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+# 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
+
+## 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 (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][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`, `crates/ra_parser`
+
+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 parser, which
+  produces a sequence of events like "start node X", "finish not 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.
+- `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](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 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_ide_api_light`
+
+All IDE features which can be implemented if you only have access to a single
+file. `ra_ide_api_light` 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_ide_api_light` is just a bunch of pure functions which take a
+syntax tree as input.
+
+The tests for `ra_ide_api_light` are `#[cfg(test)] mod tests` unit-tests spread
+throughout its modules.
+
+
+### `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
+
+