From 809acd2a8058f3d7f284c33ccfe1716b96136393 Mon Sep 17 00:00:00 2001
From: lcnr <rust@lcnr.de>
Date: Fri, 12 Jul 2024 11:58:56 +0200
Subject: [PATCH] `SearchGraphDelegate`: use `PhantomData` + comment

---
 .../rustc_next_trait_solver/src/solve/search_graph.rs     | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_next_trait_solver/src/solve/search_graph.rs b/compiler/rustc_next_trait_solver/src/solve/search_graph.rs
index 47a82e0d684d7..3d287d5875e05 100644
--- a/compiler/rustc_next_trait_solver/src/solve/search_graph.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/search_graph.rs
@@ -3,13 +3,17 @@ use rustc_type_ir::inherent::Predicate;
 use rustc_type_ir::search_graph::{self, CycleKind, UsageKind};
 use rustc_type_ir::solve::{CanonicalInput, Certainty, QueryResult};
 use rustc_type_ir::Interner;
+use std::marker::PhantomData;
 
 use super::inspect;
 use super::{inspect::ProofTreeBuilder, FIXPOINT_STEP_LIMIT};
 
+/// This type is never constructed. We only use it to implement `search_graph::Delegate`
+/// for all types which impl `SolverDelegate` and doing it directly fails in coherence.
+pub(super) struct SearchGraphDelegate<D: SolverDelegate> {
+    _marker: PhantomData<D>,
+}
 pub(super) type SearchGraph<D> = search_graph::SearchGraph<SearchGraphDelegate<D>>;
-
-pub(super) struct SearchGraphDelegate<D: SolverDelegate>(D);
 impl<D, I> search_graph::Delegate for SearchGraphDelegate<D>
 where
     D: SolverDelegate<Interner = I>,