rust-analyzer/ARCHITECTURE.md

# Architecture

This document describes high-level architecture of rust-analyzer.
If you want to familiarize yourself with the code base, you are just
in the right place!

## The Big Picture

![](https://user-images.githubusercontent.com/1711539/50114578-e8a34280-0255-11e9-902c-7cfc70747966.png)

On the highest level, rust-analyzer is a thing which accepts input source code
from the client and produces a structured semantic model of the code.

More specifically, input data consists of a set of test files (`(PathBuf,
String)` pairs) and an information about project structure, the so called
`CrateGraph`. Crate graph specifies which files are crate roots, which cfg flags
are specified for each crate (TODO: actually implement this) and what are
dependencies between the crate. The analyzer keeps all these input data in
memory and never does any IO. Because the input data is source code, which
typically measures in tens of megabytes at most, keeping all input data in
memory is OK.

A "structured semantic model" is basically an object-oriented representation of
modules, functions and types which appear in the source code. This representation
is fully "resolved": all expressions have types, all references are bound to
declarations, etc.

The client can submit a small delta of input data (typically, a change to a
single file) and get a fresh code model which accounts for changes.

Underlying engine makes sure that model is computed lazily (on-demand) and can
be quickly updated for small modifications.


## Code generation

Some of the components of this repository are generated through automatic
processes. These are outlined below:

- `gen-syntax`: The kinds of tokens are reused in several places, so a generator
  is used. We use tera templates to generate the files listed below, based on
  the grammar described in [grammar.ron]:
  - [ast/generated.rs][ast generated] in `ra_syntax` based on
    [ast/generated.tera.rs][ast source]
  - [syntax_kinds/generated.rs][syntax_kinds generated] in `ra_syntax` based on
    [syntax_kinds/generated.tera.rs][syntax_kinds source]

[tera]: https://tera.netlify.com/
[grammar.ron]: ./crates/ra_syntax/src/grammar.ron
[ast generated]: ./crates/ra_syntax/src/ast/generated.rs
[ast source]: ./crates/ra_syntax/src/ast/generated.rs.tera
[syntax_kinds generated]: ./crates/ra_syntax/src/syntax_kinds/generated.rs
[syntax_kinds source]: ./crates/ra_syntax/src/syntax_kinds/generated.rs.tera


## Code Walk-Through

### `crates/ra_syntax`

Rust syntax tree structure and parser. See
[RFC](https://github.com/rust-lang/rfcs/pull/2256) for some design
notes.

- [rowan](https://github.com/rust-analyzer/rowan) library is used for constructing syntax trees.
- `grammar` module is the actual parser. It is a hand-written recursive descent parsers, which
  produces a sequence of events like "start node X", "finish not Y". It works similarly to  [kotlin parser](https://github.com/JetBrains/kotlin/blob/4d951de616b20feca92f3e9cc9679b2de9e65195/compiler/frontend/src/org/jetbrains/kotlin/parsing/KotlinParsing.java),
  which is a good source for inspiration for dealing with syntax errors and incomplete input. Original [libsyntax parser](https://github.com/rust-lang/rust/blob/6b99adeb11313197f409b4f7c4083c2ceca8a4fe/src/libsyntax/parse/parser.rs)
  is what we use for the definition of the Rust language.
- `parser_api/parser_impl` bridges the tree-agnostic parser from `grammar` with `rowan` trees.
  This is the thing that turns a flat list of events into a tree (see `EventProcessor`)
- `ast` a type safe API on top of the raw `rowan` tree.
- `grammar.ron` RON description of the grammar, which is used to
  generate `syntax_kinds` and `ast` modules, using `cargo gen-syntax` command.
- `algo`: generic tree algorithms, including `walk` for O(1) stack
  space tree traversal (this is cool) and `visit` for type-driven
  visiting the nodes (this is double plus cool, if you understand how
  `Visitor` works, you understand rust-analyzer).

