Remove THIR::ExprKind::SelfRef

ExprKind::SelfRef was used to express accessing `self` in
the desugared Closure/Generator struct when lowering captures in THIR.

Since we handle captures in MIR now, we don't need `ExprKind::Self`.

compiler/rustc_mir_build/src/build/expr/as_place.rs

@@ -184,7 +184,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 this.lower_closure_capture(block, capture_index, *upvar_id)
             }
 
-            ExprKind::SelfRef => block.and(PlaceBuilder::from(Local::new(1))),
             ExprKind::VarRef { id } => {
                 let place_builder = if this.is_bound_var_in_guard(id) {
                     let index = this.var_local_id(id, RefWithinGuard);

compiler/rustc_mir_build/src/build/expr/as_rvalue.rs

@@ -250,7 +250,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Deref { .. }
             | ExprKind::Index { .. }
             | ExprKind::VarRef { .. }
-            | ExprKind::SelfRef
             | ExprKind::UpvarRef { .. }
             | ExprKind::Break { .. }
             | ExprKind::Continue { .. }

compiler/rustc_mir_build/src/build/expr/category.rs

@@ -38,7 +38,6 @@ impl Category {
             ExprKind::Field { .. }
             | ExprKind::Deref { .. }
             | ExprKind::Index { .. }
-            | ExprKind::SelfRef
             | ExprKind::UpvarRef { .. }
             | ExprKind::VarRef { .. }
             | ExprKind::PlaceTypeAscription { .. }

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -400,7 +400,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
             // Avoid creating a temporary
             ExprKind::VarRef { .. }
-            | ExprKind::SelfRef
             | ExprKind::UpvarRef { .. }
             | ExprKind::PlaceTypeAscription { .. }
             | ExprKind::ValueTypeAscription { .. } => {

compiler/rustc_mir_build/src/thir/mod.rs

@@ -219,8 +219,6 @@ crate enum ExprKind<'tcx> {
         /// HirId of the root variable
         var_hir_id: hir::HirId,
     },
-    /// first argument, used for self in a closure
-    SelfRef,
     Borrow {
         borrow_kind: BorrowKind,
         arg: ExprRef<'tcx>,