feat: implement willSaveWaitUntil method to prevent format-on-save issues

handlers.v

@@ -455,11 +455,6 @@ fn (mut app App) on_did_change(request Request) ?Notification {
 		log('on_did_change() no params')
 		return none
 	}
-	// If the first content change is an empty text, treat as no-op
-	if params.content_changes.len > 0 && params.content_changes[0].text == '' {
-		log('on_did_change() empty text')
-		return none
-	}
 	uri := params.text_document.uri
 	mut content := app.open_files[uri] or { '' }
 	for change in params.content_changes {
@@ -495,26 +490,56 @@ fn (mut app App) on_did_save(request Request) ?Notification {
 	}
 	uri := params.text_document.uri
 	mut content := app.open_files[uri] or { '' }
-	if text := params.text {
-		content = text
-		app.open_files[uri] = text
-		app.text = text
-		app.open_files_generation++
-	}
 	if content == '' {
-		real_path := uri_to_path(uri)
-		content = os.read_file(real_path) or {
-			$if debug { log('on_did_save: failed to read file ${real_path}: ${err}') }
-			return none
+		if text := params.text {
+			content = text
+			app.open_files[uri] = text
+			app.text = text
+			app.open_files_generation++
+		} else {
+			real_path := uri_to_path(uri)
+			content = os.read_file(real_path) or {
+				$if debug { log('on_did_save: failed to read file ${real_path}: ${err}') }
+				return none
+			}
+			app.open_files[uri] = content
+			app.text = content
+			app.open_files_generation++
 		}
-		app.open_files[uri] = content
-		app.text = content
-		app.open_files_generation++
 	}
 	notification := app.build_diagnostics_notification(uri, content)
 	return notification
 }
 
+// on_will_save_wait_until handles willSaveWaitUntil by formatting the document
+// before it is saved, returning the edits to apply atomically with the save.
+fn (mut app App) on_will_save_wait_until(request Request) Response {
+	params := json.decode(WillSaveTextDocumentParams, request.params) or {
+		$if debug { log('Failed to decode WillSaveTextDocumentParams: ${err}') }
+		return Response{
+			id:     request.id
+			result: []TextEdit{}
+		}
+	}
+	uri := params.text_document.uri
+	content := app.open_files[uri] or {
+		return Response{
+			id:     request.id
+			result: []TextEdit{}
+		}
+	}
+	edits, formatted := app.format_content(uri, content)
+	if formatted != '' {
+		app.open_files[uri] = formatted
+		app.text = formatted
+		app.open_files_generation++
+	}
+	return Response{
+		id:     request.id
+		result: edits
+	}
+}
+
 // handle_prepare_rename handles textDocument/prepareRename by returning the range
 // and placeholder text for the identifier under the cursor, or an empty result
 // when the cursor is not on a renameable symbol.
@@ -1311,70 +1336,42 @@ fn search_doc_in_vlib_dir(dir string, symbol string) string {
 	return ''
 }
 
-// handle_formatting handles the LSP formatting request, returning edits to format the document.
-fn (mut app App) handle_formatting(request Request) Response {
-	params := json.decode(DocumentFormattingParams, request.params) or {
-		log('Failed to decode DocumentFormattingParams: ${err}')
-		return Response{
-			id:     request.id
-			result: []TextEdit{}
-		}
-	}
-	path := params.text_document.uri
-	real_path := uri_to_path(path)
-
-	// Get the current content of the file
-	content := app.open_files[path] or {
-		os.read_file(real_path) or {
-			log('Failed to read file for formatting: ${err}')
-			return Response{
-				id:     request.id
-				result: []TextEdit{}
-			}
-		}
-	}
+// format_content formats the given content via v fmt and returns the TextEdits
+// needed to replace the document with its formatted version, plus the formatted
+// text. Returns empty edits if the content is already properly formatted.
+fn (mut app App) format_content(uri string, content string) ([]TextEdit, string) {
+	real_path := uri_to_path(uri)
 
-	// Write content to a temp file
 	temp_file := os.join_path(os.temp_dir(), 'vls_fmt_${os.getpid()}_${os.file_name(real_path)}')
 	os.write_file(temp_file, content) or {
 		log('Failed to write temp file for formatting: ${err}')
-		return Response{
-			id:     request.id
-			result: []TextEdit{}
-		}
+		return []TextEdit{}, ''
 	}
 
-	// Run fmt
+	// With -w flag, v fmt writes the formatted content back to the temp file.
+	// Read from there instead of relying on stdout capture, which is
+	// unreliable on Windows MSYS2.
 	result := os.execute(ensure_stderr_captured(build_v_fmt_cmd(temp_file)))
 
-	// Clean up temp file
+	mut formatted := os.read_file(temp_file) or { result.output }
+
 	os.rm(temp_file) or {
 		$if debug { log('Failed to remove temp file: ${err}') }
 	}
 
