docs(span): document validity of ModuleKind::Unambiguous (#6423)

relates #6249

crates/oxc_span/src/source_type/mod.rs

@@ -55,7 +55,8 @@ pub enum ModuleKind {
     ///
     /// ESM syntax includes `import` statement, `export` statement and `import.meta`.
     ///
-    /// Note: Dynamic import expression is not ESM syntax.
+    /// Note1: This value is only valid as a parser input, and does not appear on a valid AST's `Program::source_type`.
+    /// Note2: Dynamic import expression is not ESM syntax.
     ///
     /// See <https://babel.dev/docs/options#misc-options>
     Unambiguous = 2,

tasks/coverage/src/driver.rs

@@ -67,11 +67,14 @@ impl CompilerInterface for Driver {
     }
 
     fn after_parse(&mut self, parser_return: &mut ParserReturn) -> ControlFlow<()> {
-        let ParserReturn { program, trivias, panicked, .. } = parser_return;
+        let ParserReturn { program, trivias, panicked, errors } = parser_return;
         self.panicked = *panicked;
         if self.check_comments(trivias) {
             return ControlFlow::Break(());
         }
+        if (errors.is_empty() || !*panicked) && program.source_type.is_unambiguous() {
+            self.errors.push(OxcDiagnostic::error("SourceType must not be unambiguous."));
+        }
         // Make sure serialization doesn't crash; also for code coverage.
         let _serializer = program.serializer();
         ControlFlow::Continue(())