitably permissive license. There is an automatic check which
 inspects the Cargo metadata to ensure this.
 
+<a name=er>
+
+# How to structure your PR
+
+How you prepare the commits in your PR can make a big difference for the reviewer.
+Here are some tips.
+
+**Isolate "pure refactorings" into their own commit.** For example, if
+you rename a method, then put that rename into its own commit, along
+with the renames of all the uses.
+
+**More commits is usually better.** If you are doing a large change,
+it's almost always better to break it up into smaller steps that can
+be independently understood. The one thing to be aware of is that if
+you introduce some code following one strategy, then change it
+dramatically (versus adding to it) in a later commit, that
+'back-and-forth' can be confusing.
+
+**If you run rustfmt and the file was not already formatted, isolate
+that into its own commit.** This is really the same as the previous
+rule, but it's worth highlighting. It's ok to rustfmt files, but since
+we do not currently run rustfmt all the time, that can introduce a lot
+of noise into your commit. Please isolate that into its own
+commit. This also makes rebases a lot less painful, since rustfmt
+tends to cause a lot of merge conflicts, and having those isolated
+into their own commit makes htem easier to resolve.
+
+**No merges.** We do not allow merge commits into our history, other
+than those by bors. If you get a merge conflict, rebase instead via a
+command like `git rebase -i rust-lang/master` (presuming you use the
+name `rust-lang` for your remote).
+
+**Individual commits do not have to build (but it's nice).** We do not
+require that every intermediate commit successfully builds -- we only
+expect to be able to bisect at a PR level. However, if you *can* make
+individual commits build, that is always helpful.
+