From f4261772d8d4e2d4fb7e9dd4685b995a7176e506 Mon Sep 17 00:00:00 2001
From: Ellen <supbscripter@gmail.com>
Date: Wed, 27 Jan 2021 14:46:43 +0000
Subject: [PATCH] comments

---
 .../rustc_trait_selection/src/traits/const_evaluatable.rs     | 4 +++-
 .../const_evaluatable_checked/nested-abstract-consts-1.rs     | 2 ++
 .../const_evaluatable_checked/nested-abstract-consts-2.rs     | 2 ++
 .../const_evaluatable_checked/nested_uneval_unification-1.rs  | 2 ++
 .../const_evaluatable_checked/nested_uneval_unification-2.rs  | 2 ++
 5 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs b/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
index 272588ad516c2..b587ed6487e3c 100644
--- a/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
+++ b/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
@@ -612,6 +612,9 @@ pub(super) fn try_unify<'tcx>(
     mut a: AbstractConst<'tcx>,
     mut b: AbstractConst<'tcx>,
 ) -> bool {
+    // We substitute generics repeatedly to allow AbstractConsts to unify where a
+    // ConstKind::Unevalated could be turned into an AbstractConst that would unify e.g.
+    // Param(N) should unify with Param(T), substs: [Unevaluated("T2", [Unevaluated("T3", [Param(N)])])]
     while let Node::Leaf(a_ct) = a.root() {
         let a_ct = a_ct.subst(tcx, a.substs);
         match AbstractConst::from_const(tcx, a_ct) {
@@ -620,7 +623,6 @@ pub(super) fn try_unify<'tcx>(
             Err(_) => return true,
         }
     }
-
     while let Node::Leaf(b_ct) = b.root() {
         let b_ct = b_ct.subst(tcx, b.substs);
         match AbstractConst::from_const(tcx, b_ct) {
diff --git a/src/test/ui/const-generics/const_evaluatable_checked/nested-abstract-consts-1.rs b/src/test/ui/const-generics/const_evaluatable_checked/nested-abstract-consts-1.rs
index 124cd317da565..0fe84c1cd2a72 100644
--- a/src/test/ui/const-generics/const_evaluatable_checked/nested-abstract-consts-1.rs
+++ b/src/test/ui/const-generics/const_evaluatable_checked/nested-abstract-consts-1.rs
@@ -20,3 +20,5 @@ where
 fn main() {
     assert_eq!(caller::<4>(), 5);
 }
+
+// Test that the ``(N1 + 1) + 1`` bound on ``caller`` satisfies the ``M2 + 1`` bound on ``callee``
diff --git a/src/test/ui/const-generics/const_evaluatable_checked/nested-abstract-consts-2.rs b/src/test/ui/const-generics/const_evaluatable_checked/nested-abstract-consts-2.rs
index 5936662dadb20..fe5c2237a7603 100644
--- a/src/test/ui/const-generics/const_evaluatable_checked/nested-abstract-consts-2.rs
+++ b/src/test/ui/const-generics/const_evaluatable_checked/nested-abstract-consts-2.rs
@@ -30,3 +30,5 @@ where
 fn main() {
     assert_eq!((Generic::<10>).bar(), 11);
 }
+
+// Test that the ``ConstU64<{ K + 1 - 1}>`` bound on ``bar``'s impl block satisfies the ``ConstU64<{K - 1}>`` bound on ``foo``'s impl block
diff --git a/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-1.rs b/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-1.rs
index ff0f2efaee9cb..f686bfd1d2b02 100644
--- a/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-1.rs
+++ b/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-1.rs
@@ -30,3 +30,5 @@ where
 fn main() {
     assert_eq!(substs3::<2>().0, [0; 3]);
 }
+
+// Test that the ``{ (L - 1) * 2 + 1 }`` bound on ``substs3`` satisfies the ``{ N + 1 }`` bound on ``Substs1``
diff --git a/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-2.rs b/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-2.rs
index 806f7ba920310..4cdfb107417a6 100644
--- a/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-2.rs
+++ b/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-2.rs
@@ -24,3 +24,5 @@ fn substs3<const L: usize>() -> Substs1<{ (L) }> {
 fn main() {
     assert_eq!(substs3::<2>().0, [0; 2]);
 }
+
+// Test that the implicit ``{ (L) }`` bound on ``substs3`` satisfies the ``{ (N) }`` bound on ``Substs1``