3 files changed, 64 insertions, 6 deletions
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index b7ccfac8063..1123cab8076 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -402,7 +402,7 @@ impl ObligationCauseCode<'_> {
 
 // `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-static_assert_size!(ObligationCauseCode<'_>, 40);
+static_assert_size!(ObligationCauseCode<'_>, 48);
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 pub enum StatementAsExpression {
@@ -440,11 +440,11 @@ pub struct IfExpressionCause {
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 pub struct DerivedObligationCause<'tcx> {
-    /// The trait reference of the parent obligation that led to the
+    /// The trait predicate of the parent obligation that led to the
     /// current obligation. Note that only trait obligations lead to
-    /// derived obligations, so we just store the trait reference here
+    /// derived obligations, so we just store the trait predicate here
     /// directly.
-    pub parent_trait_ref: ty::PolyTraitRef<'tcx>,
+    pub parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
 
     /// The parent trait had this cause.
     pub parent_code: Lrc<ObligationCauseCode<'tcx>>,
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index 64c00c353ca..6174c922e2d 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -766,6 +766,17 @@ impl<'tcx> TraitPredicate<'tcx> {
             *param_env = param_env.with_constness(self.constness.and(param_env.constness()))
         }
     }
+
+    /// Remap the constness of this predicate before emitting it for diagnostics.
+    pub fn remap_constness_diag(&mut self, param_env: ParamEnv<'tcx>) {
+        // this is different to `remap_constness` that callees want to print this predicate
+        // in case of selection errors. `T: ~const Drop` bounds cannot end up here when the
+        // param_env is not const because we it is always satisfied in non-const contexts.
+        if let hir::Constness::NotConst = param_env.constness() {
+            self.constness = ty::BoundConstness::NotConst;
+        }
+    }
+
     pub fn def_id(self) -> DefId {
         self.trait_ref.def_id
     }
@@ -784,6 +795,14 @@ impl<'tcx> PolyTraitPredicate<'tcx> {
     pub fn self_ty(self) -> ty::Binder<'tcx, Ty<'tcx>> {
         self.map_bound(|trait_ref| trait_ref.self_ty())
     }
+
+    /// Remap the constness of this predicate before emitting it for diagnostics.
+    pub fn remap_constness_diag(&mut self, param_env: ParamEnv<'tcx>) {
+        *self = self.map_bound(|mut p| {
+            p.remap_constness_diag(param_env);
+            p
+        });
+    }
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index 00a1256d684..ddcc8680d83 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -2413,6 +2413,29 @@ impl<'tcx> ty::Binder<'tcx, ty::TraitRef<'tcx>> {
     }
 }
 
+#[derive(Copy, Clone, TypeFoldable, Lift)]
+pub struct TraitPredPrintModifiersAndPath<'tcx>(ty::TraitPredicate<'tcx>);
+
+impl<'tcx> fmt::Debug for TraitPredPrintModifiersAndPath<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self, f)
+    }
+}
+
+impl<'tcx> ty::TraitPredicate<'tcx> {
+    pub fn print_modifiers_and_trait_path(self) -> TraitPredPrintModifiersAndPath<'tcx> {
+        TraitPredPrintModifiersAndPath(self)
+    }
+}
+
+impl<'tcx> ty::PolyTraitPredicate<'tcx> {
+    pub fn print_modifiers_and_trait_path(
+        self,
+    ) -> ty::Binder<'tcx, TraitPredPrintModifiersAndPath<'tcx>> {
+        self.map_bound(TraitPredPrintModifiersAndPath)
+    }
+}
+
 forward_display_to_print! {
     Ty<'tcx>,
     &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
@@ -2427,6 +2450,7 @@ forward_display_to_print! {
     ty::Binder<'tcx, TraitRefPrintOnlyTraitName<'tcx>>,
     ty::Binder<'tcx, ty::FnSig<'tcx>>,
     ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
+    ty::Binder<'tcx, TraitPredPrintModifiersAndPath<'tcx>>,
     ty::Binder<'tcx, ty::SubtypePredicate<'tcx>>,
     ty::Binder<'tcx, ty::ProjectionPredicate<'tcx>>,
     ty::Binder<'tcx, ty::OutlivesPredicate<Ty<'tcx>, ty::Region<'tcx>>>,
@@ -2491,6 +2515,18 @@ define_print_and_forward_display! {
         p!(print_def_path(self.0.def_id, &[]));
     }
 
+    TraitPredPrintModifiersAndPath<'tcx> {
+        if let ty::BoundConstness::ConstIfConst = self.0.constness {
+            p!("~const ")
+        }
+
+        if let ty::ImplPolarity::Negative = self.0.polarity {
+            p!("!")
+        }
+
+        p!(print(self.0.trait_ref.print_only_trait_path()));
+    }
+
     ty::ParamTy {
         p!(write("{}", self.name))
     }
@@ -2508,8 +2544,11 @@ define_print_and_forward_display! {
     }
 
     ty::TraitPredicate<'tcx> {
-        p!(print(self.trait_ref.self_ty()), ": ",
-           print(self.trait_ref.print_only_trait_path()))
+        p!(print(self.trait_ref.self_ty()), ": ");
+        if let ty::BoundConstness::ConstIfConst = self.constness {
+            p!("~const ");
+        }
+        p!(print(self.trait_ref.print_only_trait_path()))
     }
 
     ty::ProjectionPredicate<'tcx> {