Canonicalize nested template format specs in annotations (#9076)

Use the existing f-string body unparser for template interpolation format specs, while preserving the outer interpolation source text. Add compile-only Python annotation-constant coverage.

Assisted-by: OpenAI Codex:model identifier unavailable

crates/codegen/src/unparse.rs

@@ -814,7 +814,7 @@ impl<'a, 'b, 'c> Unparser<'a, 'b, 'c> {
 
         if let Some(format_spec) = &interpolation.format_spec {
             self.p(":")?;
-            self.unparse_tstring_body(&format_spec.elements)?;
+            self.unparse_fstring_body(&format_spec.elements)?;
         }
 
         self.p("}")

extra_tests/snippets/syntax_annotations.py

@@ -8,3 +8,15 @@ hints = get_type_hints(func)
 # The order of type hints matters for certain functions
 # e.g. functools.singledispatch
 assert list(hints.items()) == [('s', str), ('return', int)]
+
+
+# Template format specs use canonical expression text inside future annotations.
+for template, expected in (
+    ("t'{value:04}'", "t'{value:04}'"),
+    ("t'{value:{(width)}}'", "t'{value:{width}}'"),
+    ("t'{(value):{(width)!r}}'", "t'{(value):{width!r}}'"),
+):
+    annotation_code = compile(
+        "from __future__ import annotations\nx: " + template, "<test>", "exec"
+    )
+    assert expected in annotation_code.co_consts