servo: Merge #18603 - fix #18594: Unnecessarily allocating an error string (from tigercosmos:allocating); r=KiChjang

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r? @asajeffrey

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #18594 (github issue number if applicable).

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- [X] These changes do not require tests because _____

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Source-Repo: https://github.com/servo/servo
Source-Revision: 35aad086191c7c4561067a1f774b3ae2d69bc375

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 50ce2afd344fbaeb4e453929b4bba0e22ab4b027

servo/components/script/document_loader.rs

@@ -130,7 +130,7 @@ impl DocumentLoader {
     pub fn finish_load(&mut self, load: &LoadType) {
         debug!("Removing blocking load {:?} ({}).", load, self.blocking_loads.len());
         let idx = self.blocking_loads.iter().position(|unfinished| *unfinished == *load);
-        self.blocking_loads.remove(idx.expect(&format!("unknown completed load {:?}", load)));
+        self.blocking_loads.remove(idx.unwrap_or_else(|| panic!("unknown completed load {:?}", load)));
     }
 
     pub fn is_blocked(&self) -> bool {