move overflow lint computation into decorator

compiler/rustc_trait_selection/src/solve/delegate.rs

@@ -515,27 +515,24 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
 
     fn emit_next_solver_overflow_fcw(&self, goal: Goal<'tcx, ty::Predicate<'tcx>>, span: Span) {
         let tcx = self.tcx;
-        let goal = self.deeply_resolve_ignoring_regions(goal);
-        let mut visitor = OverflowedGoalChain {
-            span,
-            predicates: vec![],
-            recursion_limit: usize::min(16, tcx.recursion_limit().0),
-        };
-
-        // HACK: avoid computing goal chains for dependencies by relying on the fact that
-        // `cargo` passes `lint_cap=allow` to deps. This should mitigate some of the perf/rss
-        // regression when compiling crates whose deps trigger a large number of these FCWs.
-        if !matches!(tcx.sess.opts.lint_cap, Some(rustc_lint_defs::Level::Allow)) {
-            let _ = self.with_disabled_next_solver_overflow_fcw(|| {
-                self.visit_proof_tree(goal, &mut visitor)
-            });
-        }
-
         tcx.emit_node_span_lint(
             RECURSION_DEPTH_EXCEEDING_LIMIT,
             CRATE_HIR_ID,
             span,
             rustc_errors::DiagDecorator(|diag| {
+                // We do this in the decorator as that avoids the work
+                // if we don't actually emit the lint.
+                let goal = self.deeply_resolve_ignoring_regions(goal);
+                let mut visitor = OverflowedGoalChain {
+                    span,
+                    predicates: vec![],
+                    recursion_limit: usize::min(16, tcx.recursion_limit().0),
+                };
+
+                let _ = self.with_disabled_next_solver_overflow_fcw(|| {
+                    self.visit_proof_tree(goal, &mut visitor)
+                });
+
                 // FIXME: share this with overflow error in fulfillment instead of duplicating.
                 let pred_str = |pred: ty::Predicate<'tcx>| {
                     let s = pred.to_string();