580 lines
14 KiB
Markdown
580 lines
14 KiB
Markdown
Our approach to "clean code" is two-fold:
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* We generally don't block PRs on style changes.
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* At the same time, all code in rust-analyzer is constantly refactored.
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It is explicitly OK for a reviewer to flag only some nits in the PR, and then send a follow-up cleanup PR for things which are easier to explain by example, cc-ing the original author.
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Sending small cleanup PRs (like renaming a single local variable) is encouraged.
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# General
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## Scale of Changes
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Everyone knows that it's better to send small & focused pull requests.
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The problem is, sometimes you *have* to, eg, rewrite the whole compiler, and that just doesn't fit into a set of isolated PRs.
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The main things to keep an eye on are the boundaries between various components.
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There are three kinds of changes:
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1. Internals of a single component are changed.
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Specifically, you don't change any `pub` items.
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A good example here would be an addition of a new assist.
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2. API of a component is expanded.
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Specifically, you add a new `pub` function which wasn't there before.
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A good example here would be expansion of assist API, for example, to implement lazy assists or assists groups.
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3. A new dependency between components is introduced.
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Specifically, you add a `pub use` reexport from another crate or you add a new line to the `[dependencies]` section of `Cargo.toml`.
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A good example here would be adding reference search capability to the assists crates.
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For the first group, the change is generally merged as long as:
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* it works for the happy case,
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* it has tests,
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* it doesn't panic for the unhappy case.
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For the second group, the change would be subjected to quite a bit of scrutiny and iteration.
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The new API needs to be right (or at least easy to change later).
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The actual implementation doesn't matter that much.
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It's very important to minimize the amount of changed lines of code for changes of the second kind.
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Often, you start doing a change of the first kind, only to realise that you need to elevate to a change of the second kind.
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In this case, we'll probably ask you to split API changes into a separate PR.
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Changes of the third group should be pretty rare, so we don't specify any specific process for them.
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That said, adding an innocent-looking `pub use` is a very simple way to break encapsulation, keep an eye on it!
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Note: if you enjoyed this abstract hand-waving about boundaries, you might appreciate
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https://www.tedinski.com/2018/02/06/system-boundaries.html
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## Crates.io Dependencies
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We try to be very conservative with usage of crates.io dependencies.
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Don't use small "helper" crates (exception: `itertools` is allowed).
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If there's some general reusable bit of code you need, consider adding it to the `stdx` crate.
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## Commit Style
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We don't have specific rules around git history hygiene.
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Maintaining clean git history is strongly encouraged, but not enforced.
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Use rebase workflow, it's OK to rewrite history during PR review process.
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After you are happy with the state of the code, please use [interactive rebase](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History) to squash fixup commits.
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Avoid @mentioning people in commit messages and pull request descriptions(they are added to commit message by bors).
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Such messages create a lot of duplicate notification traffic during rebases.
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## Clippy
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We don't enforce Clippy.
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A number of default lints have high false positive rate.
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Selectively patching false-positives with `allow(clippy)` is considered worse than not using Clippy at all.
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There's `cargo xtask lint` command which runs a subset of low-FPR lints.
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Careful tweaking of `xtask lint` is welcome.
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See also [rust-lang/clippy#5537](https://github.com/rust-lang/rust-clippy/issues/5537).
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Of course, applying Clippy suggestions is welcome as long as they indeed improve the code.
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# Code
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## Minimal Tests
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Most tests in rust-analyzer start with a snippet of Rust code.
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This snippets should be minimal -- if you copy-paste a snippet of real code into the tests, make sure to remove everything which could be removed.
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There are many benefits to this:
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* less to read or to scroll past
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* easier to understand what exactly is tested
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* less stuff printed during printf-debugging
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* less time to run test
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It also makes sense to format snippets more compactly (for example, by placing enum definitions like `enum E { Foo, Bar }` on a single line),
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as long as they are still readable.
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When using multiline fixtures, use unindented raw string literals:
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```rust
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#[test]
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fn inline_field_shorthand() {
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check_assist(
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inline_local_variable,
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r#"
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struct S { foo: i32}
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fn main() {
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let <|>foo = 92;
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S { foo }
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}
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"#,
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r#"
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struct S { foo: i32}
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fn main() {
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S { foo: 92 }
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}
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"#,
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);
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}
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```
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That way, you can use your editor's "number of selected characters" feature to correlate offsets with test's source code.
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## Preconditions
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Express function preconditions in types and force the caller to provide them (rather than checking in callee):
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```rust
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// Good
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fn frbonicate(walrus: Walrus) {
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...
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}
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// Not as good
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fn frobnicate(walrus: Option<Walrus>) {
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let walrus = match walrus {
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Some(it) => it,
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None => return,
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};
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...
