diff options
Diffstat (limited to 'compiler/rustc_mir_transform/src/elaborate_box_derefs.rs')
| -rw-r--r-- | compiler/rustc_mir_transform/src/elaborate_box_derefs.rs | 184 |
1 files changed, 184 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/elaborate_box_derefs.rs b/compiler/rustc_mir_transform/src/elaborate_box_derefs.rs new file mode 100644 index 000000000..44e3945d6 --- /dev/null +++ b/compiler/rustc_mir_transform/src/elaborate_box_derefs.rs @@ -0,0 +1,184 @@ +//! This pass transforms derefs of Box into a deref of the pointer inside Box. +//! +//! Box is not actually a pointer so it is incorrect to dereference it directly. + +use crate::MirPass; +use rustc_hir::def_id::DefId; +use rustc_index::vec::Idx; +use rustc_middle::mir::patch::MirPatch; +use rustc_middle::mir::visit::MutVisitor; +use rustc_middle::mir::*; +use rustc_middle::ty::subst::Subst; +use rustc_middle::ty::{Ty, TyCtxt}; + +/// Constructs the types used when accessing a Box's pointer +pub fn build_ptr_tys<'tcx>( + tcx: TyCtxt<'tcx>, + pointee: Ty<'tcx>, + unique_did: DefId, + nonnull_did: DefId, +) -> (Ty<'tcx>, Ty<'tcx>, Ty<'tcx>) { + let substs = tcx.intern_substs(&[pointee.into()]); + let unique_ty = tcx.bound_type_of(unique_did).subst(tcx, substs); + let nonnull_ty = tcx.bound_type_of(nonnull_did).subst(tcx, substs); + let ptr_ty = tcx.mk_imm_ptr(pointee); + + (unique_ty, nonnull_ty, ptr_ty) +} + +// Constructs the projection needed to access a Box's pointer +pub fn build_projection<'tcx>( + unique_ty: Ty<'tcx>, + nonnull_ty: Ty<'tcx>, + ptr_ty: Ty<'tcx>, +) -> [PlaceElem<'tcx>; 3] { + [ + PlaceElem::Field(Field::new(0), unique_ty), + PlaceElem::Field(Field::new(0), nonnull_ty), + PlaceElem::Field(Field::new(0), ptr_ty), + ] +} + +struct ElaborateBoxDerefVisitor<'tcx, 'a> { + tcx: TyCtxt<'tcx>, + unique_did: DefId, + nonnull_did: DefId, + local_decls: &'a mut LocalDecls<'tcx>, + patch: MirPatch<'tcx>, +} + +impl<'tcx, 'a> MutVisitor<'tcx> for ElaborateBoxDerefVisitor<'tcx, 'a> { + fn tcx(&self) -> TyCtxt<'tcx> { + self.tcx + } + + fn visit_place( + &mut self, + place: &mut Place<'tcx>, + context: visit::PlaceContext, + location: Location, + ) { + let tcx = self.tcx; + + let base_ty = self.local_decls[place.local].ty; + + // Derefer ensures that derefs are always the first projection + if place.projection.first() == Some(&PlaceElem::Deref) && base_ty.is_box() { + let source_info = self.local_decls[place.local].source_info; + + let (unique_ty, nonnull_ty, ptr_ty) = + build_ptr_tys(tcx, base_ty.boxed_ty(), self.unique_did, self.nonnull_did); + + let ptr_local = self.patch.new_temp(ptr_ty, source_info.span); + self.local_decls.push(LocalDecl::new(ptr_ty, source_info.span)); + + self.patch.add_statement(location, StatementKind::StorageLive(ptr_local)); + + self.patch.add_assign( + location, + Place::from(ptr_local), + Rvalue::Use(Operand::Copy( + Place::from(place.local) + .project_deeper(&build_projection(unique_ty, nonnull_ty, ptr_ty), tcx), + )), + ); + + place.local = ptr_local; + + self.patch.add_statement( + Location { block: location.block, statement_index: location.statement_index + 1 }, + StatementKind::StorageDead(ptr_local), + ); + } + + self.super_place(place, context, location); + } +} + +pub struct ElaborateBoxDerefs; + +impl<'tcx> MirPass<'tcx> for ElaborateBoxDerefs { + fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { + if let Some(def_id) = tcx.lang_items().owned_box() { + let unique_did = tcx.adt_def(def_id).non_enum_variant().fields[0].did; + + let Some(nonnull_def) = tcx.type_of(unique_did).ty_adt_def() else { + span_bug!(tcx.def_span(unique_did), "expected Box to contain Unique") + }; + + let nonnull_did = nonnull_def.non_enum_variant().fields[0].did; + + let patch = MirPatch::new(body); + + let local_decls = &mut body.local_decls; + + let mut visitor = + ElaborateBoxDerefVisitor { tcx, unique_did, nonnull_did, local_decls, patch }; + + for (block, BasicBlockData { statements, terminator, .. }) in + body.basic_blocks.as_mut().iter_enumerated_mut() + { + let mut index = 0; + for statement in statements { + let location = Location { block, statement_index: index }; + visitor.visit_statement(statement, location); + index += 1; + } + + if let Some(terminator) = terminator + && !matches!(terminator.kind, TerminatorKind::Yield{..}) + { + let location = Location { block, statement_index: index }; + visitor.visit_terminator(terminator, location); + } + + let location = Location { block, statement_index: index }; + match terminator { + // yielding into a box is handled when lowering generators + Some(Terminator { kind: TerminatorKind::Yield { value, .. }, .. }) => { + visitor.visit_operand(value, location); + } + Some(terminator) => { + visitor.visit_terminator(terminator, location); + } + None => {} + } + } + + visitor.patch.apply(body); + + for debug_info in body.var_debug_info.iter_mut() { + if let VarDebugInfoContents::Place(place) = &mut debug_info.value { + let mut new_projections: Option<Vec<_>> = None; + let mut last_deref = 0; + + for (i, (base, elem)) in place.iter_projections().enumerate() { + let base_ty = base.ty(&body.local_decls, tcx).ty; + + if elem == PlaceElem::Deref && base_ty.is_box() { + let new_projections = new_projections.get_or_insert_default(); + + let (unique_ty, nonnull_ty, ptr_ty) = + build_ptr_tys(tcx, base_ty.boxed_ty(), unique_did, nonnull_did); + + new_projections.extend_from_slice(&base.projection[last_deref..]); + new_projections.extend_from_slice(&build_projection( + unique_ty, nonnull_ty, ptr_ty, + )); + new_projections.push(PlaceElem::Deref); + + last_deref = i; + } + } + + if let Some(mut new_projections) = new_projections { + new_projections.extend_from_slice(&place.projection[last_deref..]); + place.projection = tcx.intern_place_elems(&new_projections); + } + } + } + } else { + // box is not present, this pass doesn't need to do anything + } + } +} |