Only compute Obligation cache_key once in register_obligation_at

Author: estebank

compiler/rustc_data_structures/src/obligation_forest/mod.rs

@@ -336,12 +336,13 @@ impl<O: ForestObligation> ObligationForest<O> {
 
     // Returns Err(()) if we already know this obligation failed.
     fn register_obligation_at(&mut self, obligation: O, parent: Option<usize>) -> Result<(), ()> {
-        if self.done_cache.contains(&obligation.as_cache_key()) {
+        let cache_key = obligation.as_cache_key();
+        if self.done_cache.contains(&cache_key) {
             debug!("register_obligation_at: ignoring already done obligation: {:?}", obligation);
             return Ok(());
         }
 
-        match self.active_cache.entry(obligation.as_cache_key()) {
+        match self.active_cache.entry(cache_key.clone()) {
             Entry::Occupied(o) => {
                 let node = &mut self.nodes[*o.get()];
                 if let Some(parent_index) = parent {
@@ -365,7 +366,7 @@ impl<O: ForestObligation> ObligationForest<O> {
                     && self
                         .error_cache
                         .get(&obligation_tree_id)
-                        .map(|errors| errors.contains(&obligation.as_cache_key()))
+                        .map(|errors| errors.contains(&cache_key))
                         .unwrap_or(false);
 
                 if already_failed {