1 files changed, 133 insertions, 0 deletions

diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
new file mode 100644
index 000000000..b497cc5d7
--- /dev/null
+++ b/ARCHITECTURE.md
@@ -0,0 +1,133 @@
+# 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!
+
+
+## Code generation
+
+Some of the components of this repository are generated through automatic
+processes. These are outlined below:
+
+- `gen-kinds`: 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-kinds` 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_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 an input.
+
+The tests for `ra_editor` are `#[cfg(test)] mod tests` unit-tests spread
+throughout its modules.
+
+### `crates/salsa`
+
+An implementation of red-green incremental compilation algorithm from
+rust compiler. It makes all rust-analyzer features on-demand. To be replaced
+with `salsa-rs/salsa` soon.
+
+
+### `crates/ra_analysis`
+
+A stateful library for analyzing many Rust files as they change.
+`AnalysisHost` is a mutable entity (clojure's atom) which holds
+current state, incorporates changes and handles out `Analysis` --- an
+immutable consistent snapshot of world state at a point in time, which
+actually powers analysis.
+
+
+### `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/cli`
+
+A CLI interface to rust-analyzer.
+
+### `crate/tools`
+
+Custom Cargo tasks used to develop rust-analyzer:
+
+- `cargo gen-kinds` -- 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`.
+