Review feedback: Move match to before cause computation

Author: amandasystems

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -399,10 +399,6 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
 
         let origin_longer = self.regioncx.definitions[longer_fr].origin;
 
-        // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
-        let cause =
-            self.regioncx.best_blame_constraint(longer_fr, origin_longer, error_vid).0.cause;
-
         let Placeholder(placeholder) = origin_longer else {
             bug!("Expected {longer_fr:?} to come from placeholder!");
         };
@@ -413,6 +409,10 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             Placeholder(other_placeholder) => Some(other_placeholder),
         };
 
+        // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
+        let cause =
+            self.regioncx.best_blame_constraint(longer_fr, origin_longer, error_vid).0.cause;
+
         // FIXME these methods should have better names, and also probably not be this generic.
         // FIXME note that we *throw away* the error element here! We probably want to
         // thread it through the computation further down and use it, but there currently isn't