Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+# Guide to rust-analyzer
+
+## About the guide
+
+This guide describes the current state of rust-analyzer as of 2019-01-20 (git
+tag [guide-2019-01]). Its purpose is to document various problems and
+architectural solutions related to the problem of building IDE-first compiler
+for Rust. There is a video version of this guide as well:
+https://youtu.be/ANKBNiSWyfc.
+
+[guide-2019-01]: https://github.com/rust-lang/rust-analyzer/tree/guide-2019-01
+
+## The big picture
+
+On the highest possible level, rust-analyzer is a stateful component. A client may
+apply changes to the analyzer (new contents of `foo.rs` file is "fn main() {}")
+and it may ask semantic questions about the current state (what is the
+definition of the identifier with offset 92 in file `bar.rs`?). Two important
+properties hold:
+
+* Analyzer does not do any I/O. It starts in an empty state and all input data is
+  provided via `apply_change` API.
+
+* Only queries about the current state are supported. One can, of course,
+  simulate undo and redo by keeping a log of changes and inverse changes respectively.
+
+## IDE API
+
+To see the bigger picture of how the IDE features work, let's take a look at the [`AnalysisHost`] and
+[`Analysis`] pair of types. `AnalysisHost` has three methods:
+
+* `default()` for creating an empty analysis instance
+* `apply_change(&mut self)` to make changes (this is how you get from an empty
+  state to something interesting)
+* `analysis(&self)` to get an instance of `Analysis`
+
+`Analysis` has a ton of methods for IDEs, like `goto_definition`, or
+`completions`. Both inputs and outputs of `Analysis`' methods are formulated in
+terms of files and offsets, and **not** in terms of Rust concepts like structs,
+traits, etc. The "typed" API with Rust specific types is slightly lower in the
+stack, we'll talk about it later.
+
+[`AnalysisHost`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ide_api/src/lib.rs#L265-L284
+[`Analysis`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ide_api/src/lib.rs#L291-L478
+
+The reason for this separation of `Analysis` and `AnalysisHost` is that we want to apply
+changes "uniquely", but we might also want to fork an `Analysis` and send it to
+another thread for background processing. That is, there is only a single
+`AnalysisHost`, but there may be several (equivalent) `Analysis`.
+
+Note that all of the `Analysis` API return `Cancellable<T>`. This is required to
+be responsive in an IDE setting. Sometimes a long-running query is being computed
+and the user types something in the editor and asks for completion. In this
+case, we cancel the long-running computation (so it returns `Err(Cancelled)`),
+apply the change and execute request for completion. We never use stale data to
+answer requests. Under the cover, `AnalysisHost` "remembers" all outstanding
+`Analysis` instances. The `AnalysisHost::apply_change` method cancels all
+`Analysis`es, blocks until all of them are `Dropped` and then applies changes
+in-place. This may be familiar to Rustaceans who use read-write locks for interior
+mutability.
+
+Next, let's talk about what the inputs to the `Analysis` are, precisely.
+
+## Inputs
+
+Rust Analyzer never does any I/O itself, all inputs get passed explicitly via
+the `AnalysisHost::apply_change` method, which accepts a single argument, a
+`Change`. [`Change`] is a builder for a single change
+"transaction", so it suffices to study its methods to understand all of the
+input data.
+
+[`Change`]: https://github.com/rust-lang/rust-analyzer/blob/master/crates/base_db/src/change.rs#L14-L89
+
+The `(add|change|remove)_file` methods control the set of the input files, where
+each file has an integer id (`FileId`, picked by the client), text (`String`)
+and a filesystem path. Paths are tricky; they'll be explained below, in source roots
+section, together with the `add_root` method. The `add_library` method allows us to add a
+group of files which are assumed to rarely change. It's mostly an optimization
+and does not change the fundamental picture.