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}
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```
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Avoid preconditions that span across function boundaries:
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```rust
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// Good
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fn main() {
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let s: &str = ...;
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if let Some(contents) = string_literal_contents(s) {
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}
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}
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fn string_literal_contents(s: &str) -> Option<&str> {
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if s.starts_with('"') && s.ends_with('"') {
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Some(&s[1..s.len() - 1])
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} else {
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None
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}
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}
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// Not as good
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fn main() {
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let s: &str = ...;
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if is_string_literal(s) {
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let contents = &s[1..s.len() - 1];
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}
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}
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fn is_string_literal(s: &str) -> bool {
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s.starts_with('"') && s.ends_with('"')
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}
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```
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In the "Not as good" version, the precondition that `1` is a valid char boundary is checked in `is_string_literal` and used in `foo`.
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In the "Good" version, the precondition check and usage are checked in the same block, and then encoded in the types.
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When checking a boolean precondition, prefer `if !invariant` to `if negated_invariant`:
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```rust
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// Good
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if !(idx < len) {
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return None;
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}
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// Not as good
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if idx >= len {
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return None;
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}
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```
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## Getters & Setters
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If a field can have any value without breaking invariants, make the field public.
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Conversely, if there is an invariant, document it, enforce it in the "constructor" function, make the field private, and provide a getter.
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Never provide setters.
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Getters should return borrowed data:
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```rust
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struct Person {
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// Invariant: never empty
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first_name: String,
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middle_name: Option<String>
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}
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// Good
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impl Person {
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fn first_name(&self) -> &str { self.first_name.as_str() }
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fn middle_name(&self) -> Option<&str> { self.middle_name.as_ref() }
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}
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// Not as good
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impl Person {
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fn first_name(&self) -> String { self.first_name.clone() }
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fn middle_name(&self) -> &Option<String> { &self.middle_name }
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}
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```
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## Constructors
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Prefer `Default` to zero-argument `new` function
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```rust
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// Good
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#[derive(Default)]
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struct Foo {
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bar: Option<Bar>
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}
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// Not as good
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struct Foo {
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bar: Option<Bar>
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}
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impl Foo {
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fn new() -> Foo {
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Foo { bar: None }
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}
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}
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```
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Prefer `Default` even it has to be implemented manually.
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## Functions Over Objects
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Avoid creating "doer" objects.
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That is, objects which are created only to execute a single action.
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```rust
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// Good
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do_thing(arg1, arg2);
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// Not as good
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ThingDoer::new(arg1, arg2).do();
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```
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Note that this concerns only outward API.
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When implementing `do_thing`, it might be very useful to create a context object.
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```rust
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pub fn do_thing(arg1: Arg1, arg2: Arg2) -> Res {
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let mut ctx = Ctx { arg1, arg2 }
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ctx.run()
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}
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struct Ctx {
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arg1: Arg1, arg2: Arg2
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}
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impl Ctx {
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fn run(self) -> Res {
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...
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}
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}
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```
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The difference is that `Ctx` is an impl detail here.
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Sometimes a middle ground is acceptable if this can save some busywork:
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```rust
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ThingDoer::do(arg1, arg2);
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pub struct ThingDoer {
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arg1: Arg1, arg2: Arg2,
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}
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impl ThingDoer {
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pub fn do(arg1: Arg1, arg2: Arg2) -> Res {
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ThingDoer { arg1, arg2 }.run()
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}
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fn run(self) -> Res {
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...
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}
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}
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```
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## Avoid Monomorphization
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Rust uses monomorphization to compile generic code, meaning that for each instantiation of a generic functions with concrete types, the function is compiled afresh, *per crate*.
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This allows for exceptionally good performance, but leads to increased compile times.
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Runtime performance obeys 80%/20% rule -- only a small fraction of code is hot.
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Compile time **does not** obey this rule -- all code has to be compiled.
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For this reason, avoid making a lot of code type parametric, *especially* on the boundaries between crates.
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```rust
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// Good
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fn frbonicate(f: impl FnMut()) {
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frobnicate_impl(&mut f)
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}
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fn frobnicate_impl(f: &mut dyn FnMut()) {
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// lots of code
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}
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// Not as good
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fn frbonicate(f: impl FnMut()) {
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// lots of code
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}
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```
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Avoid `AsRef` polymorphism, it pays back only for widely used libraries:
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```rust
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// Good
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fn frbonicate(f: &Path) {
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}
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// Not as good
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fn frbonicate(f: impl AsRef<Path>) {
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}
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```
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# Premature Pessimization
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## Avoid Allocations
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Avoid writing code which is slower than it needs to be.
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Don't allocate a `Vec` where an iterator would do, don't allocate strings needlessly.