-	// Check for errors
 	if result.exit_code != 0 {
 		$if debug { log('v fmt failed with code ${result.exit_code}: ${result.output}') }
-		return Response{
-			id:     request.id
-			result: []TextEdit{}
-		}
+		return []TextEdit{}, ''
 	}
 
-	// If content is unchanged, return empty edits
-	if result.output == content {
-		return Response{
-			id:     request.id
-			result: []TextEdit{}
-		}
+	if formatted == '' || formatted == content {
+		return []TextEdit{}, ''
 	}
 
-	// Calculate the range of the entire document
 	lines := content.split_into_lines()
 	last_line := lines.len - 1
 	last_char := if lines.len > 0 { lines[last_line].len } else { 0 }
 
-	// Return a single TextEdit that replaces the entire document
 	edit := TextEdit{
 		range:    LSPRange{
 			start: Position{
@@ -1386,12 +1383,37 @@ fn (mut app App) handle_formatting(request Request) Response {
 				char: last_char
 			}
 		}
-		new_text: result.output
+		new_text: formatted
 	}
+	return [edit], formatted
+}
 
+// handle_formatting handles the LSP formatting request, returning edits to format the document.
+fn (mut app App) handle_formatting(request Request) Response {
+	params := json.decode(DocumentFormattingParams, request.params) or {
+		log('Failed to decode DocumentFormattingParams: ${err}')
+		return Response{
+			id:     request.id
+			result: []TextEdit{}
+		}
+	}
+	path := params.text_document.uri
+	real_path := uri_to_path(path)
+
+	content := app.open_files[path] or {
+		os.read_file(real_path) or {
+			log('Failed to read file for formatting: ${err}')
+			return Response{
+				id:     request.id
+				result: []TextEdit{}
+			}
+		}
+	}
+
+	edits, _ := app.format_content(path, content)
 	return Response{
 		id:     request.id
-		result: [edit]
+		result: edits
 	}
 }
 

handlers_test.v

@@ -352,7 +352,7 @@ fn test_on_did_change_empty_text() {
 		cleanup_test_app(app)
 	}
 
-	// Request with empty text should return none
+	// Request with empty text (deletion) should be processed and return diagnostics
 	request := Request{
 		params: json.encode(Params{
 			content_changes: [ContentChange{
@@ -362,7 +362,13 @@ fn test_on_did_change_empty_text() {
 	}
 
 	result := app.on_did_change(request)
-	assert result == none
+	if notif := result {
+		assert notif.method == 'textDocument/publishDiagnostics'
+		assert notif.params.uri == ''
+		assert notif.params.diagnostics.len == 0
+	} else {
+		assert false, 'expected a notification'
+	}
 }
 
 fn test_on_did_change_returns_notification() {

integration_test.v

@@ -514,7 +514,7 @@ fn test_integration_diagnostics_empty_file() {
 		})
 	})
 
-	// Empty content should return none
+	// Empty content should be processed and return diagnostics for the empty file
 	result := app.on_did_change(Request{
 		params: json.encode(Params{
 			text_document:   TextDocumentIdentifier{
@@ -526,7 +526,12 @@ fn test_integration_diagnostics_empty_file() {
 		})
 	})
 
-	assert result == none
+	if notif := result {
+		assert notif.method == 'textDocument/publishDiagnostics'
+		assert notif.params.uri == uri
+	} else {
+		assert false, 'expected a notification for empty file'
+	}
 }
 
 fn test_integration_completion_request() {

interop.v

@@ -40,8 +40,10 @@ fn ensure_stderr_captured(cmd string) string {
 }
 
 fn shell_quote(s string) string {
-	escaped := s.replace("'", '\'"\'"\'')
-	return "'${escaped}'"
+	// Use double quotes for cross-platform compatibility.
+	// Windows CMD does not treat single quotes as string delimiters.
+	escaped := s.replace('"', '\\"')
+	return '"${escaped}"'
 }
 
 fn build_v_check_cmd_single(file_to_check string) string {
@@ -62,7 +64,7 @@ fn build_v_line_info_cmd_single(file_to_check string, line_info string, compile_
 }
 
 fn build_v_fmt_cmd(temp_file string) string {
-	return 'v fmt -inprocess ${shell_quote(temp_file)}'
+	return 'v fmt -inprocess -w ${shell_quote(temp_file)}'
 }
 
 fn execute_in_dir(dir string, cmd string) os.Result {

interop_test.v

@@ -204,8 +204,13 @@ fn test_execute_in_dir_restores_working_directory() {
 		os.rmdir_all(work_dir) or {}
 	}
 
