rust/src/librustc_mir/dataflow/move_paths/builder.rs

use rustc_index::vec::IndexVec;
use rustc_middle::mir::tcx::RvalueInitializationState;
use rustc_middle::mir::*;
use rustc_middle::ty::{self, TyCtxt};
use smallvec::{smallvec, SmallVec};

use std::convert::TryInto;
use std::mem;

use super::abs_domain::Lift;
use super::IllegalMoveOriginKind::*;
use super::{Init, InitIndex, InitKind, InitLocation, LookupResult, MoveError};
use super::{
    LocationMap, MoveData, MoveOut, MoveOutIndex, MovePath, MovePathIndex, MovePathLookup,
};

struct MoveDataBuilder<'a, 'tcx> {
    body: &'a Body<'tcx>,
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    data: MoveData<'tcx>,
    errors: Vec<(Place<'tcx>, MoveError<'tcx>)>,
}

impl<'a, 'tcx> MoveDataBuilder<'a, 'tcx> {
    fn new(body: &'a Body<'tcx>, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Self {
        let mut move_paths = IndexVec::new();
        let mut path_map = IndexVec::new();
        let mut init_path_map = IndexVec::new();

        MoveDataBuilder {
            body,
            tcx,
            param_env,
            errors: Vec::new(),
            data: MoveData {
                moves: IndexVec::new(),
                loc_map: LocationMap::new(body),
                rev_lookup: MovePathLookup {
                    locals: body
                        .local_decls
                        .indices()
                        .map(|i| {
                            Self::new_move_path(
                                &mut move_paths,
                                &mut path_map,
                                &mut init_path_map,
                                None,
                                Place::from(i),
                            )
                        })
                        .collect(),
                    projections: Default::default(),
                },
                move_paths,
                path_map,
                inits: IndexVec::new(),
                init_loc_map: LocationMap::new(body),
                init_path_map,
            },
        }
    }

    fn new_move_path(
        move_paths: &mut IndexVec<MovePathIndex, MovePath<'tcx>>,
        path_map: &mut IndexVec<MovePathIndex, SmallVec<[MoveOutIndex; 4]>>,
        init_path_map: &mut IndexVec<MovePathIndex, SmallVec<[InitIndex; 4]>>,
        parent: Option<MovePathIndex>,
        place: Place<'tcx>,
    ) -> MovePathIndex {
        let move_path =
            move_paths.push(MovePath { next_sibling: None, first_child: None, parent, place });

        if let Some(parent) = parent {
            let next_sibling = mem::replace(&mut move_paths[parent].first_child, Some(move_path));
            move_paths[move_path].next_sibling = next_sibling;
        }

        let path_map_ent = path_map.push(smallvec![]);
        assert_eq!(path_map_ent, move_path);

        let init_path_map_ent = init_path_map.push(smallvec![]);
        assert_eq!(init_path_map_ent, move_path);

        move_path
    }
}

impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
    /// This creates a MovePath for a given place, returning an `MovePathError`
    /// if that place can't be moved from.
    ///
    /// NOTE: places behind references *do not* get a move path, which is
    /// problematic for borrowck.
    ///
    /// Maybe we should have separate "borrowck" and "moveck" modes.
    fn move_path_for(&mut self, place: &Place<'tcx>) -> Result<MovePathIndex, MoveError<'tcx>> {
        debug!("lookup({:?})", place);
        let mut base = self.builder.data.rev_lookup.locals[place.local];

