phase29an(p3): add freeze unstructured tag

src/mir/builder/control_flow/plan/planner/freeze.rs

@@ -39,6 +39,14 @@ impl Freeze {
         }
     }
 
+    pub(in crate::mir::builder) fn unstructured(message: impl Into<String>) -> Self {
+        Self {
+            tag: "unstructured",
+            message: message.into(),
+            hint: None,
+        }
+    }
+
     pub(in crate::mir::builder) fn bug(message: impl Into<String>) -> Self {
         Self {
             tag: "bug",
@@ -64,3 +72,14 @@ impl fmt::Display for Freeze {
 }
 
 impl std::error::Error for Freeze {}
+
+#[cfg(test)]
+mod tests {
+    use super::Freeze;
+
+    #[test]
+    fn unstructured_tag_formats_as_expected() {
+        let text = Freeze::unstructured("x").to_string();
+        assert!(text.contains("[plan/freeze:unstructured] x"));
+    }
+}