+
+The `set_crate_graph` method allows us to control how the input files are partitioned
+into compilation units -- crates. It also controls (in theory, not implemented
+yet) `cfg` flags. `CrateGraph` is a directed acyclic graph of crates. Each crate
+has a root `FileId`, a set of active `cfg` flags and a set of dependencies. Each
+dependency is a pair of a crate and a name. It is possible to have two crates
+with the same root `FileId` but different `cfg`-flags/dependencies. This model
+is lower than Cargo's model of packages: each Cargo package consists of several
+targets, each of which is a separate crate (or several crates, if you try
+different feature combinations).
+
+Procedural macros should become inputs as well, but currently they are not
+supported. Procedural macro will be a black box `Box<dyn Fn(TokenStream) -> TokenStream>`
+function, and will be inserted into the crate graph just like dependencies.
+
+Soon we'll talk how we build an LSP server on top of `Analysis`, but first,
+let's deal with that paths issue.
+
+## Source roots (a.k.a. "Filesystems are horrible")
+
+This is a non-essential section, feel free to skip.
+
+The previous section said that the filesystem path is an attribute of a file,
+but this is not the whole truth. Making it an absolute `PathBuf` will be bad for
+several reasons. First, filesystems are full of (platform-dependent) edge cases:
+
+* It's hard (requires a syscall) to decide if two paths are equivalent.
+* Some filesystems are case-sensitive (e.g. macOS).
+* Paths are not necessarily UTF-8.
+* Symlinks can form cycles.
+
+Second, this might hurt the reproducibility and hermeticity of builds. In theory,
+moving a project from `/foo/bar/my-project` to `/spam/eggs/my-project` should
+not change a bit in the output. However, if the absolute path is a part of the
+input, it is at least in theory observable, and *could* affect the output.
+
+Yet another problem is that we really *really* want to avoid doing I/O, but with
+Rust the set of "input" files is not necessarily known up-front. In theory, you
+can have `#[path="/dev/random"] mod foo;`.
+
+To solve (or explicitly refuse to solve) these problems rust-analyzer uses the
+concept of a "source root". Roughly speaking, source roots are the contents of a
+directory on a file systems, like `/home/matklad/projects/rustraytracer/**.rs`.
+
+More precisely, all files (`FileId`s) are partitioned into disjoint
+`SourceRoot`s. Each file has a relative UTF-8 path within the `SourceRoot`.
+`SourceRoot` has an identity (integer ID). Crucially, the root path of the
+source root itself is unknown to the analyzer: A client is supposed to maintain a
+mapping between `SourceRoot` IDs (which are assigned by the client) and actual
+`PathBuf`s. `SourceRoot`s give a sane tree model of the file system to the
+analyzer.
+
+Note that `mod`, `#[path]` and `include!()` can only reference files from the
+same source root. It is of course possible to explicitly add extra files to
+the source root, even `/dev/random`.
+
+## Language Server Protocol
+
+Now let's see how the `Analysis` API is exposed via the JSON RPC based language server protocol. The
+hard part here is managing changes (which can come either from the file system
+or from the editor) and concurrency (we want to spawn background jobs for things
+like syntax highlighting). We use the event loop pattern to manage the zoo, and
+the loop is the [`main_loop_inner`] function. The [`main_loop`] does a one-time
+initialization and tearing down of the resources.
+
+[`main_loop`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L51-L110
+[`main_loop_inner`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L156-L258
+
+
+Let's walk through a typical analyzer session!
+
+First, we need to figure out what to analyze. To do this, we run `cargo
+metadata` to learn about Cargo packages for current workspace and dependencies,
+and we run `rustc --print sysroot` and scan the "sysroot" (the directory containing the current Rust toolchain's files) to learn about crates like
+`std`. Currently we load this configuration once at the start of the server, but
+it should be possible to dynamically reconfigure it later without restart.