-	result := execute_in_dir(work_dir, 'pwd')
-	assert result.output.trim_space() == work_dir
+	mut result := execute_in_dir(work_dir, 'pwd')
+	$if windows {
+		// On Windows, os.execute uses cmd.exe which understands echo %cd%
+		result = execute_in_dir(work_dir, 'echo %cd%')
+	}
+	// Use real_path to resolve symlinks (e.g. /tmp -> /private/tmp on macOS)
+	assert os.real_path(result.output.trim_space()) == os.real_path(work_dir)
 	assert os.getwd() == original
 }
 
@@ -221,18 +226,18 @@ fn test_execute_in_dir_returns_error_when_directory_missing() {
 
 fn test_shell_quote_handles_single_quotes() {
 	quoted := shell_quote("a'b")
-	assert quoted == '\'a\'"\'"\'b\''
+	assert quoted == '"a\'b"'
 }
 
 fn test_build_v_check_cmd_single_quotes_path() {
 	cmd := build_v_check_cmd_single('/tmp/a b/test.v')
-	assert cmd.contains("'/tmp/a b/test.v'")
+	assert cmd.contains('"/tmp/a b/test.v"')
 	assert cmd.contains('-vls-mode')
 }
 
 fn test_build_v_fmt_cmd_quotes_temp_file() {
 	cmd := build_v_fmt_cmd('/tmp/fmt file.v')
-	assert cmd == "v fmt -inprocess '/tmp/fmt file.v'"
+	assert cmd == 'v fmt -inprocess -w "/tmp/fmt file.v"'
 }
 
 // ============================================================================

lsp.v

@@ -647,9 +647,11 @@ struct SaveOptions {
 
 // TextDocumentSyncOptions describes document synchronization options.
 struct TextDocumentSyncOptions {
-	open_close bool @[json: 'openClose']
-	change     int         // 1 for Full, 2 for Incremental
-	save       SaveOptions // emit {"includeText":true} to receive text in didSave
+	open_close           bool @[json: 'openClose']
+	change               int         // 1 for Full, 2 for Incremental
+	save                 SaveOptions // emit {"includeText":true} to receive text in didSave
+	will_save            bool @[json: 'willSave']
+	will_save_wait_until bool @[json: 'willSaveWaitUntil']
 }
 
 // SignatureHelpOptions describes signature help trigger characters.
@@ -829,20 +831,22 @@ enum Method {
 	selection_range                         @['textDocument/selectionRange']
 	semantic_tokens_range                   @['textDocument/semanticTokens/range']
 	range_formatting                        @['textDocument/rangeFormatting']
-	did_change_watched_files                @['workspace/didChangeWatchedFiles']
-	code_lens             @['textDocument/codeLens']
-	code_lens_resolve     @['codeLens/resolve']
-	execute_command       @['workspace/executeCommand']
-	inline_value          @['textDocument/inlineValue']
-	linked_editing_range  @['textDocument/linkedEditingRange']
-	will_create_files     @['workspace/willCreateFiles']
-	will_rename_files     @['workspace/willRenameFiles']
-	will_delete_files     @['workspace/willDeleteFiles']
-	on_type_formatting    @['textDocument/onTypeFormatting']
-	set_trace             @['$/setTrace']
-	cancel_request        @['$/cancelRequest']
-	shutdown              @['shutdown']
-	exit                  @['exit']
+	will_save                 @['textDocument/willSave']
+	will_save_wait_until      @['textDocument/willSaveWaitUntil']
+	did_change_watched_files  @['workspace/didChangeWatchedFiles']
+	code_lens                 @['textDocument/codeLens']
+	code_lens_resolve         @['codeLens/resolve']
+	execute_command           @['workspace/executeCommand']
+	inline_value              @['textDocument/inlineValue']
+	linked_editing_range      @['textDocument/linkedEditingRange']
+	will_create_files         @['workspace/willCreateFiles']
+	will_rename_files         @['workspace/willRenameFiles']
+	will_delete_files         @['workspace/willDeleteFiles']
+	on_type_formatting        @['textDocument/onTypeFormatting']
+	set_trace                 @['$/setTrace']
+	cancel_request            @['$/cancelRequest']
+	shutdown                  @['shutdown']
+	exit                      @['exit']
 }
 
 // TextDocumentPositionParams for position-based requests
@@ -882,6 +886,13 @@ struct DidSaveTextDocumentParams {
 	text          ?string
 }
 
+// WillSaveTextDocumentParams for willSave/willSaveWaitUntil.
+// Reason: 1 = Manual, 2 = AfterDelay, 3 = FocusOut.
+struct WillSaveTextDocumentParams {
+	text_document TextDocumentIdentifier @[json: 'textDocument']
+	reason        int
+}
+
 // ReferenceContext carries the includeDeclaration flag for references requests.
 struct ReferenceContext {
 	include_declaration bool @[json: 'includeDeclaration']