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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-analyzer/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 works, 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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_ide_api/src/lib.rs#L265-L284
-[`Analysis`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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 `Cancelable<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(Canceled)`),
-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
-`AnalysisChange`. [`AnalysisChange`] is a builder for a single change
-"transaction", so it suffices to study its methods to understand all of the
-input data.
-[`AnalysisChange`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_ide_api/src/lib.rs#L119-L167
-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 unites -- 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. on macOS)
-* paths are not necessary UTF-8
-* symlinks can form cycles
-Second, this might hurt 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 necessary 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 is 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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L51-L110
-[`main_loop_inner`]: https://github.com/rust-analyzer/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-analyzer/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 `AnalysisChange`. 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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/project_model.rs#L16-L20
-[`ServerWorldState::new`]: https://github.com/rust-analyzer/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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L194)
-* [Changes from the editor](https://github.com/rust-analyzer/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 `AnalysisChange` and
-[apply] it. This always happens on the main thread and blocks the loop.
-[apply]: https://github.com/rust-analyzer/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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/server_world.rs#L216
-[`schedule`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L426-L455
-[The task]: https://github.com/rust-analyzer/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, lets
-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 is 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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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 `AnalysisChange` structure:
-indeed, what `apply_change` does is it sets the values of input queries.
-[`FilesDatabase`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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 isn't a one-to-one mapping between 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 be included as a module into
-several crate, 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 position into a semantic model is inherently imprecise for
-this reason, and is handled by the [`source_binder`].
-[`source_binder`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/code_model_api.rs
-[the list of queries]: https://github.com/rust-analyzer/rust-analyzer/blob/7e84440e25e19529e4ff8a66e521d1b06349c6ec/crates/ra_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-analyzer/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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_db/src/loc2id.rs#L65-L71
-[interners]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/nameres/lower.rs#L113-L117
-[loop]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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
-an 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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/nameres/lower.rs#L52-L59
-[`SourceMap`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/nameres/lower.rs#L52-L59
-[projection query]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/nameres/lower.rs#L97-L103
-[uses]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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-analyzer/rust-analyzer/pull/327
-[lower the AST]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/expr.rs
-[positional ID]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/expr.rs#L13-L15
-[a source map]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/expr.rs#L41-L44
-[type inference]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_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 also place where we [catch] canceled errors if, immediately after completion, the
-client sends some modification.
-In [the handler] we a deserialize LSP request into the rust-analyzer specific data
-types (by converting a file url into a numeric `FileId`), [ask analysis for
-completion] and serializer results to 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
-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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L203
-[schedule it on the threadpool]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_lsp_server/src/main_loop.rs#L428
-[catch]: https://github.com/rust-analyzer/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-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_ide_api/src/lib.rs#L439-L444
-[completion implementation]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_ide_api/src/completion.rs#L46-L62
-[`CompletionContext`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_ide_api/src/completion/completion_context.rs#L14-L37
-["IntelliJ Trick"]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_ide_api/src/completion/completion_context.rs#L72-L75
-[find an ancestor `fn` node]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_ide_api/src/completion/completion_context.rs#L116-L120
-[semantic model]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_ide_api/src/completion/completion_context.rs#L123
-[series of independent completion routines]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_ide_api/src/completion.rs#L52-L59
-[`complete_dot`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_ide_api/src/completion/complete_dot.rs#L6-L22