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```rust
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// Good
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use itertools::Itertools;
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let (first_word, second_word) = match text.split_ascii_whitespace().collect_tuple() {
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Some(it) => it,
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None => return,
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}
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// Not as good
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let words = text.split_ascii_whitespace().collect::<Vec<_>>();
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if words.len() != 2 {
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return
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}
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```
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## Push Allocations to the Call Site
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If allocation is inevitable, let the caller allocate the resource:
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```rust
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// Good
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fn frobnicate(s: String) {
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...
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}
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// Not as good
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fn frobnicate(s: &str) {
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let s = s.to_string();
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...
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}
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```
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This is better because it reveals the costs.
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It is also more efficient when the caller already owns the allocation.
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## Collection types
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Prefer `rustc_hash::FxHashMap` and `rustc_hash::FxHashSet` instead of the ones in `std::collections`.
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They use a hasher that's slightly faster and using them consistently will reduce code size by some small amount.
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# Style
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## Order of Imports
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Separate import groups with blank lines.
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Use one `use` per crate.
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```rust
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mod x;
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mod y;
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// First std.
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use std::{ ... }
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// Second, external crates (both crates.io crates and other rust-analyzer crates).
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use crate_foo::{ ... }
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use crate_bar::{ ... }
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// Then current crate.
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use crate::{}
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// Finally, parent and child modules, but prefer `use crate::`.
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use super::{}
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```
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Module declarations come before the imports.
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Order them in "suggested reading order" for a person new to the code base.
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## Import Style
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Qualify items from `hir` and `ast`.
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```rust
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// Good
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use syntax::ast;
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fn frobnicate(func: hir::Function, strukt: ast::StructDef) {}
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// Not as good
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use hir::Function;
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use syntax::ast::StructDef;
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fn frobnicate(func: Function, strukt: StructDef) {}
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```
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Avoid local `use MyEnum::*` imports.
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Prefer `use crate::foo::bar` to `use super::bar`.
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When implementing `Debug` or `Display`, import `std::fmt`:
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```rust
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// Good
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use std::fmt;
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impl fmt::Display for RenameError {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { .. }
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}
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// Not as good
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impl std::fmt::Display for RenameError {
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fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { .. }
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}
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```
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## Order of Items
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Optimize for the reader who sees the file for the first time, and wants to get a general idea about what's going on.
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People read things from top to bottom, so place most important things first.
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Specifically, if all items except one are private, always put the non-private item on top.
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```rust
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// Good
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pub(crate) fn frobnicate() {
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Helper::act()
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}
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#[derive(Default)]
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struct Helper { stuff: i32 }
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impl Helper {
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fn act(&self) {
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}
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}
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// Not as good
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#[derive(Default)]
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struct Helper { stuff: i32 }
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pub(crate) fn frobnicate() {
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Helper::act()
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}
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impl Helper {
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fn act(&self) {
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}
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}
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```
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If there's a mixture of private and public items, put public items first.
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If function bodies are folded in the editor, the source code should read as documentation for the public API.
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Put `struct`s and `enum`s first, functions and impls last. Order types declarations in top-down manner.
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```rust
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// Good
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struct Parent {
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children: Vec<Child>
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}
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struct Child;
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impl Parent {
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}
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impl Child {
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}
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// Not as good
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struct Child;
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impl Child {
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}
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struct Parent {
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children: Vec<Child>
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}
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impl Parent {
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}
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```
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## Variable Naming
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Use boring and long names for local variables ([yay code completion](https://github.com/rust-analyzer/rust-analyzer/pull/4162#discussion_r417130973)).
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The default name is a lowercased name of the type: `global_state: GlobalState`.
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Avoid ad-hoc acronyms and contractions, but use the ones that exist consistently (`db`, `ctx`, `acc`).
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Prefer American spelling (color, behavior).
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Default names:
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* `res` -- "result of the function" local variable
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* `it` -- I don't really care about the name
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* `n_foo` -- number of foos
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* `foo_idx` -- index of `foo`
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Many names in rust-analyzer conflict with keywords.
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We use mangled names instead of `r#ident` syntax:
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```
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struct -> strukt
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crate -> krate
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impl -> imp
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trait -> trait_
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fn -> func
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enum -> enum_
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mod -> module
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```
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## Early Returns
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Do use early returns
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```rust
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// Good
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fn foo() -> Option<Bar> {
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if !condition() {
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return None;
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}
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Some(...)
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}
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// Not as good
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fn foo() -> Option<Bar> {
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if condition() {
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Some(...)
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} else {
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None
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}
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}
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```
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## Comparisons
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Use `<`/`<=`, avoid `>`/`>=`.
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Less-then comparisons are more intuitive, they correspond spatially to [real line](https://en.wikipedia.org/wiki/Real_line)
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```rust
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// Good
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assert!(lo <= x && x <= hi);
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// Not as good
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assert!(x >= lo && x <= hi>);
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```
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## Documentation
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For `.md` and `.adoc` files, prefer a sentence-per-line format, don't wrap lines.
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If the line is too long, you want to split the sentence in two :-)
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