no-unnecessary-non-null-assertion.ts

import type { Rule } from '@tsslint/config';

export function create(): Rule {
	return ({ typescript: ts, sourceFile, languageService, reportWarning }) => {
		ts.forEachChild(sourceFile, function walk(node) {
			if (ts.isNonNullExpression(node)) {
				const typeChecker = languageService.getProgram()!.getTypeChecker();
				const type = typeChecker.getTypeAtLocation(node.expression);
				if (
					typeChecker.typeToString(type, undefined, ts.TypeFormatFlags.NoTruncation)
					=== typeChecker.typeToString(type.getNonNullableType(), undefined, ts.TypeFormatFlags.NoTruncation)
				) {
					reportWarning(
						`Unnecessary non-null assertion.`,
						node.getStart(sourceFile),
						node.getEnd()
					).withFix(
						'Remove unnecessary non-null assertion',
						() => [{
							fileName: sourceFile.fileName,
							textChanges: [
								{
									newText: '',
									span: {
										start: node.expression.getEnd(),
										length: node.getEnd() - node.expression.getEnd(),
									},
								}
							],
						}]
					);
				}
			}
			ts.forEachChild(node, walk);
		});
	};
}