[ty] Fix panic for unclosed string literal in type annotation position (#21592)

crates/ruff_python_ast/src/nodes.rs

@@ -773,11 +773,15 @@ pub trait StringFlags: Copy {
     }
 
     /// The total length of the string's closer.
-    /// This is always equal to `self.quote_len()`,
-    /// but is provided here for symmetry with the `opener_len()` method.
+    /// This is always equal to `self.quote_len()`, except when the string is unclosed,
+    /// in which case the length is zero.
     fn closer_len(self) -> TextSize {
+        if self.is_unclosed() {
+            TextSize::default()
+        } else {
             self.quote_len()
         }
+    }
 
     fn as_any_string_flags(self) -> AnyStringFlags {
         AnyStringFlags::new(self.prefix(), self.quote_style(), self.triple_quotes())

crates/ty_python_semantic/resources/mdtest/assignment/annotations.md

@@ -715,3 +715,17 @@ def _(a: int, b: str, c: int | str):
     x9: int | str | None = f(lst(c))
     reveal_type(x9)  # revealed: int | str | None
 ```
+
+## Forward annotation with unclosed string literal
+
+Regression test for [#1611](https://github.com/astral-sh/ty/issues/1611).
+
+<!-- blacken-docs:off -->
+
+```py
+# error: [invalid-syntax]
+# error: [invalid-syntax-in-forward-annotation]
+a:'
+```
+
+<!-- blacken-docs:on -->