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+# 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`.
+
+## 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($0)] struct S;`
+
+### Unresolved Questions
+
+* Where exactly are `SnippetTextEdit`s allowed (only in code actions at the moment)?
+* Can snippets span multiple files (so far, no)?
+
+## Join Lines
+
+**Issue:** https://github.com/microsoft/language-server-protocol/issues/992
+
+**Server Capability:** `{ "joinLines": boolean }`
+
+This request is send from client to server to handle "Join Lines" editor action.
+
+**Method:** `experimental/JoinLines`
+
+**Request:**
+
+```typescript
+interface JoinLinesParams {
+    textDocument: TextDocumentIdentifier,
+    /// Currently active selections/cursor offsets.
+    /// This is an array to support multiple cursors.
+    ranges: Range[],
+}
+```
+
+**Response:**
+
+```typescript
+TextEdit[]
+```
+
+### Example
+
+```rust
+fn main() {
+    /*cursor here*/let x = {
+        92
+    };
+}
+```
+
+`experimental/joinLines` yields (curly braces are automagiacally removed)
+
+```rust
+fn main() {
+    let x = 92;
+}
+```
+
+### Unresolved Question
+
+* What is the position of the cursor after `joinLines`?
+  Currently this is left to editor's discretion, but it might be useful to specify on the server via snippets.
+  However, it then becomes unclear how it works with multi cursor.
+
+## Structural Search Replace (SSR)
+
+**Server Capability:** `{ "ssr": boolean }`
+
+This request is send from client to server to handle structural search replace -- automated syntax tree based transformation of the source.
+
+**Method:** `experimental/ssr`
+
+**Request:**
+
+```typescript
+interface SsrParams {
+    /// Search query.
+    /// The specific syntax is specified outside of the protocol.
+    query: string,
+    /// If true, only check the syntax of the query and don't compute the actual edit.
+    parseOnly: bool,
+}
+```
+
+**Response:**
+
+```typescript
+WorkspaceEdit
+```
+
+### Example
+
+SSR with query `foo($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.
+
+## `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?