2 files changed, 59 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/add_subtyping_projections.rs b/compiler/rustc_mir_transform/src/add_subtyping_projections.rs
new file mode 100644
index 00000000000..e51f4a2ff21
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/add_subtyping_projections.rs
@@ -0,0 +1,57 @@
+use crate::MirPass;
+use rustc_index::IndexVec;
+use rustc_middle::mir::patch::MirPatch;
+use rustc_middle::mir::visit::MutVisitor;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+
+pub struct Subtyper;
+
+pub struct SubTypeCheker<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    patcher: MirPatch<'tcx>,
+    local_decls: &'a IndexVec<Local, LocalDecl<'tcx>>,
+}
+
+impl<'a, 'tcx> MutVisitor<'tcx> for SubTypeCheker<'a, 'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn visit_assign(
+        &mut self,
+        place: &mut Place<'tcx>,
+        rvalue: &mut Rvalue<'tcx>,
+        location: Location,
+    ) {
+        let place_ty = place.ty(self.local_decls, self.tcx);
+        let rval_ty = rvalue.ty(self.local_decls, self.tcx);
+        if place_ty.ty != rval_ty {
+            let temp = self
+                .patcher
+                .new_temp(rval_ty, self.local_decls[place.as_ref().local].source_info.span);
+            let new_place =
+                Place::from(temp).project_deeper(&[ProjectionElem::Subtype(place_ty.ty)], self.tcx);
+            self.patcher.add_assign(location, new_place, rvalue.clone());
+            let new_rval = Rvalue::Use(Operand::Move(new_place));
+            *rvalue = new_rval;
+        }
+    }
+}
+
+pub fn subtype_finder<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+    let patch = MirPatch::new(body);
+    let mut checker = SubTypeCheker { tcx, patcher: patch, local_decls: &body.local_decls };
+
+    for (bb, data) in body.basic_blocks.as_mut_preserves_cfg().iter_enumerated_mut() {
+        checker.visit_basic_block_data(bb, data);
+    }
+
+    checker.patcher.apply(body);
+}
+
+impl<'tcx> MirPass<'tcx> for Subtyper {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        subtype_finder(tcx, body);
+    }
+}
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index 754f2ee8376..22381844d6d 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -54,6 +54,7 @@ mod check_packed_ref;
 pub mod check_unsafety;
 mod remove_place_mention;
 // This pass is public to allow external drivers to perform MIR cleanup
+mod add_subtyping_projections;
 pub mod cleanup_post_borrowck;
 mod const_debuginfo;
 mod const_goto;
@@ -466,6 +467,7 @@ pub fn run_analysis_to_runtime_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'
 /// After this series of passes, no lifetime analysis based on borrowing can be done.
 fn run_analysis_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let passes: &[&dyn MirPass<'tcx>] = &[
+        &add_subtyping_projections::Subtyper,
         &cleanup_post_borrowck::CleanupPostBorrowck,
         &remove_noop_landing_pads::RemoveNoopLandingPads,
         &simplify::SimplifyCfg::EarlyOpt,