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, 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

The Big Picture

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 (TODO: actually implement this) 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 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.

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. These are outlined below:

• gen-syntax: The kinds of tokens that 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 in ra_syntax based on ast/generated.tera.rs
  • syntax_kinds/generated.rs in ra_syntax based on syntax_kinds/generated.tera.rs

Code Walk-Through

crates/ra_syntax, crates/ra_parser

Rust syntax tree structure and parser. See RFC for some design notes.

• 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 not Y". It works similarly to kotlin's parser, which is a good source of inspiration for dealing with syntax errors and incomplete input. Original libsyntax parser 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 provides 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 the design of syntax trees).

Tests 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 for an example PR which fixes a bug in the grammar.

crates/ra_db

We use the 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 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 module is responsible for guessing a HIR for a particular source position.

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

crates/ra_ide_api

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/ra_lsp_server

An LSP implementation which wraps ra_ide_api into a langauge server protocol.

ra_vfs

Although hir and ra_ide_api don't do any IO, we need to be able to read files from disk at the end of the day. This is what ra_vfs does. It also manages overlays: "dirty" files in the editor, whose "true" contents is different from data on disk. This is more or less the single really platform-dependent component, so it lives in a separate repository and has an extensive cross-platform CI testing.

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.

Testing Infrastructure

Rust Analyzer has three interesting systems boundaries to concentrate tests on.

The outermost boundary is the ra_lsp_server 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 ra_ide_api. Unlike ra_lsp_server, which exposes API, ide_api 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_api, but the basic testing setup is the same: we create a database, run some queries, assert result.

For comparisons, we use insta library 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.