2 files changed, 40 insertions, 10 deletions

diff --git a/compiler/rustc_traits/src/chalk/lowering.rs b/compiler/rustc_traits/src/chalk/lowering.rs
index e4d94fd7beb..6f143c1c607 100644
--- a/compiler/rustc_traits/src/chalk/lowering.rs
+++ b/compiler/rustc_traits/src/chalk/lowering.rs
@@ -1034,10 +1034,6 @@ impl<'tcx> TypeFolder<'tcx> for ParamsSubstitutor<'tcx> {
 
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
         match *t.kind() {
-            // FIXME(chalk): currently we convert params to placeholders starting at
-            // index `0`. To support placeholders, we'll actually need to do a
-            // first pass to collect placeholders. Then we can insert params after.
-            ty::Placeholder(_) => unimplemented!(),
             ty::Param(param) => match self.list.iter().position(|r| r == &param) {
                 Some(idx) => self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType {
                     universe: ty::UniverseIndex::from_usize(0),
@@ -1053,15 +1049,15 @@ impl<'tcx> TypeFolder<'tcx> for ParamsSubstitutor<'tcx> {
                     }))
                 }
             },
-
             _ => t.super_fold_with(self),
         }
     }
 
     fn fold_region(&mut self, r: Region<'tcx>) -> Region<'tcx> {
         match r {
-            // FIXME(chalk) - jackh726 - this currently isn't hit in any tests.
-            // This covers any region variables in a goal, right?
+            // FIXME(chalk) - jackh726 - this currently isn't hit in any tests,
+            // since canonicalization will already change these to canonical
+            // variables (ty::ReLateBound).
             ty::ReEarlyBound(_re) => match self.named_regions.get(&_re.def_id) {
                 Some(idx) => {
                     let br = ty::BoundRegion {
@@ -1084,6 +1080,39 @@ impl<'tcx> TypeFolder<'tcx> for ParamsSubstitutor<'tcx> {
     }
 }
 
+crate struct ReverseParamsSubstitutor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    params: rustc_data_structures::fx::FxHashMap<usize, rustc_middle::ty::ParamTy>,
+}
+
+impl<'tcx> ReverseParamsSubstitutor<'tcx> {
+    crate fn new(
+        tcx: TyCtxt<'tcx>,
+        params: rustc_data_structures::fx::FxHashMap<usize, rustc_middle::ty::ParamTy>,
+    ) -> Self {
+        Self { tcx, params }
+    }
+}
+
+impl<'tcx> TypeFolder<'tcx> for ReverseParamsSubstitutor<'tcx> {
+    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+        match *t.kind() {
+            ty::Placeholder(ty::PlaceholderType { universe: ty::UniverseIndex::ROOT, name }) => {
+                match self.params.get(&name.as_usize()) {
+                    Some(param) => self.tcx.mk_ty(ty::Param(*param)),
+                    None => t,
+                }
+            }
+
+            _ => t.super_fold_with(self),
+        }
+    }
+}
+
 /// Used to collect `Placeholder`s.
 crate struct PlaceholdersCollector {
     universe_index: ty::UniverseIndex,
diff --git a/compiler/rustc_traits/src/chalk/mod.rs b/compiler/rustc_traits/src/chalk/mod.rs
index 8c42b019554..3c2a266dab9 100644
--- a/compiler/rustc_traits/src/chalk/mod.rs
+++ b/compiler/rustc_traits/src/chalk/mod.rs
@@ -23,7 +23,7 @@ use rustc_infer::traits::{self, CanonicalChalkEnvironmentAndGoal};
 
 use crate::chalk::db::RustIrDatabase as ChalkRustIrDatabase;
 use crate::chalk::lowering::LowerInto;
-use crate::chalk::lowering::{ParamsSubstitutor, PlaceholdersCollector};
+use crate::chalk::lowering::{ParamsSubstitutor, PlaceholdersCollector, ReverseParamsSubstitutor};
 
 use chalk_solve::Solution;
 
@@ -44,7 +44,7 @@ crate fn evaluate_goal<'tcx>(
     let mut params_substitutor =
         ParamsSubstitutor::new(tcx, placeholders_collector.next_ty_placeholder);
     let obligation = obligation.fold_with(&mut params_substitutor);
-    let _params: FxHashMap<usize, ParamTy> = params_substitutor.params;
+    let params: FxHashMap<usize, ParamTy> = params_substitutor.params;
 
     let max_universe = obligation.max_universe.index();
 
@@ -101,8 +101,9 @@ crate fn evaluate_goal<'tcx>(
         use rustc_middle::infer::canonical::CanonicalVarInfo;
 
         let mut var_values: IndexVec<BoundVar, GenericArg<'tcx>> = IndexVec::new();
+        let mut reverse_param_substitutor = ReverseParamsSubstitutor::new(tcx, params);
         subst.as_slice(interner).iter().for_each(|p| {
-            var_values.push(p.lower_into(interner));
+            var_values.push(p.lower_into(interner).fold_with(&mut reverse_param_substitutor));
         });
         let variables: Vec<_> = binders
             .iter(interner)