rust/docs/dev/lsp-extensions.md

# LSP Extensions

This document describes LSP extensions used by rust-analyzer.
It's a best effort document, when in doubt, consult the source (and send a PR with clarification ;-) ).
We aim to upstream all non Rust-specific extensions to the protocol, but this is not a top priority.
All capabilities are enabled via `experimental` field of `ClientCapabilities` or `ServerCapabilities`.
Requests which we hope to upstream live under `experimental/` namespace.
Requests, which are likely to always remain specific to `rust-analyzer` are under `rust-analyzer/` namespace.

## Snippet `TextEdit`

**Issue:** https://github.com/microsoft/language-server-protocol/issues/724

**Client Capability:** `{ "snippetTextEdit": boolean }`

If this capability is set, `WorkspaceEdit`s returned from `codeAction` requests might contain `SnippetTextEdit`s instead of usual `TextEdit`s:

```typescript
interface SnippetTextEdit extends TextEdit {
    insertTextFormat?: InsertTextFormat;
}
```

```typescript
export interface TextDocumentEdit {
	textDocument: VersionedTextDocumentIdentifier;
	edits: (TextEdit | SnippetTextEdit)[];
}
```

When applying such code action, the editor should insert snippet, with tab stops and placeholder.
At the moment, rust-analyzer guarantees that only a single edit will have `InsertTextFormat.Snippet`.

### Example

"Add `derive`" code action transforms `struct S;` into `#[derive($0)] struct S;`

### Unresolved Questions

* Where exactly are `SnippetTextEdit`s allowed (only in code actions at the moment)?
* Can snippets span multiple files (so far, no)?

## `CodeAction` Groups

**Issue:** https://github.com/microsoft/language-server-protocol/issues/994

**Client Capability:** `{ "codeActionGroup": boolean }`

If this capability is set, `CodeAction` returned from the server contain an additional field, `group`:

```typescript
interface CodeAction {
    title: string;
    group?: string;
    ...
}
```

All code-actions with the same `group` should be grouped under single (extendable) entry in lightbulb menu.
The set of actions `[ { title: "foo" }, { group: "frobnicate", title: "bar" }, { group: "frobnicate", title: "baz" }]` should be rendered as

```
💡
  +-------------+
  | foo         |
  +-------------+-----+
  | frobnicate >| bar |
  +-------------+-----+
                | baz |
                +-----+
```

Alternatively, selecting `frobnicate` could present a user with an additional menu to choose between `bar` and `baz`.

### Example

```rust
fn main() {
    let x: Entry/*cursor here*/ = todo!();
}
```

Invoking code action at this position will yield two code actions for importing `Entry` from either `collections::HashMap` or `collection::BTreeMap`, grouped under a single "import" group.

### Unresolved Questions

* Is a fixed two-level structure enough?
* Should we devise a general way to encode custom interaction protocols for GUI refactorings?

## Join Lines

**Issue:** https://github.com/microsoft/language-server-protocol/issues/992

**Server Capability:** `{ "joinLines": boolean }`

This request is send from client to server to handle "Join Lines" editor action.

**Method:** `experimental/joinLines`

**Request:**

```typescript
interface JoinLinesParams {
    textDocument: TextDocumentIdentifier,
    /// Currently active selections/cursor offsets.
    /// This is an array to support multiple cursors.
    ranges: Range[],
}
```

**Response:**

```typescript
TextEdit[]
```

### Example

```rust
fn main() {
    /*cursor here*/let x = {
        92
    };
}
```

`experimental/joinLines` yields (curly braces are automagiacally removed)

```rust
fn main() {
    let x = 92;
}
```

### Unresolved Question

* What is the position of the cursor after `joinLines`?
  Currently this is left to editor's discretion, but it might be useful to specify on the server via snippets.
  However, it then becomes unclear how it works with multi cursor.

## On Enter

**Issue:** https://github.com/microsoft/language-server-protocol/issues/1001

**Server Capability:** `{ "onEnter": boolean }`

This request is send from client to server to handle <kbd>Enter</kbd> keypress.

