Flesh out `CONTRIBUTING.md`

Add code guidelines and instructions.

CONTRIBUTING.md

@@ -3,20 +3,155 @@
 Contributions are very welcome. When contributing code, please follow these
 simple guidelines.
 
-* Make sure you run `rustfmt` on your code. Also ensure all trailing whitespace is trimmed.
-* Run the tests with `cargo test --all`.
-* Before adding any code dependencies, check with the maintainers if this is okay.
-* Write properly formatted comments: they should be English sentences, eg:
+* Follow the coding guidelines when proposing code changes ([see below](#code-style)).
+* Write properly formatted git commits messages ([see below](#writing-git-commit-messages)).
+* Read the DCO and if you are contributing a significant amount of code, make sure your commits are signed off, using `git commit -s`.
 
-        // Return the current UNIX time.
+## Linting & formatting
 
-* Read the DCO and make sure all commits are signed off, using `git commit -s`.
-* Follow the guidelines when proposing code changes (see below).
-* Write properly formatted git commits (see below).
+Always check your code with the linter (`clippy`), by running:
 
-Proposing changes
------------------
-When proposing changes via a pull-request or patch:
+    $ cargo clippy --all --tests
+
+And make sure your code is formatted with, using:
+
+    $ cargo fmt
+
+Finally, ensure there is no trailing whitespace anywhere.
+
+## Running tests
+
+Make sure all tests are passing with:
+
+    $ cargo test --all
+
+## Checking the docs
+
+If you make documentation changes, you may want to check whether there are any
+warnings or errors:
+
+    $ cargo doc --all --all-features
+
+## Code style
+
+The following code guidelines will help make code review smoother.
+
+### Use of `unwrap` and `expect`
+
+Use `unwrap` only in either of three circumstances:
+
+1. Based on manual static anaylsis, you've concluded that it's impossible for
+the code to panic; so unwrapping is *safe*. An example would be:
+
+        let list = vec![a, b, c];
+        let first = list.first().unwrap();
+
+2. The panic caused by `unwrap` would indicate a bug in the software, and it
+would be impossible to continue in that case.
+
+3. The `unwrap` is part of test code, ie. `cfg!(test)` is `true`.
+
+In the first and second case, document `unwrap` call sites with a comment prefixed
+with `SAFETY:` that explains why it's safe to unwrap, eg.
+
+    // SAFETY: Node IDs are valid ref strings.
+    let r = RefString::try_from(node.to_string()).unwrap();
+
+Use `expect` only if the function expects certain invariants that were not met,
+either due to bad inputs, or a problem with the environment; and include the
+expectation in the message. For example:
+
+    logger::init(log::Level::Debug)
+        .expect("logger must only be initialized once");
+
+### Module imports
+
+Modules are declared at the top of the file, before the imports. Public modules
+are separated from private modules with a blank line:
+
+    mod git;
+    mod storage;
+
+    pub mod refs;
+
+    use std::time;
+    use std::process;
+
+    ...
+
+Imports are organized in groups, from least specific to more specific:
+
+    use std::collections::HashMap;   // First, `std` imports.
+    use std::process;
+    use std::time;
+
+    use git_ref_format as format;    // Then, external dependencies.
+    use once_cell::sync::Lazy;
+
+    use crate::crypto::PublicKey;    // Finally, local crate imports.
+    use crate::storage::refs::Refs;
+    use crate::storage::RemoteId;
+
+### Variable naming
+
+Use short 1-letter names when the variable scope is only a few lines, or the context is
+obvious, eg.
+
+    if let Some(e) = result.err() {
+        ...
+    }
+
+Use 1-word names for function parameters or variables that have larger scopes:
+
+    pub fn commit(repo: &Repository, sig: &Signature) -> Result<Commit, Error> {
+        ...
+    }
+
+Use the most descriptive names for globals:
+
+    pub const KEEP_ALIVE_DELTA: LocalDuration = LocalDuration::from_secs(30);
+
+### Function naming
+
+Stay concise. Use the function doc comment to describe
+what the function does, not the name. Keep in mind functions are in the
+context of the parent module and/or object and repeating that would be
+redundant.
+
+### Logging
+
+When writing log statements, always include a `target` and include enough
+context in the log message that it is useful on its own, eg.
+
+    debug!(target: "service", "Routing table updated for {rid} with seed {nid}");
+
+Check the file you are working on for what the `target` name should be; most
+logs should be at the *debug* level.
+
+## Dependencies
+
+Before adding any code dependencies, check with the maintainers if this is okay.
+In general, we try not to add external dependencies unless it's necessary.
+Dependencies increase counter-party risk, build-time, attack surface, and
+make code harder to audit.
+
+## Documentation
+
+Public types and functions should be documented. Modules *may* be documented,
+if you see the need.
+
+Code comments should usually be full english sentences, and add missing context
+for the reader:
+
+    // Ensure that our inventory is recorded in our routing table, and we are tracking
+    // all of it. It can happen that inventory is not properly tracked if for eg. the
+    // user creates a new repository while the node is stopped.
+    for rid in self.storage.inventory()? {
+        ...
+
+## Proposing changes
+
+When proposing changes via a patch:
 
 * Isolate changes in separate commits to make the review process easier.
 * Don't make unrelated changes, unless it happens to be an obvious improvement to
@@ -28,8 +163,8 @@ When proposing changes via a pull-request or patch:
   codebase, write a proposal in English first, and get consensus on that before
   proposing the code changes.
 
-Writing Git commit messages
----------------------------
+## Writing Git commit messages
+
 A properly formed git commit subject line should always be able to complete the
 following sentence: