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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!
+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.
+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:
+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.
+Note that the guide and videos are pretty dated, this document should be, in general, fresher.
 
-## The Big Picture
+See also these implementation-related blog posts:
+
+* https://rust-analyzer.github.io/blog/2019/11/13/find-usages.html
+* https://rust-analyzer.github.io/blog/2020/07/20/three-architectures-for-responsive-ide.html
+* https://rust-analyzer.github.io/blog/2020/09/16/challeging-LR-parsing.html
+* https://rust-analyzer.github.io/blog/2020/09/28/how-to-make-a-light-bulb.html
+* https://rust-analyzer.github.io/blog/2020/10/24/introducing-ungrammar.html
+
+## Bird's Eye View
 
 ![](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.
+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.
+This is the input (ground) state.
+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 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.
+This is derived state.
+
+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.
 
-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.
+## Code Map
 
-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.
+This section talks briefly about various important directories and data structures.
+Pay attention to the **Architecture Invariant** sections.
+They often talk about things which are deliberately absent in the source code.
 
-The underlying engine makes sure that model is computed lazily (on-demand) and
-can be quickly updated for small modifications.
+Note also which crates are **API Boundaries**.
+Remember, [rules at the boundary are different](https://www.tedinski.com/2018/02/06/system-boundaries.html).
 
+### `xtask`
 
-## Code generation
+This is rust-analyzer's "build system".
+We use cargo to compile rust code, but there are also various other tasks, like release management or local installation.
+They are handled by Rust code in the xtask directory.
 
-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.
+### `editors/code`
 
-In particular, `cargo xtask codegen` generates:
+VS Code plugin.
 
-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.
+### `libs/`
 
-2. [`ast/generated`](https://github.com/rust-analyzer/rust-analyzer/blob/a0be39296d2925972cacd9fbf8b5fb258fad6947/crates/ra_syntax/src/ast/generated.rs)
-  -- AST data structure.
+rust-analyzer independent libraries which we publish to crates.io.
+It's not heavily utilized at the moment.
 
-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.
+### `crates/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).
+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.
 
-## Code Walk-Through
+**Architecture Invariant:** the parser is independent of the particular tree structure and particular representation of the tokens.
+It transforms one flat stream of events into another flat stream of events.
+Token independence allows us to pares out both text-based source code and `tt`-based macro input.
+Tree independence allows us to more easily vary the syntax tree implementation.
+It should also unlock efficient light-parsing approaches.
+For example, you can extract the set of names defined in a file (for typo correction) without building a syntax tree.
 
-### `crates/ra_syntax`, `crates/parser`
+**Architecture Invariant:** parsing never fails, the parser produces `(T, Vec<Error>)` rather than `Result<T, Error>`.
 
-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.
+### `crates/syntax`
+
+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.
+- `ungrammar` 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.
+
+To update test data, run with `UPDATE_EXPECT` variable:
 
-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.
+```bash
+env UPDATE_EXPECT=1 cargo qt
+```
 
-Note
-[`api_walkthrough`](https://github.com/rust-analyzer/rust-analyzer/blob/2fb6af89eb794f775de60b82afe56b6f986c2a40/crates/ra_syntax/src/lib.rs#L190-L348)
+After adding a new inline test you need to run `cargo xtest codegen` and also update the test data as described above.
+
+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.
+See [#93](https://github.com/rust-analyzer/rust-analyzer/pull/93) for an example PR which fixes a bug in the grammar.
+
+**Architecture Invariant:** `syntax` crate is completely independent from the rest of rust-analyzer. It knows nothing about salsa or LSP.
+This is important because it is possible to make useful tooling using only the syntax tree.
+Without semantic information, you don't need to be able to _build_ code, which makes the tooling more robust.
+See also https://web.stanford.edu/~mlfbrown/paper.pdf.
+You can view the `syntax` crate as an entry point to rust-analyzer.
+`syntax` crate is an **API Boundary**.
+
+**Architecture Invariant:** syntax tree is a value type.
+The tree is fully determined by the contents of its syntax nodes, it doesn't need global context (like an interner) and doesn't store semantic info.
+Using the tree as a store for semantic info is convenient in traditional compilers, but doesn't work nicely in the IDE.
+Specifically, assists and refactors require transforming syntax trees, and that becomes awkward if you need to do something with the semantic info.
+
+**Architecture Invariant:** syntax tree is built for a single file.
+This is to enable parallel parsing of all files.
+
+**Architecture Invariant:**  Syntax trees are by design incomplete and do not enforce well-formedness.
+If an AST method returns an `Option`, it *can* be `None` at runtime, even if this is forbidden by 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.
+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 can also 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.
+
+**Architecture Invariant:** particularities of the build system are *not* the part of the ground state.
+In particular, `base_db` knows nothing about cargo.
+The `CrateGraph` structure is used to represent the dependencies between the crates abstractly.
+
+**Architecture Invariant:** `base_db` doesn't know about file system and file paths.
+Files are represented with opaque `FileId`, there's no operation to get an `std::path::Path` out of the `FileId`.
+
+### `crates/hir_expand`, `crates/hir_def`, `crates/hir_ty`
+
+These crates are the *brain* of rust-analyzer.
+This is the compiler part of the IDE.
+
+`hir_xxx` crates have a strong ECS flavor, in that they work with raw ids and directly query the database.
+There's little abstraction here.
+These crates integrate deeply with salsa and chalk.
+
+Name resolution, macro expansion and type inference all happen here.
+These crates also define various intermediate representations of the core.
+
+`ItemTree` condenses a single `SyntaxTree` into a "summary" data structure, which is stable over modifications to function bodies.
 
-### `crates/hir*` crates
+`DefMap` contains the module tree of a crate and stores module scopes.
 
-HIR provides high-level "object oriented" access to Rust code.
+`Body` stores information about expressions.
 
-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.
+**Architecture Invariant:** these crates are not, and will never be, an api boundary.
 
-Underneath, HIR works on top of salsa, using a `HirDatabase` trait.
+**Architecture Invariant:** these crates explicitly care about being incremental.
+The core invariant we maintain is "typing inside a function's body never invalidates global derived data".
+i.e., if you change the body of `foo`, all facts about `bar` should remain intact.
 
-`hir_xxx` crates have a strong ECS flavor, in that they work with raw ids and
-directly query the database.
+**Architecture Invariant:** hir exists only in context of particular crate instance with specific CFG flags.
+The same syntax may produce several instances of HIR if the crate participates in the crate graph more than once.
 
-The top-level `hir` façade crate wraps ids into a more OO-flavored API.
+### `crates/hir`
+
+The top-level `hir` crate is an **API Boundary**.
+If you think about "using rust-analyzer as a library", `hir` crate is most likely the façade you'll be talking to.
+
+It wraps ECS-style internal API into a more OO-flavored API (with an extra `db` argument for each call).
+
+**Architecture Invariant:** `hir` provides a static, fully resolved view of the code.
+While internal `hir_*` crates _compute_ things, `hir`, from the outside, looks like an inert data structure.
+
+`hir` also handles the delicate task of going from syntax to the corresponding `hir`.
+Remember that the mapping here is one-to-many.
+See `Semantics` type and `source_to_def` module.
+
+Note in particular a curious recursive structure in `source_to_def`.
+We first resolve the parent _syntax_ node to the parent _hir_ element.
+Then we ask the _hir_ parent what _syntax_ children does it have.
+Then we look for our node in the set of children.
+
+This is the heart of many IDE features, like goto definition, which start with figuring out the hir node at the cursor.
+This is some kind of (yet unnamed) uber-IDE pattern, as it is present in Roslyn and Kotlin as well.
 
 ### `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.
+The `ide` crate builds on top of `hir` semantic model to provide high-level IDE features like completion or goto definition.
+It is an **API Boundary**.
+If you want to use IDE parts of rust-analyzer via LSP, custom flatbuffers-based protocol or just as a library in your text editor, this is the right API.
+
+**Architecture Invariant:** `ide` crate's API is build out of POD types with public fields.
+The API uses editor's terminology, it talks about offsets and string labels rather than in terms of definitions or types.
+It is effectively the view in MVC and viewmodel in [MVVM](https://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93viewmodel).
+All arguments and return types are conceptually serializable.
+In particular, syntax tress and and hir types are generally absent from the API (but are used heavily in the implementation).
+Shout outs to LSP developers for popularizing the idea that "UI" is a good place to draw a boundary at.
+
+`ide` is also the first crate which has the notion of change over time.
+`AnalysisHost` is a state to which you can transactionally `apply_change`.
+`Analysis` is an immutable snapshot of the state.
 