+
+[main_loop.rs#L62-L70](https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L62-L70)
+
+The [`ProjectModel`] we get after this step is very Cargo and sysroot specific,
+it needs to be lowered to get the input in the form of `Change`. This
+happens in [`ServerWorldState::new`] method. Specifically
+
+* Create a `SourceRoot` for each Cargo package and sysroot.
+* Schedule a filesystem scan of the roots.
+* Create an analyzer's `Crate` for each Cargo **target** and sysroot crate.
+* Setup dependencies between the crates.
+
+[`ProjectModel`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/project_model.rs#L16-L20
+[`ServerWorldState::new`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/server_world.rs#L38-L160
+
+The results of the scan (which may take a while) will be processed in the body
+of the main loop, just like any other change. Here's where we handle:
+
+* [File system changes](https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L194)
+* [Changes from the editor](https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L377)
+
+After a single loop's turn, we group the changes into one `Change` and
+[apply] it. This always happens on the main thread and blocks the loop.
+
+[apply]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/server_world.rs#L216
+
+To handle requests, like ["goto definition"], we create an instance of the
+`Analysis` and [`schedule`] the task (which consumes `Analysis`) on the
+threadpool. [The task] calls the corresponding `Analysis` method, while
+massaging the types into the LSP representation. Keep in mind that if we are
+executing "goto definition" on the threadpool and a new change comes in, the
+task will be canceled as soon as the main loop calls `apply_change` on the
+`AnalysisHost`.
+
+["goto definition"]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/server_world.rs#L216
+[`schedule`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L426-L455
+[The task]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop/handlers.rs#L205-L223
+
+This concludes the overview of the analyzer's programing *interface*. Next, let's
+dig into the implementation!
+
+## Salsa
+
+The most straightforward way to implement an "apply change, get analysis, repeat"
+API would be to maintain the input state and to compute all possible analysis
+information from scratch after every change. This works, but scales poorly with
+the size of the project. To make this fast, we need to take advantage of the
+fact that most of the changes are small, and that analysis results are unlikely
+to change significantly between invocations.
+
+To do this we use [salsa]: a framework for incremental on-demand computation.
+You can skip the rest of the section if you are familiar with `rustc`'s red-green
+algorithm (which is used for incremental compilation).
+
+[salsa]: https://github.com/salsa-rs/salsa
+
+It's better to refer to salsa's docs to learn about it. Here's a small excerpt:
+
+The key idea of salsa is that you define your program as a set of queries. Every
+query is used like a function `K -> V` that maps from some key of type `K` to a value
+of type `V`. Queries come in two basic varieties:
+
+* **Inputs**: the base inputs to your system. You can change these whenever you
+  like.
+
+* **Functions**: pure functions (no side effects) that transform your inputs
+  into other values. The results of queries are memoized to avoid recomputing
+  them a lot. When you make changes to the inputs, we'll figure out (fairly
+  intelligently) when we can re-use these memoized values and when we have to
+  recompute them.
+
+For further discussion, its important to understand one bit of "fairly
+intelligently". Suppose we have two functions, `f1` and `f2`, and one input,
+`z`. We call `f1(X)` which in turn calls `f2(Y)` which inspects `i(Z)`. `i(Z)`
+returns some value `V1`, `f2` uses that and returns `R1`, `f1` uses that and
+returns `O`. Now, let's change `i` at `Z` to `V2` from `V1` and try to compute
+`f1(X)` again. Because `f1(X)` (transitively) depends on `i(Z)`, we can't just
+reuse its value as is. However, if `f2(Y)` is *still* equal to `R1` (despite
+`i`'s change), we, in fact, *can* reuse `O` as result of `f1(X)`. And that's how
+salsa works: it recomputes results in *reverse* order, starting from inputs and
+progressing towards outputs, stopping as soon as it sees an intermediate value
+that hasn't changed. If this sounds confusing to you, don't worry: it is
+confusing. This illustration by @killercup might help:
+
+<img alt="step 1" src="https://user-images.githubusercontent.com/1711539/51460907-c5484780-1d6d-11e9-9cd2-d6f62bd746e0.png" width="50%">
+
+<img alt="step 2" src="https://user-images.githubusercontent.com/1711539/51460915-c9746500-1d6d-11e9-9a77-27d33a0c51b5.png" width="50%">
+
+<img alt="step 3" src="https://user-images.githubusercontent.com/1711539/51460920-cda08280-1d6d-11e9-8d96-a782aa57a4d4.png" width="50%">
+
+<img alt="step 4" src="https://user-images.githubusercontent.com/1711539/51460927-d1340980-1d6d-11e9-851e-13c149d5c406.png" width="50%">
+
+## Salsa Input Queries
+
+All analyzer information is stored in a salsa database. `Analysis` and
+`AnalysisHost` types are newtype wrappers for [`RootDatabase`] -- a salsa
+database.
