Move a span_delayed_bug.

So it's only called if necessary.

Author: nnethercote

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -857,12 +857,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 Ok(r)
             })
             .or_else(|error| {
-                let guar = self
-                    .dcx()
-                    .span_delayed_bug(span, "method resolution should've emitted an error");
                 let result = match error {
                     method::MethodError::PrivateMatch(kind, def_id, _) => Ok((kind, def_id)),
-                    _ => Err(guar),
+                    _ => {
+                        let guar = self
+                            .dcx()
+                            .span_delayed_bug(span, "method resolution should've emitted an error");
+                        Err(guar)
+                    }
                 };
 
                 let trait_missing_method =