-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.
+Internally, `ide` is split across several crates. `ide_assists`, `ide_completion` and `ide_ssr` implement large isolated features.
+`ide_db` implements common IDE functionality (notably, reference search is implemented here).
+The `ide` contains a public API/façade, as well as implementation for a plethora of smaller features.
 
-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`.
+**Architecture Invariant:** `ide` crate strives to provide a _perfect_ API.
+Although at the moment it has only one consumer, the LSP server, LSP *does not* influence it's API design.
+Instead, we keep in mind a hypothetical _ideal_ client -- an IDE tailored specifically for rust, every nook and cranny of which is packed with Rust-specific goodies.
 
 ### `crates/rust-analyzer`
 
-An LSP implementation which wraps `ide` into a language server protocol.
+This crate defines the `rust-analyzer` binary, so it is the **entry point**.
+It implements the language server.
+
+**Architecture Invariant:** `rust-analyzer` is the only crate that knows about LSP and JSON serialization.
+If you want to expose a datastructure `X` from ide to LSP, don't make it serializable.
+Instead, create a serializable counterpart in `rust-analyzer` crate and manually convert between the two.
+
+`GlobalState` is the state of the server.
+The `main_loop` defines the server event loop which accepts requests and sends responses.
+Requests that modify the state or might block user's typing are handled on the main thread.
+All other requests are processed in background.
+
+**Architecture Invariant:** the server is stateless, a-la HTTP.
+Sometimes state needs to be preserved between requests.
+For example, "what is the `edit` for the fifth completion item of the last completion edit?".
+For this, the second request should include enough info to re-create the context from scratch.
+This generally means including all the parameters of the original request.
+
+`reload` module contains the code that handles configuration and Cargo.toml changes.
+This is a tricky business.
+
+**Architecture Invariant:** `rust-analyzer` should be partially available even when the build is broken.
+Reloading process should not prevent IDE features from working.
+
+### `crates/toolchain`, `crates/project_model`, `crates/flycheck`
+
+These crates deal with invoking `cargo` to learn about project structure and get compiler errors for the "check on save" feature.
+
+They use `crates/path` heavily instead of `std::path`.
+A single `rust-analyzer` process can serve many projects, so it is important that server's current directory does not leak.
+
+### `crates/mbe`, `crates/tt`, `crates/proc_macro_api`, `crates/proc_macro_srv`
+
+These crates implement macros as token tree -> token tree transforms.
+They are independent from the rest of the code.
+
+### `crates/cfg`
+
+This crate is responsible for parsing, evaluation and general definition of `cfg` attributes.
 
-### `crates/vfs`
+### `crates/vfs`, `crates/vfs-notify`
 
-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. 
+These crates implement a virtual file system.
+They provide consistent snapshots of the underlying file system and insulate messy OS paths.
 
-## Testing Infrastructure
+**Architecture Invariant:** vfs doesn't assume a single unified file system.
+i.e., a single rust-analyzer process can act as a remote server for two different machines, where the same `/tmp/foo.rs` path points to different files.
+For this reason, all path APIs generally take some existing path as a "file system witness".
 
-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.
+### `crates/stdx`
 
-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.
+This crate contains various non-rust-analyzer specific utils, which could have been in std, as well
+as copies of unstable std items we would like to make use of already, like `std::str::split_once`.
 