+
+[`RootDatabase`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ide_api/src/db.rs#L88-L134
+
+Salsa input queries are defined in [`FilesDatabase`] (which is a part of
+`RootDatabase`). They closely mirror the familiar `Change` structure:
+indeed, what `apply_change` does is it sets the values of input queries.
+
+[`FilesDatabase`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/base_db/src/input.rs#L150-L174
+
+## From text to semantic model
+
+The bulk of the rust-analyzer is transforming input text into a semantic model of
+Rust code: a web of entities like modules, structs, functions and traits.
+
+An important fact to realize is that (unlike most other languages like C# or
+Java) there is not a one-to-one mapping between the source code and the semantic model. A
+single function definition in the source code might result in several semantic
+functions: for example, the same source file might get included as a module in
+several crates or a single crate might be present in the compilation DAG
+several times, with different sets of `cfg`s enabled. The IDE-specific task of
+mapping source code into a semantic model is inherently imprecise for
+this reason and gets handled by the [`source_binder`].
+
+[`source_binder`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/source_binder.rs
+
+The semantic interface is declared in the [`code_model_api`] module. Each entity is
+identified by an integer ID and has a bunch of methods which take a salsa database
+as an argument and returns other entities (which are also IDs). Internally, these
+methods invoke various queries on the database to build the model on demand.
+Here's [the list of queries].
+
+[`code_model_api`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/code_model_api.rs
+[the list of queries]: https://github.com/rust-lang/rust-analyzer/blob/7e84440e25e19529e4ff8a66e521d1b06349c6ec/crates/hir/src/db.rs#L20-L106
+
+The first step of building the model is parsing the source code.
+
+## Syntax trees
+
+An important property of the Rust language is that each file can be parsed in
+isolation. Unlike, say, `C++`, an `include` can't change the meaning of the
+syntax. For this reason, rust-analyzer can build a syntax tree for each "source
+file", which could then be reused by several semantic models if this file
+happens to be a part of several crates.
+
+The representation of syntax trees that rust-analyzer uses is similar to that of `Roslyn`
+and Swift's new [libsyntax]. Swift's docs give an excellent overview of the
+approach, so I skip this part here and instead outline the main characteristics
+of the syntax trees:
+
+* Syntax trees are fully lossless. Converting **any** text to a syntax tree and
+  back is a total identity function. All whitespace and comments are explicitly
+  represented in the tree.
+
+* Syntax nodes have generic `(next|previous)_sibling`, `parent`,
+  `(first|last)_child` functions. You can get from any one node to any other
+  node in the file using only these functions.
+
+* Syntax nodes know their range (start offset and length) in the file.
+
+* Syntax nodes share the ownership of their syntax tree: if you keep a reference
+  to a single function, the whole enclosing file is alive.
+
+* Syntax trees are immutable and the cost of replacing the subtree is
+  proportional to the depth of the subtree. Read Swift's docs to learn how
+  immutable + parent pointers + cheap modification is possible.
+
+* Syntax trees are build on best-effort basis. All accessor methods return
+  `Option`s. The tree for `fn foo` will contain a function declaration with
+  `None` for parameter list and body.