        // The move path index of the first union that we find. Once this is
        // some we stop creating child move paths, since moves from unions
        // move the whole thing.
        // We continue looking for other move errors though so that moving
        // from `*(u.f: &_)` isn't allowed.
        let mut union_path = None;

        for (i, elem) in place.projection.iter().enumerate() {
            let proj_base = &place.projection[..i];
            let body = self.builder.body;
            let tcx = self.builder.tcx;
            let place_ty = Place::ty_from(place.local, proj_base, body, tcx).ty;
            match place_ty.kind {
                ty::Ref(..) | ty::RawPtr(..) => {
                    let proj = &place.projection[..i + 1];
                    return Err(MoveError::cannot_move_out_of(
                        self.loc,
                        BorrowedContent {
                            target_place: Place {
                                local: place.local,
                                projection: tcx.intern_place_elems(proj),
                            },
                        },
                    ));
                }
                ty::Adt(adt, _) if adt.has_dtor(tcx) && !adt.is_box() => {
                    return Err(MoveError::cannot_move_out_of(
                        self.loc,
                        InteriorOfTypeWithDestructor { container_ty: place_ty },
                    ));
                }
                ty::Adt(adt, _) if adt.is_union() => {
                    union_path.get_or_insert(base);
                }
                ty::Slice(_) => {
                    return Err(MoveError::cannot_move_out_of(
                        self.loc,
                        InteriorOfSliceOrArray {
                            ty: place_ty,
                            is_index: match elem {
                                ProjectionElem::Index(..) => true,
                                _ => false,
                            },
                        },
                    ));
                }

                ty::Array(..) => {
                    if let ProjectionElem::Index(..) = elem {
                        return Err(MoveError::cannot_move_out_of(
                            self.loc,
                            InteriorOfSliceOrArray { ty: place_ty, is_index: true },
                        ));
                    }
                }

                _ => {}
            };

            if union_path.is_none() {
                base = self.add_move_path(base, elem, |tcx| Place {
                    local: place.local,
                    projection: tcx.intern_place_elems(&place.projection[..i + 1]),
                });
            }
        }

        if let Some(base) = union_path {
            // Move out of union - always move the entire union.
            Err(MoveError::UnionMove { path: base })
        } else {
            Ok(base)
        }
    }

    fn add_move_path(
        &mut self,
        base: MovePathIndex,
        elem: &PlaceElem<'tcx>,
        mk_place: impl FnOnce(TyCtxt<'tcx>) -> Place<'tcx>,
    ) -> MovePathIndex {
        let MoveDataBuilder {
            data: MoveData { rev_lookup, move_paths, path_map, init_path_map, .. },
            tcx,
            ..
        } = self.builder;
        *rev_lookup.projections.entry((base, elem.lift())).or_insert_with(move || {
            MoveDataBuilder::new_move_path(
                move_paths,
                path_map,
                init_path_map,
                Some(base),
                mk_place(*tcx),
            )
        })
    }

    fn create_move_path(&mut self, place: &Place<'tcx>) {
        // This is an non-moving access (such as an overwrite or
        // drop), so this not being a valid move path is OK.
        let _ = self.move_path_for(place);
    }
}

impl<'a, 'tcx> MoveDataBuilder<'a, 'tcx> {
    fn finalize(
        self,
    ) -> Result<MoveData<'tcx>, (MoveData<'tcx>, Vec<(Place<'tcx>, MoveError<'tcx>)>)> {
        debug!("{}", {
            debug!("moves for {:?}:", self.body.span);
            for (j, mo) in self.data.moves.iter_enumerated() {
                debug!("    {:?} = {:?}", j, mo);
            }
            debug!("move paths for {:?}:", self.body.span);
            for (j, path) in self.data.move_paths.iter_enumerated() {
                debug!("    {:?} = {:?}", j, path);
            }
            "done dumping moves"
        });

        if !self.errors.is_empty() { Err((self.data, self.errors)) } else { Ok(self.data) }
    }
}

pub(super) fn gather_moves<'tcx>(
    body: &Body<'tcx>,
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
) -> Result<MoveData<'tcx>, (MoveData<'tcx>, Vec<(Place<'tcx>, MoveError<'tcx>)>)> {
    let mut builder = MoveDataBuilder::new(body, tcx, param_env);

    builder.gather_args();

    for (bb, block) in body.basic_blocks().iter_enumerated() {
        for (i, stmt) in block.statements.iter().enumerate() {
            let source = Location { block: bb, statement_index: i };
            builder.gather_statement(source, stmt);
        }