-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.
+### `crates/profile`
 
-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.
+This crate contains utilities for CPU and memory profiling.
+
+
+## Cross-Cutting Concerns
+
+This sections talks about the things which are everywhere and nowhere in particular.
+
+### Code generation
+
+Some of the components of this repository are generated through automatic processes.
+`cargo xtask codegen` runs all generation tasks.
+Generated code is generally committed to the git repository.
+There are tests to check that the generated code is fresh.
+
+In particular, we generate:
+
+* API for working with syntax trees (`syntax::ast`, the [`ungrammar`](https://github.com/rust-analyzer/ungrammar) crate).
+* Various sections of the manual:
+
+    * features
+    * assists
+    * config
+
+* Documentation tests for assists
+
+**Architecture Invariant:** we avoid bootstrapping.
+For codegen we need to parse Rust code.
+Using rust-analyzer for that would work and would be fun, but it would also complicate the build process a lot.
+For that reason, we use syn and manual string parsing.
+
+### Cancellation
+
+Let's say that the IDE is in the process of computing syntax highlighting, when the user types `foo`.
+What should happen?
+`rust-analyzer`s answer is that the highlighting process should be cancelled -- its results are now stale, and it also blocks modification of the inputs.
+
+The salsa database maintains a global revision counter.
+When applying a change, salsa bumps this counter and waits until all other threads using salsa finish.
+If a thread does salsa-based computation and notices that the counter is incremented, it panics with a special value (see `Canceled::throw`).
+That is, rust-analyzer requires unwinding.
+
+`ide` is the boundary where the panic is caught and transformed into a `Result<T, Cancelled>`.
+
+### Testing
+
+Rust Analyzer has three interesting [system 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.
+Heavy tests are only run when `RUN_SLOW_TESTS` env var is set.
+
+The middle, and most important, boundary is `ide`.
+Unlike `rust-analyzer`, which exposes API, `ide` uses Rust API and is intended for use by various tools.
+A 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.
+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.
+
+**Architecture Invariant:** rust-analyzer tests do not use libcore or libstd.
+All required library code must be a part of the tests.
+This ensures fast test execution.
+
+**Architecture Invariant:** tests are data driven and do not test the API.
+Tests which directly call various API functions are a liability, because they make refactoring the API significantly more complicated.
+So most of the tests look like this:
+
+```rust
+fn check(input: &str, expect: expect_test::Expect) {
+    // The single place that actually exercises a particular API
+}
+
+
+#[test]
+fn foo() {
+    check("foo", expect![["bar"]]);
+}
+
+#[test]
+fn spam() {
+    check("spam", expect![["eggs"]]);
+}
+// ...and a hundred more tests that don't care about the specific API at all.
+```
+
+To specify input data, we use a single string literal in a special format, which can describe a set of rust files.
+See the `Fixture` type.
+
+**Architecture Invariant:** all code invariants are tested by `#[test]` tests.
+There's no additional checks in CI, formatting and tidy tests are run with `cargo test`.
+
+**Architecture Invariant:** tests do not depend on any kind of external resources, they are perfectly reproducible.
+
+### Error Handling
+
+**Architecture Invariant:** core parts of rust-analyzer (`ide`/`hir`) don't interact with the outside world and thus can't fail.
+Only parts touching LSP are allowed to do IO.
+
+Internals of rust-analyzer need to deal with broken code, but this is not an error condition.
+rust-analyzer is robust: various analysis compute `(T, Vec<Error>)` rather than `Result<T, Error>`.
+
+rust-analyzer is a complex long-running process.
+It will always have bugs and panics.
+But a panic in an isolated feature should not bring down the whole process.
+Each LSP-request is protected by a `catch_unwind`.
+We use `always` and `never` macros instead of `assert` to gracefully recover from impossible conditions.
+
+### Observability
+
+rust-analyzer is a long-running process, so its important to understand what's going on inside.
+We have several instruments for that.
+
+The event loop that runs rust-analyzer is very explicit.
+Rather than spawning futures or scheduling callbacks (open), the event loop accepts an `enum` of possible events (closed).
+It's easy to see all the things that trigger rust-analyzer processing, together with their performance
+
+rust-analyzer includes a simple hierarchical profiler (`hprof`).
+It is enabled with `RA_PROFILE='*>50` env var (log all (`*`) actions which take more than `50` ms) and produces output like:
+
+```
+85ms - handle_completion
+    68ms - import_on_the_fly
+        67ms - import_assets::search_for_relative_paths
+             0ms - crate_def_map:wait (804 calls)
+             0ms - find_path (16 calls)
+             2ms - find_similar_imports (1 calls)
+             0ms - generic_params_query (334 calls)
+            59ms - trait_solve_query (186 calls)
+         0ms - Semantics::analyze_impl (1 calls)
+         1ms - render_resolution (8 calls)
+     0ms - Semantics::analyze_impl (5 calls)
+```
+
+This is cheap enough to enable in production.
+
+
+Similarly, we save live object counting (`RA_COUNT=1`).
+It is not cheap enough to enable in prod, and this is a bug which should be fixed.