+
+* Syntax trees do not know the file they are built from, they only know about
+  the text.
+
+The implementation is based on the generic [rowan] crate on top of which a
+[rust-specific] AST is generated.
+
+[libsyntax]: https://github.com/apple/swift/tree/5e2c815edfd758f9b1309ce07bfc01c4bc20ec23/lib/Syntax
+[rowan]: https://github.com/rust-analyzer/rowan/tree/100a36dc820eb393b74abe0d20ddf99077b61f88
+[rust-specific]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_syntax/src/ast/generated.rs
+
+The next step in constructing the semantic model is ...
+
+## Building a Module Tree
+
+The algorithm for building a tree of modules is to start with a crate root
+(remember, each `Crate` from a `CrateGraph` has a `FileId`), collect all `mod`
+declarations and recursively process child modules. This is handled by the
+[`module_tree_query`], with two slight variations.
+
+[`module_tree_query`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/module_tree.rs#L116-L123
+
+First, rust-analyzer builds a module tree for all crates in a source root
+simultaneously. The main reason for this is historical (`module_tree` predates
+`CrateGraph`), but this approach also enables accounting for files which are not
+part of any crate. That is, if you create a file but do not include it as a
+submodule anywhere, you still get semantic completion, and you get a warning
+about a free-floating module (the actual warning is not implemented yet).
+
+The second difference is that `module_tree_query` does not *directly* depend on
+the "parse" query (which is confusingly called `source_file`). Why would calling
+the parse directly be bad? Suppose the user changes the file slightly, by adding
+an insignificant whitespace. Adding whitespace changes the parse tree (because
+it includes whitespace), and that means recomputing the whole module tree.
+
+We deal with this problem by introducing an intermediate [`submodules_query`].
+This query processes the syntax tree and extracts a set of declared submodule
+names. Now, changing the whitespace results in `submodules_query` being
+re-executed for a *single* module, but because the result of this query stays
+the same, we don't have to re-execute [`module_tree_query`]. In fact, we only
+need to re-execute it when we add/remove new files or when we change mod
+declarations.
+
+[`submodules_query`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/module_tree.rs#L41
+
+We store the resulting modules in a `Vec`-based indexed arena. The indices in
+the arena becomes module IDs. And this brings us to the next topic:
+assigning IDs in the general case.
+
+## Location Interner pattern
+
+One way to assign IDs is how we've dealt with modules: Collect all items into a
+single array in some specific order and use the index in the array as an ID. The
+main drawback of this approach is that these IDs are not stable: Adding a new item can
+shift the IDs of all other items. This works for modules, because adding a module is
+a comparatively rare operation, but would be less convenient for, for example,
+functions.
+
+Another solution here is positional IDs: We can identify a function as "the
+function with name `foo` in a ModuleId(92) module". Such locations are stable:
+adding a new function to the module (unless it is also named `foo`) does not
+change the location. However, such "ID" types ceases to be a `Copy`able integer and in
+general can become pretty large if we account for nesting (for example: "third parameter of
+the `foo` function of the `bar` `impl` in the `baz` module").
+
+[`LocationInterner`] allows us to combine the benefits of positional and numeric
+IDs. It is a bidirectional append-only map between locations and consecutive
+integers which can "intern" a location and return an integer ID back. The salsa
+database we use includes a couple of [interners]. How to "garbage collect"
+unused locations is an open question.
+
+[`LocationInterner`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/base_db/src/loc2id.rs#L65-L71
+[interners]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/db.rs#L22-L23
+
+For example, we use `LocationInterner` to assign IDs to definitions of functions,
+structs, enums, etc. The location, [`DefLoc`] contains two bits of information:
+
+* the ID of the module which contains the definition,
+* the ID of the specific item in the modules source code.
