SearchGraphDelegate: use PhantomData + comment

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>,