**Method:** `experimental/onEnter`

**Request:**: `TextDocumentPositionParams`

**Response:**

```typescript
SnippetTextEdit[]
```

### Example

```rust
fn main() {
    // Some /*cursor here*/ docs
    let x = 92;
}
```

`experimental/onEnter` returns the following snippet

```rust
fn main() {
    // Some
    // $0 docs
    let x = 92;
}
```

The primary goal of `onEnter` is to handle automatic indentation when opening a new line.
This is not yet implemented.
The secondary goal is to handle fixing up syntax, like continuing doc strings and comments, and escaping `\n` in string literals.

As proper cursor positioning is raison-d'etat for `onEnter`, it uses `SnippetTextEdit`.

### Unresolved Question

* How to deal with synchronicity of the request?
  One option is to require the client to block until the server returns the response.
  Another option is to do a OT-style merging of edits from client and server.
  A third option is to do a record-replay: client applies heuristic on enter immediatelly, then applies all user's keypresses.
  When the server is ready with the response, the client rollbacks all the changes and applies the recorded actions on top of the correct response.
* How to deal with multiple carets?
* Should we extend this to arbitrary typed events and not just `onEnter`?

## Structural Search Replace (SSR)

**Server Capability:** `{ "ssr": boolean }`

This request is send from client to server to handle structural search replace -- automated syntax tree based transformation of the source.

**Method:** `experimental/ssr`

**Request:**

```typescript
interface SsrParams {
    /// Search query.
    /// The specific syntax is specified outside of the protocol.
    query: string,
    /// If true, only check the syntax of the query and don't compute the actual edit.
    parseOnly: bool,
}
```

**Response:**

```typescript
WorkspaceEdit
```

### Example

SSR with query `foo($a:expr, $b:expr) ==>> ($a).foo($b)` will transform, eg `foo(y + 5, z)` into `(y + 5).foo(z)`.

### Unresolved Question

* Probably needs search without replace mode
* Needs a way to limit the scope to certain files.

## Matching Brace

**Issue:** https://github.com/microsoft/language-server-protocol/issues/999

**Server Capability:** `{ "matchingBrace": boolean }`

This request is send from client to server to handle "Matching Brace" editor action.

**Method:** `experimental/matchingBrace`

**Request:**

```typescript
interface MatchingBraceParams {
    textDocument: TextDocumentIdentifier,
    /// Position for each cursor
    positions: Position[],
}
```

**Response:**

```typescript
Position[]
```

### Example

```rust
fn main() {
    let x: Vec<()>/*cursor here*/ = vec![]
}
```

`experimental/matchingBrace` yields the position of `<`.
In many cases, matching braces can be handled by the editor.
However, some cases (like disambiguating between generics and comparison operations) need a real parser.
Moreover, it would be cool if editors didn't need to implement even basic language parsing

### Unresolved Question

* Should we return a a nested brace structure, to allow paredit-like actions of jump *out* of the current brace pair?
  This is how `SelectionRange` request works.
* Alternatively, should we perhaps flag certain `SelectionRange`s as being brace pairs?

## Analyzer Status

**Method:** `rust-analyzer/analyzerStatus`

**Request:** `null`

**Response:** `string`

Returns internal status message, mostly for debugging purposes.

## Collect Garbage

**Method:** `rust-analyzer/collectGarbage`

**Request:** `null`

**Response:** `null`

Frees some caches. For internal use, and is mostly broken at the moment.

## Syntax Tree

**Method:** `rust-analyzer/syntaxTree`

**Request:**

```typescript
interface SyntaxTeeParams {
    textDocument: TextDocumentIdentifier,
    range?: Range,
}
```

**Response:** `string`

Returns textual representation of a parse tree for the file/selected region.
Primarily for debugging, but very useful for all people working on rust-analyzer itself.

## Expand Macro

**Method:** `rust-analyzer/expandMacro`

**Request:**

```typescript
interface ExpandMacroParams {
    textDocument: TextDocumentIdentifier,
    position?: Position,
}
```

**Response:**

```typescript
interface ExpandedMacro {
    name: string,
    expansion: string,
}
```

Expands macro call at a given position.