+
+We "could" use a text offset for the location of a particular item, but that would play
+badly with salsa: offsets change after edits. So, as a rule of thumb, we avoid
+using offsets, text ranges or syntax trees as keys and values for queries. What
+we do instead is we store "index" of the item among all of the items of a file
+(so, a positional based ID, but localized to a single file).
+
+[`DefLoc`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/ids.rs#L127-L139
+
+One thing we've glossed over for the time being is support for macros. We have
+only proof of concept handling of macros at the moment, but they are extremely
+interesting from an "assigning IDs" perspective.
+
+## Macros and recursive locations
+
+The tricky bit about macros is that they effectively create new source files.
+While we can use `FileId`s to refer to original files, we can't just assign them
+willy-nilly to the pseudo files of macro expansion. Instead, we use a special
+ID, [`HirFileId`] to refer to either a usual file or a macro-generated file:
+
+```rust
+enum HirFileId {
+    FileId(FileId),
+    Macro(MacroCallId),
+}
+```
+
+`MacroCallId` is an interned ID that specifies a particular macro invocation.
+Its `MacroCallLoc` contains:
+
+* `ModuleId` of the containing module
+* `HirFileId` of the containing file or pseudo file
+* an index of this particular macro invocation in this file (positional id
+  again).
+
+Note how `HirFileId` is defined in terms of `MacroCallLoc` which is defined in
+terms of `HirFileId`! This does not recur infinitely though: any chain of
+`HirFileId`s bottoms out in `HirFileId::FileId`, that is, some source file
+actually written by the user.
+
+[`HirFileId`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/ids.rs#L18-L125
+
+Now that we understand how to identify a definition, in a source or in a
+macro-generated file, we can discuss name resolution a bit.
+
+## Name resolution
+
+Name resolution faces the same problem as the module tree: if we look at the
+syntax tree directly, we'll have to recompute name resolution after every
+modification. The solution to the problem is the same: We [lower] the source code of
+each module into a position-independent representation which does not change if
+we modify bodies of the items. After that we [loop] resolving all imports until
+we've reached a fixed point.
+
+[lower]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/nameres/lower.rs#L113-L117
+[loop]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/nameres.rs#L186-L196
+
+And, given all our preparation with IDs and a position-independent representation,
+it is satisfying to [test] that typing inside function body does not invalidate
+name resolution results.
+
+[test]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/nameres/tests.rs#L376
+
+An interesting fact about name resolution is that it "erases" all of the
+intermediate paths from the imports: in the end, we know which items are defined
+and which items are imported in each module, but, if the import was `use
+foo::bar::baz`, we deliberately forget what modules `foo` and `bar` resolve to.
+
+To serve "goto definition" requests on intermediate segments we need this info
+in the IDE, however. Luckily, we need it only for a tiny fraction of imports, so we just ask
+the module explicitly, "What does the path `foo::bar` resolve to?". This is a
+general pattern: we try to compute the minimal possible amount of information
+during analysis while allowing IDE to ask for additional specific bits.
+
+Name resolution is also a good place to introduce another salsa pattern used
+throughout the analyzer:
+
+## Source Map pattern
+
+Due to an obscure edge case in completion, IDE needs to know the syntax node of
+a use statement which imported the given completion candidate. We can't just
+store the syntax node as a part of name resolution: this will break
+incrementality, due to the fact that syntax changes after every file
+modification.
+
+We solve this problem during the lowering step of name resolution. The lowering
+query actually produces a *pair* of outputs: `LoweredModule` and [`SourceMap`].
+The `LoweredModule` module contains [imports], but in a position-independent form.
+The `SourceMap` contains a mapping from position-independent imports to
+(position-dependent) syntax nodes.
+
+The result of this basic lowering query changes after every modification. But
+there's an intermediate [projection query] which returns only the first
+position-independent part of the lowering. The result of this query is stable.
+Naturally, name resolution [uses] this stable projection query.