Test for ra_syntax are mostly data-driven: `tests/data/parser` contains a bunch of `.rs`
(test vectors) and `.txt` files with corresponding syntax trees. During testing, we check
`.rs` against `.txt`. If the `.txt` file is missing, it is created (this is how you update
tests). Additionally, running `cargo gen-tests` will walk the grammar module and collect
all `//test test_name` comments into files inside `tests/data` directory.

See [#93](https://github.com/rust-analyzer/rust-analyzer/pull/93) for an example PR which
fixes a bug in the grammar.

### `crates/ra_db`

We use the [salsa](https://github.com/salsa-rs/salsa) crate for incremental and
on-demand computation. Roughly, you can think of salsa as a key-value store, but
it also can compute derived values using specified functions. The `ra_db` crate
provides a basic infrastructure for interacting with salsa. Crucially, it
defines most of the "input" queries: facts supplied by the client of the
analyzer. Reading the docs of the `ra_db::input` module should be useful:
everything else is strictly derived from those inputs.

### `crates/ra_hir`

HIR provides high-level "object oriented" access to Rust code.

The principal difference between HIR and syntax trees is that HIR is bound to a
particular crate instance. That is, it has cfg flags and features applied (in
theory, in practice this is to be implemented). So, the relation between
syntax and HIR is many-to-one. The `source_binder` modules is responsible for
guessing a HIR for a particular source position.

Underneath, HIR works on top of salsa, using a `HirDatabase` trait.

### `crates/ra_analysis`

A stateful library for analyzing many Rust files as they change.
`AnalysisHost` is a mutable entity (clojure's atom) which holds the
current state, incorporates changes and handles out `Analysis` --- an
immutable and consistent snapshot of world state at a point in time, which
actually powers analysis.

One interesting aspect of analysis is its support for cancellation. When a change
is applied to `AnalysisHost`, first all currently active snapshots are
cancelled. Only after all snapshots are dropped the change actually affects the
database.

### `crates/ra_lsp_server`

An LSP implementation which uses `ra_analysis` for managing state and
`ra_editor` for actually doing useful stuff.

See [#79](https://github.com/rust-analyzer/rust-analyzer/pull/79/) as an
example of PR which adds a new feature to `ra_editor` and exposes it
to `ra_lsp_server`.

### `crates/ra_editor`

All IDE features which can be implemented if you only have access to a
single file. `ra_editor` could be used to enhance editing of Rust code
without the need to fiddle with build-systems, file
synchronization and such.

In a sense, `ra_editor` is just a bunch of pure functions which take a
syntax tree as input.

The tests for `ra_editor` are `#[cfg(test)] mod tests` unit-tests spread
throughout its modules.

### `crates/gen_lsp_server`

A language server scaffold, exposing a synchronous crossbeam-channel based API.
This crate handles protocol handshaking and parsing messages, while you
control the message dispatch loop yourself.

Run with `RUST_LOG=sync_lsp_server=debug` to see all the messages.

### `crates/ra_cli`

A CLI interface to rust-analyzer.

### `crate/tools`

Custom Cargo tasks used to develop rust-analyzer:

- `cargo gen-syntax` -- generate `ast` and `syntax_kinds`
- `cargo gen-tests` -- collect inline tests from grammar
- `cargo install-code` -- build and install VS Code extension and server

### `editors/code`

VS Code plugin


## Common workflows

To try out VS Code extensions, run `cargo install-code`.  This installs both the
`ra_lsp_server` binary and VS Code extension. To install only the binary, use
`cargo install --path crates/ra_lsp_server --force`

To see logs from the language server, set `RUST_LOG=info` env variable. To see
all communication between the server and the client, use
`RUST_LOG=gen_lsp_server=debug` (will print quite a bit of stuff).

To run tests, just `cargo test`.

To work on VS Code extension, launch code inside `editors/code` and use `F5` to
launch/debug. To automatically apply formatter and linter suggestions, use `npm
run fix`.