Improve CONTRIBUTING.md

Author: emilk

CONTRIBUTING.md

@@ -1,11 +1,13 @@
-# Contributing guidelines
+# Contribution Guidelines
 
 ## Introduction
 
 `egui` has been an on-and-off weekend project of mine since late 2018. I am grateful to any help I can get, but bare in mind that sometimes I can be slow to respond because I am busy with other things!
 
 / Emil
 
+## How to contribute to egui
+You want to contribute to egui, but don't know how? First of all: thank you! I created a special issue just for that: <https://github.com/emilk/egui/issues/3742>. But make sure you still read this file first :)
 
 ## Discussion
 
@@ -25,7 +27,7 @@ If you are filing a bug, please provide a way to reproduce it.
 
 ## Making a PR
 
-First file an issue (or find an existing one) and announce that you plan to work on something. That way we will avoid having several people doing double work. Please ask for feedback before you start working on something non-trivial!
+For small things, just go ahead an open a PR. For bigger things, please file an issue first (or find an existing one) and announce that you plan to work on something. That way we will avoid having several people doing double work, and you might get useful feedback on the issue before you start working.
 
 Browse through [`ARCHITECTURE.md`](ARCHITECTURE.md) to get a sense of how all pieces connects.
 
@@ -34,76 +36,115 @@ You can test your code locally by running `./scripts/check.sh`.
 When you have something that works, open a draft PR. You may get some helpful feedback early!
 When you feel the PR is ready to go, do a self-review of the code, and then open it for review.
 
-Please keep pull requests small and focused.
-
 Don't worry about having many small commits in the PR - they will be squashed to one commit once merged.
 
-Do not include the `.js` and `.wasm` build artifacts generated for building for web.
-`git` is not great at storing large files like these, so we only commit a new web demo after a new egui release.
+Please keep pull requests small and focused. The smaller it is, the more likely it is to get merged.
+
+## PR review
+
+Most PR reviews are done by me, Emil, but I very much appreciate any help I can get reviewing PRs!
 
+It is very easy to add complexity to a project, but remember that each line of code added is code that needs to be maintained in perpituity, so we have a high bar on what get merged!
+
+When reviewing, we look for:
+* The PR title and description should be helpful
+* Breaking changes are documented in the PR description
+* The code should be readable
+* The code should have helpful docstrings
+* The code should follow the [Code Style](CONTRIBUTING.md#code-style)
+
+Note that each new egui release have some breaking changes, so we don't mind having a few of those in a PR. Of course, we still try to avoid them if we can, and if we can't we try to first deprecate old code using the `#[deprecated]` attribute.
 
 ## Creating an integration for egui
 
-If you make an integration for `egui` for some engine or renderer, please share it with the world!
-I will add a link to it from the `egui` README.md so others can easily find it.
+See <https://docs.rs/egui/latest/egui/#integrating-with-egui> for how to write your own egui integration.
 
-Read the section on integrations at <https://github.com/emilk/egui#integrations>.
+If you make an integration for `egui` for some engine or renderer, please share it with the world!
+Make a PR to add it as a link to [`README.md`](README.md#integrations) so others can easily find it.
 
 
 ## Testing the web viewer
-* Install some tools with `scripts/setup_web.sh`
 * Build with `scripts/build_demo_web.sh`
 * Host with `scripts/start_server.sh`
 * Open <http://localhost:8888/index.html>
 
 
-## Code Conventions
-Conventions unless otherwise specified:
-
-* angles are in radians and clock-wise
-* `Vec2::X` is right and `Vec2::Y` is down.
-* `Pos2::ZERO` is left top.
-
+## Code Style
 While using an immediate mode gui is simple, implementing one is a lot more tricky. There are many subtle corner-case you need to think through. The `egui` source code is a bit messy, partially because it is still evolving.
 