        let terminator_loc = Location { block: bb, statement_index: block.statements.len() };
        builder.gather_terminator(terminator_loc, block.terminator());
    }

    builder.finalize()
}

impl<'a, 'tcx> MoveDataBuilder<'a, 'tcx> {
    fn gather_args(&mut self) {
        for arg in self.body.args_iter() {
            let path = self.data.rev_lookup.locals[arg];

            let init = self.data.inits.push(Init {
                path,
                kind: InitKind::Deep,
                location: InitLocation::Argument(arg),
            });

            debug!("gather_args: adding init {:?} of {:?} for argument {:?}", init, path, arg);

            self.data.init_path_map[path].push(init);
        }
    }

    fn gather_statement(&mut self, loc: Location, stmt: &Statement<'tcx>) {
        debug!("gather_statement({:?}, {:?})", loc, stmt);
        (Gatherer { builder: self, loc }).gather_statement(stmt);
    }

    fn gather_terminator(&mut self, loc: Location, term: &Terminator<'tcx>) {
        debug!("gather_terminator({:?}, {:?})", loc, term);
        (Gatherer { builder: self, loc }).gather_terminator(term);
    }
}

struct Gatherer<'b, 'a, 'tcx> {
    builder: &'b mut MoveDataBuilder<'a, 'tcx>,
    loc: Location,
}

impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
    fn gather_statement(&mut self, stmt: &Statement<'tcx>) {
        match stmt.kind {
            StatementKind::Assign(box (ref place, ref rval)) => {
                self.create_move_path(place);
                if let RvalueInitializationState::Shallow = rval.initialization_state() {
                    // Box starts out uninitialized - need to create a separate
                    // move-path for the interior so it will be separate from
                    // the exterior.
                    self.create_move_path(&self.builder.tcx.mk_place_deref(place.clone()));
                    self.gather_init(place.as_ref(), InitKind::Shallow);
                } else {
                    self.gather_init(place.as_ref(), InitKind::Deep);
                }
                self.gather_rvalue(rval);
            }
            StatementKind::FakeRead(_, ref place) => {
                self.create_move_path(place);
            }
            StatementKind::LlvmInlineAsm(ref asm) => {
                for (output, kind) in asm.outputs.iter().zip(&asm.asm.outputs) {
                    if !kind.is_indirect {
                        self.gather_init(output.as_ref(), InitKind::Deep);
                    }
                }
                for (_, input) in asm.inputs.iter() {
                    self.gather_operand(input);
                }
            }
            StatementKind::StorageLive(_) => {}
            StatementKind::StorageDead(local) => {
                self.gather_move(&Place::from(local));
            }
            StatementKind::SetDiscriminant { .. } => {
                span_bug!(
                    stmt.source_info.span,
                    "SetDiscriminant should not exist during borrowck"
                );
            }
            StatementKind::Retag { .. }
            | StatementKind::AscribeUserType(..)
            | StatementKind::Nop => {}
        }
    }

    fn gather_rvalue(&mut self, rvalue: &Rvalue<'tcx>) {
        match *rvalue {
            Rvalue::Use(ref operand)
            | Rvalue::Repeat(ref operand, _)
            | Rvalue::Cast(_, ref operand, _)
            | Rvalue::UnaryOp(_, ref operand) => self.gather_operand(operand),
            Rvalue::BinaryOp(ref _binop, ref lhs, ref rhs)
            | Rvalue::CheckedBinaryOp(ref _binop, ref lhs, ref rhs) => {
                self.gather_operand(lhs);
                self.gather_operand(rhs);
            }
            Rvalue::Aggregate(ref _kind, ref operands) => {
                for operand in operands {
                    self.gather_operand(operand);
                }
            }
            Rvalue::Ref(..)
            | Rvalue::AddressOf(..)
            | Rvalue::Discriminant(..)
            | Rvalue::Len(..)
            | Rvalue::NullaryOp(NullOp::SizeOf, _)
            | Rvalue::NullaryOp(NullOp::Box, _) => {
                // This returns an rvalue with uninitialized contents. We can't
                // move out of it here because it is an rvalue - assignments always
                // completely initialize their place.
                //
                // However, this does not matter - MIR building is careful to
                // only emit a shallow free for the partially-initialized
                // temporary.
                //
                // In any case, if we want to fix this, we have to register a
                // special move and change the `statement_effect` functions.
            }
        }
    }

