src/vs/editor/common/languages/enterAction.ts/getEnterAction
export function getEnterAction(
	autoIndent: EditorAutoIndentStrategy,
	model: ITextModel,
	range: Range,
	languageConfigurationService: ILanguageConfigurationService
): CompleteEnterAction | null {
	const scopedLineTokens = getScopedLineTokens(model, range.startLineNumber, range.startColumn);
	const richEditSupport = languageConfigurationService.getLanguageConfiguration(scopedLineTokens.languageId);
	if (!richEditSupport) {
		return null;
	}

	const scopedLineText = scopedLineTokens.getLineContent();
	const beforeEnterText = scopedLineText.substr(0, range.startColumn - 1 - scopedLineTokens.firstCharOffset);

	// selection support
	let afterEnterText: string;
	if (range.isEmpty()) {
		afterEnterText = scopedLineText.substr(range.startColumn - 1 - scopedLineTokens.firstCharOffset);
	} else {
		const endScopedLineTokens = getScopedLineTokens(model, range.endLineNumber, range.endColumn);
		afterEnterText = endScopedLineTokens.getLineContent().substr(range.endColumn - 1 - scopedLineTokens.firstCharOffset);
	}

	let previousLineText = '';
	if (range.startLineNumber > 1 && scopedLineTokens.firstCharOffset === 0) {
		// This is not the first line and the entire line belongs to this mode
		const oneLineAboveScopedLineTokens = getScopedLineTokens(model, range.startLineNumber - 1);
		if (oneLineAboveScopedLineTokens.languageId === scopedLineTokens.languageId) {
			// The line above ends with text belonging to the same mode
			previousLineText = oneLineAboveScopedLineTokens.getLineContent();
		}
	}

	const enterResult = richEditSupport.onEnter(autoIndent, previousLineText, beforeEnterText, afterEnterText);
	if (!enterResult) {
		return null;
	}

	const indentAction = enterResult.indentAction;
	let appendText = enterResult.appendText;
	const removeText = enterResult.removeText || 0;

	// Here we add `\t` to appendText first because enterAction is leveraging appendText and removeText to change indentation.
	if (!appendText) {
		if (
			(indentAction === IndentAction.Indent) ||
			(indentAction === IndentAction.IndentOutdent)
		) {
			appendText = '\t';
		} else {
			appendText = '';
		}
	} else if (indentAction === IndentAction.Indent) {
		appendText = '\t' + appendText;
	}

	let indentation = getIndentationAtPosition(model, range.startLineNumber, range.startColumn);
	if (removeText) {
		indentation = indentation.substring(0, indentation.length - removeText);
	}

	return {
		indentAction: indentAction,
		appendText: appendText,
		removeText: removeText,
		indentation: indentation
	};
}