fix(tvix/eval): substring: propagate catchables r/7205

Change-Id: Ia9b7858c817fbc9c95a3d1c2855b2445f7830e8d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10326
Tested-by: BuildkiteCI
Autosubmit: Adam Joseph <adam@westernsemico.com>
Reviewed-by: tazjin <tazjin@tvl.su>

1 files changed, 5 insertions, 4 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 554fc9d7e329..27b7d5648f48 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -1002,10 +1002,11 @@ mod pure_builtins {
         let beg = start.as_int()?;
         let len = len.as_int()?;
         let span = generators::request_span(&co).await;
-        let x = s
-            .coerce_to_string(co, CoercionKind::Weak, span)
-            .await?
-            .to_str()?;
+        let x = s.coerce_to_string(co, CoercionKind::Weak, span).await?;
+        if x.is_catchable() {
+            return Ok(x);
+        }
+        let x = x.to_str()?;
 
         if beg < 0 {
             return Err(ErrorKind::IndexOutOfBounds { index: beg });