1 //! This pass transforms derefs of Box into a deref of the pointer inside Box.
3 //! Box is not actually a pointer so it is incorrect to dereference it directly.
6 use rustc_hir::def_id::DefId;
7 use rustc_index::vec::Idx;
8 use rustc_middle::mir::patch::MirPatch;
9 use rustc_middle::mir::visit::MutVisitor;
10 use rustc_middle::mir::*;
11 use rustc_middle::ty::{Ty, TyCtxt};
13 /// Constructs the types used when accessing a Box's pointer
14 pub fn build_ptr_tys<'tcx>(
19 ) -> (Ty<'tcx>, Ty<'tcx>, Ty<'tcx>) {
20 let substs = tcx.intern_substs(&[pointee.into()]);
21 let unique_ty = tcx.bound_type_of(unique_did).subst(tcx, substs);
22 let nonnull_ty = tcx.bound_type_of(nonnull_did).subst(tcx, substs);
23 let ptr_ty = tcx.mk_imm_ptr(pointee);
25 (unique_ty, nonnull_ty, ptr_ty)
28 // Constructs the projection needed to access a Box's pointer
29 pub fn build_projection<'tcx>(
33 ) -> [PlaceElem<'tcx>; 3] {
35 PlaceElem::Field(Field::new(0), unique_ty),
36 PlaceElem::Field(Field::new(0), nonnull_ty),
37 PlaceElem::Field(Field::new(0), ptr_ty),
41 struct ElaborateBoxDerefVisitor<'tcx, 'a> {
45 local_decls: &'a mut LocalDecls<'tcx>,
46 patch: MirPatch<'tcx>,
49 impl<'tcx, 'a> MutVisitor<'tcx> for ElaborateBoxDerefVisitor<'tcx, 'a> {
50 fn tcx(&self) -> TyCtxt<'tcx> {
56 place: &mut Place<'tcx>,
57 context: visit::PlaceContext,
62 let base_ty = self.local_decls[place.local].ty;
64 // Derefer ensures that derefs are always the first projection
65 if place.projection.first() == Some(&PlaceElem::Deref) && base_ty.is_box() {
66 let source_info = self.local_decls[place.local].source_info;
68 let (unique_ty, nonnull_ty, ptr_ty) =
69 build_ptr_tys(tcx, base_ty.boxed_ty(), self.unique_did, self.nonnull_did);
71 let ptr_local = self.patch.new_internal(ptr_ty, source_info.span);
73 self.patch.add_assign(
75 Place::from(ptr_local),
76 Rvalue::Use(Operand::Copy(
77 Place::from(place.local)
78 .project_deeper(&build_projection(unique_ty, nonnull_ty, ptr_ty), tcx),
82 place.local = ptr_local;
85 self.super_place(place, context, location);
89 pub struct ElaborateBoxDerefs;
91 impl<'tcx> MirPass<'tcx> for ElaborateBoxDerefs {
92 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
93 if let Some(def_id) = tcx.lang_items().owned_box() {
94 let unique_did = tcx.adt_def(def_id).non_enum_variant().fields[0].did;
96 let Some(nonnull_def) = tcx.type_of(unique_did).ty_adt_def() else {
97 span_bug!(tcx.def_span(unique_did), "expected Box to contain Unique")
100 let nonnull_did = nonnull_def.non_enum_variant().fields[0].did;
102 let patch = MirPatch::new(body);
104 let local_decls = &mut body.local_decls;
107 ElaborateBoxDerefVisitor { tcx, unique_did, nonnull_did, local_decls, patch };
109 for (block, data) in body.basic_blocks.as_mut_preserves_cfg().iter_enumerated_mut() {
110 visitor.visit_basic_block_data(block, data);
113 visitor.patch.apply(body);
115 for debug_info in body.var_debug_info.iter_mut() {
116 if let VarDebugInfoContents::Place(place) = &mut debug_info.value {
117 let mut new_projections: Option<Vec<_>> = None;
118 let mut last_deref = 0;
120 for (i, (base, elem)) in place.iter_projections().enumerate() {
121 let base_ty = base.ty(&body.local_decls, tcx).ty;
123 if elem == PlaceElem::Deref && base_ty.is_box() {
124 let new_projections = new_projections.get_or_insert_default();
126 let (unique_ty, nonnull_ty, ptr_ty) =
127 build_ptr_tys(tcx, base_ty.boxed_ty(), unique_did, nonnull_did);
129 new_projections.extend_from_slice(&base.projection[last_deref..]);
130 new_projections.extend_from_slice(&build_projection(
131 unique_ty, nonnull_ty, ptr_ty,
133 new_projections.push(PlaceElem::Deref);
139 if let Some(mut new_projections) = new_projections {
140 new_projections.extend_from_slice(&place.projection[last_deref..]);
141 place.projection = tcx.intern_place_elems(&new_projections);
146 // box is not present, this pass doesn't need to do anything