Add textDocument/prepareRename request (#30)

* Add textDocument/prepareRename request

NOTE: the type of response is not modelled fully as it's a bit involved. I hope supporting additional response shapes
can be done in a follow up if needed.

* Fix textDocument/prepareRename default behavior

notImplemented cannot be used because the server will fail on rename.
None cannot be used because it marks an invalid rename.

We need to respond with a {defaultBehavior=true} so that those servers
that didn't override prepareRename still have the same rename behavior
as before.

NOTE: I needed to model the response fully to make things work.

* misc: Fix formatting

Author: artempyanykh

src/LanguageServerProtocol.fs

@@ -193,6 +193,7 @@ module Server =
       "textDocument/completion", requestHandling (fun s p -> s.TextDocumentCompletion(p))
       "completionItem/resolve", requestHandling (fun s p -> s.CompletionItemResolve(p))
       "textDocument/rename", requestHandling (fun s p -> s.TextDocumentRename(p))
+      "textDocument/prepareRename", requestHandling (fun s p -> s.TextDocumentPrepareRename(p))
       "textDocument/definition", requestHandling (fun s p -> s.TextDocumentDefinition(p))
       "textDocument/typeDefinition", requestHandling (fun s p -> s.TextDocumentTypeDefinition(p))
       "textDocument/implementation", requestHandling (fun s p -> s.TextDocumentImplementation(p))

src/Server.fs

@@ -92,6 +92,13 @@ type LspServer() =
   abstract member TextDocumentRename: RenameParams -> AsyncLspResult<WorkspaceEdit option>
   default __.TextDocumentRename(_) = notImplemented
 
+  /// The prepare rename request is sent from the client to the server to setup and test the validity of a rename operation at a given location.
+  /// If None is returned then it is deemed that a ‘textDocument/rename’ request is not valid at the given position.
+  abstract member TextDocumentPrepareRename: PrepareRenameParams -> AsyncLspResult<PrepareRenameResult option>
+
+  default __.TextDocumentPrepareRename(_) =
+    AsyncLspResult.success (Some(PrepareRenameResult.Default { DefaultBehavior = true }))
+
   /// The goto definition request is sent from the client to the server to resolve the definition location of
   /// a symbol at a given text document position.
   abstract member TextDocumentDefinition: TextDocumentPositionParams -> AsyncLspResult<GotoResult option>

src/Types.fs

@@ -1786,6 +1786,30 @@ type RenameParams =
     member this.TextDocument = this.TextDocument
     member this.Position = this.Position
 
+type PrepareRenameParams =
+  { /// The document to rename.
+    TextDocument: TextDocumentIdentifier
+
+    /// The position at which this request was sent.
+    Position: Position }
+  interface ITextDocumentPositionParams with
+    member this.TextDocument = this.TextDocument
+    member this.Position = this.Position
+
+type DefaultBehavior = { DefaultBehavior: bool }
+
+type RangeWithPlaceholder = { Range: Range; Placeholder: string }
+
+[<ErasedUnion>]
+[<RequireQualifiedAccess>]
+type PrepareRenameResult =
+  /// A range of the string to rename.
+  | Range of Range
+  /// A range of the string to rename and a placeholder text of the string content to be renamed.
+  | RangeWithPlaceholder of RangeWithPlaceholder
+  /// The rename position is valid and the client should use its default behavior to compute the rename range.
+  | Default of DefaultBehavior
+
 [<ErasedUnion>]
 [<RequireQualifiedAccess>]
 type GotoResult =