+
+[imports]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/nameres/lower.rs#L52-L59
+[`SourceMap`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/nameres/lower.rs#L52-L59
+[projection query]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/nameres/lower.rs#L97-L103
+[uses]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/query_definitions.rs#L49
+
+## Type inference
+
+First of all, implementation of type inference in rust-analyzer was spearheaded
+by [@flodiebold]. [#327] was an awesome Christmas present, thank you, Florian!
+
+Type inference runs on per-function granularity and uses the patterns we've
+discussed previously.
+
+First, we [lower the AST] of a function body into a position-independent
+representation. In this representation, each expression is assigned a
+[positional ID]. Alongside the lowered expression, [a source map] is produced,
+which maps between expression ids and original syntax. This lowering step also
+deals with "incomplete" source trees by replacing missing expressions by an
+explicit `Missing` expression.
+
+Given the lowered body of the function, we can now run [type inference] and
+construct a mapping from `ExprId`s to types.
+
+[@flodiebold]: https://github.com/flodiebold
+[#327]: https://github.com/rust-lang/rust-analyzer/pull/327
+[lower the AST]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/expr.rs
+[positional ID]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/expr.rs#L13-L15
+[a source map]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/expr.rs#L41-L44
+[type inference]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/hir/src/ty.rs#L1208-L1223
+
+## Tying it all together: completion
+
+To conclude the overview of the rust-analyzer, let's trace the request for
+(type-inference powered!) code completion!
+
+We start by [receiving a message] from the language client. We decode the
+message as a request for completion and [schedule it on the threadpool]. This is
+the place where we [catch] canceled errors if, immediately after completion, the
+client sends some modification.
+
+In [the handler], we deserialize LSP requests into rust-analyzer specific data
+types (by converting a file url into a numeric `FileId`), [ask analysis for
+completion] and serialize results into the LSP.
+
+The [completion implementation] is finally the place where we start doing the actual
+work. The first step is to collect the `CompletionContext` -- a struct which
+describes the cursor position in terms of Rust syntax and semantics. For
+example, `function_syntax: Option<&'a ast::FnDef>` stores a reference to
+the enclosing function *syntax*, while `function: Option<hir::Function>` is the
+`Def` for this function.
+
+To construct the context, we first do an ["IntelliJ Trick"]: we insert a dummy
+identifier at the cursor's position and parse this modified file, to get a
+reasonably looking syntax tree. Then we do a bunch of "classification" routines
+to figure out the context. For example, we [find an ancestor `fn` node] and we get a
+[semantic model] for it (using the lossy `source_binder` infrastructure).
+
+The second step is to run a [series of independent completion routines]. Let's
+take a closer look at [`complete_dot`], which completes fields and methods in
+`foo.bar|`. First we extract a semantic function and a syntactic receiver
+expression out of the `Context`. Then we run type-inference for this single
+function and map our syntactic expression to `ExprId`. Using the ID, we figure
+out the type of the receiver expression. Then we add all fields & methods from
+the type to completion.
+
+[receiving a message]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L203
+[schedule it on the threadpool]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L428
+[catch]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L436-L442
+[the handler]: https://salsa.zulipchat.com/#narrow/stream/181542-rfcs.2Fsalsa-query-group/topic/design.20next.20steps
+[ask analysis for completion]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ide_api/src/lib.rs#L439-L444
+[completion implementation]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ide_api/src/completion.rs#L46-L62
+[`CompletionContext`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ide_api/src/completion/completion_context.rs#L14-L37
+["IntelliJ Trick"]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ide_api/src/completion/completion_context.rs#L72-L75
+[find an ancestor `fn` node]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ide_api/src/completion/completion_context.rs#L116-L120
+[semantic model]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ide_api/src/completion/completion_context.rs#L123
+[series of independent completion routines]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ide_api/src/completion.rs#L52-L59
+[`complete_dot`]: https://github.com/rust-lang/rust-analyzer/blob/guide-2019-01/crates/ide_api/src/completion/complete_dot.rs#L6-L22