-* Read some code before writing your own.
-* Follow the `egui` code style.
-* Add blank lines around all `fn`, `struct`, `enum`, etc.
-* `// Comment like this.` and not `//like this`.
-* Use `TODO` instead of `FIXME`.
-* Add your github handle to the `TODO`:s you write, e.g: `TODO(emilk): clean this up`.
-* Write idiomatic rust.
-* Avoid `unsafe`.
-* Avoid code that can cause panics.
-* Use good names for everything.
-* Add docstrings to types, `struct` fields and all `pub fn`.
-* Add some example code (doc-tests).
-* Before making a function longer, consider adding a helper function.
-* If you are only using it in one function, put the `use` statement in that function. This improves locality, making it easier to read and move the code.
-* When importing a `trait` to use it's trait methods, do this: `use Trait as _;`. That lets the reader know why you imported it, even though it seems unused.
-* Follow the [Rust API Guidelines](https://rust-lang.github.io/api-guidelines/).
-* Break the above rules when it makes sense.
+* Read some code before writing your own
+* Leave the code cleaner than how you found it
+* Write idiomatic rust
+* Follow the [Rust API Guidelines](https://rust-lang.github.io/api-guidelines/)
+* Add blank lines around all `fn`, `struct`, `enum`, etc
+* `// Comment like this.` and not `//like this`
+* Use `TODO` instead of `FIXME`
+* Add your github handle to the `TODO`:s you write, e.g: `TODO(emilk): clean this up`
+* Avoid `unsafe`
+* Avoid `unwrap` and any other code that can cause panics
+* Use good names for everything
+* Add docstrings to types, `struct` fields and all `pub fn`
+* Add some example code (doc-tests)
+* Before making a function longer, consider adding a helper function
+* If you are only using it in one function, put the `use` statement in that function. This improves locality, making it easier to read and move the code
+* When importing a `trait` to use it's trait methods, do this: `use Trait as _;`. That lets the reader know why you imported it, even though it seems unused
+* Avoid double negatives
+* Flip `if !condition {} else {}`
+* Sets of things should be lexicographically sorted (e.g. crate dependencies in `Cargo.toml`)
+* Break the above rules when it makes sense
 
 
 ### Good:
 ``` rust
 /// The name of the thing.
-fn name(&self) -> &str {
+pub fn name(&self) -> &str {
     &self.name
 }
 
 fn foo(&self) {
-    // TODO(emilk): implement
+    // TODO(emilk): this can be optimized
 }
 ```
 
 ### Bad:
 ``` rust
-//some function
-fn get_name(&self) -> &str {
+//gets the name
+pub fn get_name(&self) -> &str {
     &self.name
 }
 fn foo(&self) {
-    //FIXME: implement
+    //FIXME: this can be optimized
 }
 ```
+
+### Coordinate system
+The left-top corner of the screen is `(0.0, 0.0)`,
+with `Vec2::X` increasing to the right and `Vec2::Y` increasing downwards.
+
+`egui` uses logical _points_ as its coordinate system.
+Those related to physical _pixels_ by the `pixels_per_point` scale factor.
+For example, a high-dpi screeen can have `pixels_per_point = 2.0`,
+meaning there are two physical screen pixels for each logical point.
+
+Angles are in radians, and are measured clockwise from the X-axis, which has angle=0.
+
+
+### Avoid `unwrap`, `expect` etc.
+The code should never panic or crash, which means that any instance of `unwrap` or `expect` is a potential time-bomb. Even if you structured your code to make them impossible, any reader will have to read the code very carefully to prove to themselves that an `unwrap` won't panic. Often you can instead rewrite your code so as to avoid it. The same goes for indexing into a slice (which will panic on out-of-bounds) - it is often preferable to use `.get()`.
+
+For instance:
+
+``` rust
+let first = if vec.is_empty() {
+    return;
+} else {
+    vec[0]
+};
+```
+can be better written as:
+
+``` rust
+let Some(first) = vec.first() else {
+    return;
+};
+```