rust/docs/dev/architecture.md

# Architecture

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

See also the [guide](./guide.md), which walks through a particular snapshot of
rust-analyzer code base.

Yet another resource is this playlist with videos about various parts of the
analyzer:

https://www.youtube.com/playlist?list=PL85XCvVPmGQho7MZkdW-wtPtuJcFpzycE

Note that the guide and videos are pretty dated, this document should be in
generally fresher.

## 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 information about project structure, captured in the so
called `CrateGraph`. The crate graph specifies which files are crate roots,
which cfg flags are specified for each crate and what dependencies exist between
the crates. The analyzer keeps all this input data in memory and never does any
IO. Because the input data are source code, which typically measures in tens of
megabytes at most, keeping everything 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.

The 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. `cargo xtask codegen` runs all generation tasks. Generated code is
committed to the git repository.

In particular, `cargo xtask codegen` generates:

1. [`syntax_kind/generated`](https://github.com/rust-analyzer/rust-analyzer/blob/a0be39296d2925972cacd9fbf8b5fb258fad6947/crates/ra_parser/src/syntax_kind/generated.rs)
  -- the set of terminals and non-terminals of rust grammar.

2. [`ast/generated`](https://github.com/rust-analyzer/rust-analyzer/blob/a0be39296d2925972cacd9fbf8b5fb258fad6947/crates/ra_syntax/src/ast/generated.rs)
  -- AST data structure.

3. [`doc_tests/generated`](https://github.com/rust-analyzer/rust-analyzer/blob/a0be39296d2925972cacd9fbf8b5fb258fad6947/crates/assists/src/doc_tests/generated.rs),
  [`test_data/parser/inline`](https://github.com/rust-analyzer/rust-analyzer/tree/a0be39296d2925972cacd9fbf8b5fb258fad6947/crates/ra_syntax/test_data/parser/inline)
  -- tests for assists and the parser.

The source for 1 and 2 is in [`ast_src.rs`](https://github.com/rust-analyzer/rust-analyzer/blob/a0be39296d2925972cacd9fbf8b5fb258fad6947/xtask/src/ast_src.rs).

## Code Walk-Through

### `crates/ra_syntax`, `crates/parser`

Rust syntax tree structure and parser. See
[RFC](https://github.com/rust-lang/rfcs/pull/2256) and [./syntax.md](./syntax.md) 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 parser, which
  produces a sequence of events like "start node X", "finish node Y". It works similarly to [kotlin's parser](https://github.com/JetBrains/kotlin/blob/4d951de616b20feca92f3e9cc9679b2de9e65195/compiler/frontend/src/org/jetbrains/kotlin/parsing/KotlinParsing.java),
  which is a good source of 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.
- `TreeSink` and `TokenSource` traits bridge the tree-agnostic parser from `grammar` with `rowan` trees.
- `ast` provides a type safe API on top of the raw `rowan` tree.
- `ast_src` description of the grammar, which is used to generate `syntax_kinds`
  and `ast` modules, using `cargo xtask codegen` command.

Tests for ra_syntax are mostly data-driven: `test_data/parser` contains subdirectories with 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 xtask codegen` will walk the grammar module and collect
all `// test test_name` comments into files inside `test_data/parser/inline` directory.

Note
[`api_walkthrough`](https://github.com/rust-analyzer/rust-analyzer/blob/2fb6af89eb794f775de60b82afe56b6f986c2a40/crates/ra_syntax/src/lib.rs#L190-L348)
in particular: it shows off various methods of working with syntax tree.

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

### `crates/base_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 `base_db` crate
provides 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 `base_db::input` module should be useful:
everything else is strictly derived from those inputs.

### `crates/hir*` crates

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. So,
the relation between syntax and HIR is many-to-one. The `source_binder` module
is responsible for guessing a HIR for a particular source position.

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

`hir_xxx` crates have a strong ECS flavor, in that they work with raw ids and
directly query the database.

The top-level `hir` façade crate wraps ids into a more OO-flavored API.

### `crates/ide`

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 hands out `Analysis` --- an immutable and consistent snapshot of
the 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
canceled. Only after all snapshots are dropped the change actually affects the
database.

APIs in this crate are IDE centric: they take text offsets as input and produce
offsets and strings as output. This works on top of rich code model powered by
`hir`.

### `crates/rust-analyzer`

An LSP implementation which wraps `ide` into a language server protocol.

### `crates/vfs`

Although `hir` and `ide` don't do any IO, we need to be able to read
files from disk at the end of the day. This is what `vfs` does. It also
manages overlays: "dirty" files in the editor, whose "true" contents is
different from data on disk. 

## Testing Infrastructure

Rust Analyzer has three interesting [systems
boundaries](https://www.tedinski.com/2018/04/10/making-tests-a-positive-influence-on-design.html)
to concentrate tests on.

The outermost boundary is the `rust-analyzer` crate, which defines an LSP
interface in terms of stdio. We do integration testing of this component, by
feeding it with a stream of LSP requests and checking responses. These tests are
known as "heavy", because they interact with Cargo and read real files from
disk. For this reason, we try to avoid writing too many tests on this boundary:
in a statically typed language, it's hard to make an error in the protocol
itself if messages are themselves typed.

The middle, and most important, boundary is `ide`. Unlike
`rust-analyzer`, which exposes API, `ide` uses Rust API and is intended to
use by various tools. Typical test creates an `AnalysisHost`, calls some
`Analysis` functions and compares the results against expectation.

The innermost and most elaborate boundary is `hir`. It has a much richer
vocabulary of types than `ide`, but the basic testing setup is the same: we
create a database, run some queries, assert result.

For comparisons, we use the `expect` crate for snapshot testing.

To test various analysis corner cases and avoid forgetting about old tests, we
use so-called marks. See the `marks` module in the `test_utils` crate for more.