3 files changed, 45 insertions, 13 deletions
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index bdaa586c698..e909b2f74aa 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -1488,7 +1488,7 @@ impl<'tcx> StatementKind<'tcx> {
 ///////////////////////////////////////////////////////////////////////////
 // Places
 
-impl<V, T> ProjectionElem<V, T> {
+impl<V, T, U> ProjectionElem<V, T, U> {
     /// Returns `true` if the target of this projection may refer to a different region of memory
     /// than the base.
     fn is_indirect(&self) -> bool {
@@ -1517,7 +1517,7 @@ impl<V, T> ProjectionElem<V, T> {
 
 /// Alias for projections as they appear in `UserTypeProjection`, where we
 /// need neither the `V` parameter for `Index` nor the `T` for `Field`.
-pub type ProjectionKind = ProjectionElem<(), ()>;
+pub type ProjectionKind = ProjectionElem<(), (), ()>;
 
 rustc_index::newtype_index! {
     /// A [newtype'd][wrapper] index type in the MIR [control-flow graph][CFG]
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index 99e59c770d7..a6ca04f5e62 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -890,11 +890,18 @@ pub struct Place<'tcx> {
     pub projection: &'tcx List<PlaceElem<'tcx>>,
 }
 
+/// The different kinds of projections that can be used in the projection of a `Place`.
+///
+/// `T1` is the generic type for a field projection. For an actual projection on a `Place`
+/// this parameter will always be `Ty`, but the field type can be unavailable when
+/// building (by using `PlaceBuilder`) places that correspond to upvars.
+/// `T2` is the generic type for an `OpaqueCast` (is generic since it's abstracted over
+/// in dataflow analysis, see `AbstractElem`).
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[derive(TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
-pub enum ProjectionElem<V, T> {
+pub enum ProjectionElem<V, T1, T2> {
     Deref,
-    Field(Field, T),
+    Field(Field, T1),
     /// Index into a slice/array.
     ///
     /// Note that this does not also dereference, and so it does not exactly correspond to slice
@@ -950,12 +957,36 @@ pub enum ProjectionElem<V, T> {
 
     /// Like an explicit cast from an opaque type to a concrete type, but without
     /// requiring an intermediate variable.
-    OpaqueCast(T),
+    OpaqueCast(T2),
 }
 
 /// Alias for projections as they appear in places, where the base is a place
 /// and the index is a local.
-pub type PlaceElem<'tcx> = ProjectionElem<Local, Ty<'tcx>>;
+pub type PlaceElem<'tcx> = ProjectionElem<Local, Ty<'tcx>, Ty<'tcx>>;
+
+/// Alias for projections that appear in `PlaceBuilder::Upvar`, for which
+/// we cannot provide any field types.
+pub type UpvarProjectionElem<'tcx> = ProjectionElem<Local, (), Ty<'tcx>>;
+
+impl<'tcx> From<PlaceElem<'tcx>> for UpvarProjectionElem<'tcx> {
+    fn from(elem: PlaceElem<'tcx>) -> Self {
+        match elem {
+            ProjectionElem::Deref => ProjectionElem::Deref,
+            ProjectionElem::Field(field, _) => ProjectionElem::Field(field, ()),
+            ProjectionElem::Index(v) => ProjectionElem::Index(v),
+            ProjectionElem::ConstantIndex { offset, min_length, from_end } => {
+                ProjectionElem::ConstantIndex { offset, min_length, from_end }
+            }
+            ProjectionElem::Subslice { from, to, from_end } => {
+                ProjectionElem::Subslice { from, to, from_end }
+            }
+            ProjectionElem::Downcast(opt_sym, variant_idx) => {
+                ProjectionElem::Downcast(opt_sym, variant_idx)
+            }
+            ProjectionElem::OpaqueCast(ty) => ProjectionElem::OpaqueCast(ty),
+        }
+    }
+}
 
 ///////////////////////////////////////////////////////////////////////////
 // Operands
diff --git a/compiler/rustc_middle/src/mir/tcx.rs b/compiler/rustc_middle/src/mir/tcx.rs
index fa3adafd4b8..e3ca1f41d7e 100644
--- a/compiler/rustc_middle/src/mir/tcx.rs
+++ b/compiler/rustc_middle/src/mir/tcx.rs
@@ -28,8 +28,8 @@ impl<'tcx> PlaceTy<'tcx> {
     /// `place_ty.field_ty(tcx, f)` computes the type at a given field
     /// of a record or enum-variant. (Most clients of `PlaceTy` can
     /// instead just extract the relevant type directly from their
-    /// `PlaceElem`, but some instances of `ProjectionElem<V, T>` do
-    /// not carry a `Ty` for `T`.)
+    /// `PlaceElem`, but some instances of `ProjectionElem<V, T1, T2>` do
+    /// not carry a `Ty` for `T1` or `T2`.)
     ///
     /// Note that the resulting type has not been normalized.
     pub fn field_ty(self, tcx: TyCtxt<'tcx>, f: Field) -> Ty<'tcx> {
@@ -64,17 +64,18 @@ impl<'tcx> PlaceTy<'tcx> {
     /// `Ty` or downcast variant corresponding to that projection.
     /// The `handle_field` callback must map a `Field` to its `Ty`,
     /// (which should be trivial when `T` = `Ty`).
-    pub fn projection_ty_core<V, T>(
+    pub fn projection_ty_core<V, T1, T2>(
         self,
         tcx: TyCtxt<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
-        elem: &ProjectionElem<V, T>,
-        mut handle_field: impl FnMut(&Self, Field, T) -> Ty<'tcx>,
-        mut handle_opaque_cast: impl FnMut(&Self, T) -> Ty<'tcx>,
+        elem: &ProjectionElem<V, T1, T2>,
+        mut handle_field: impl FnMut(&Self, Field, T1) -> Ty<'tcx>,
+        mut handle_opaque_cast: impl FnMut(&Self, T2) -> Ty<'tcx>,
     ) -> PlaceTy<'tcx>
     where
         V: ::std::fmt::Debug,
-        T: ::std::fmt::Debug + Copy,
+        T1: ::std::fmt::Debug + Copy,
+        T2: ::std::fmt::Debug + Copy,
     {
         if self.variant_index.is_some() && !matches!(elem, ProjectionElem::Field(..)) {
             bug!("cannot use non field projection on downcasted place")