    fn gather_terminator(&mut self, term: &Terminator<'tcx>) {
        match term.kind {
            TerminatorKind::Goto { target: _ }
            | TerminatorKind::Resume
            | TerminatorKind::Abort
            | TerminatorKind::GeneratorDrop
            | TerminatorKind::FalseEdges { .. }
            | TerminatorKind::FalseUnwind { .. }
            | TerminatorKind::Unreachable => {}

            TerminatorKind::Return => {
                self.gather_move(&Place::return_place());
            }

            TerminatorKind::Assert { ref cond, .. } => {
                self.gather_operand(cond);
            }

            TerminatorKind::SwitchInt { ref discr, .. } => {
                self.gather_operand(discr);
            }

            TerminatorKind::Yield { ref value, resume_arg: ref place, .. } => {
                self.gather_operand(value);
                self.create_move_path(place);
                self.gather_init(place.as_ref(), InitKind::Deep);
            }

            TerminatorKind::Drop { ref location, target: _, unwind: _ } => {
                self.gather_move(location);
            }
            TerminatorKind::DropAndReplace { ref location, ref value, .. } => {
                self.create_move_path(location);
                self.gather_operand(value);
                self.gather_init(location.as_ref(), InitKind::Deep);
            }
            TerminatorKind::Call {
                ref func,
                ref args,
                ref destination,
                cleanup: _,
                from_hir_call: _,
            } => {
                self.gather_operand(func);
                for arg in args {
                    self.gather_operand(arg);
                }
                if let Some((ref destination, _bb)) = *destination {
                    self.create_move_path(destination);
                    self.gather_init(destination.as_ref(), InitKind::NonPanicPathOnly);
                }
            }
        }
    }

    fn gather_operand(&mut self, operand: &Operand<'tcx>) {
        match *operand {
            Operand::Constant(..) | Operand::Copy(..) => {} // not-a-move
            Operand::Move(ref place) => {
                // a move
                self.gather_move(place);
            }
        }
    }

    fn gather_move(&mut self, place: &Place<'tcx>) {
        debug!("gather_move({:?}, {:?})", self.loc, place);

        if let [ref base @ .., ProjectionElem::Subslice { from, to, from_end: false }] =
            **place.projection
        {
            // Split `Subslice` patterns into the corresponding list of
            // `ConstIndex` patterns. This is done to ensure that all move paths
            // are disjoint, which is expected by drop elaboration.
            let base_place =
                Place { local: place.local, projection: self.builder.tcx.intern_place_elems(base) };
            let base_path = match self.move_path_for(&base_place) {
                Ok(path) => path,
                Err(MoveError::UnionMove { path }) => {
                    self.record_move(place, path);
                    return;
                }
                Err(error @ MoveError::IllegalMove { .. }) => {
                    self.builder.errors.push((base_place, error));
                    return;
                }
            };
            let base_ty = base_place.ty(self.builder.body, self.builder.tcx).ty;
            let len: u32 = match base_ty.kind {
                ty::Array(_, size) => {
                    let length = size.eval_usize(self.builder.tcx, self.builder.param_env);
                    length
                        .try_into()
                        .expect("slice pattern of array with more than u32::MAX elements")
                }
                _ => bug!("from_end: false slice pattern of non-array type"),
            };
            for offset in from..to {
                let elem =
                    ProjectionElem::ConstantIndex { offset, min_length: len, from_end: false };
                let path = self.add_move_path(base_path, &elem, |tcx| {
                    tcx.mk_place_elem(base_place.clone(), elem)
                });
                self.record_move(place, path);
            }
        } else {
            match self.move_path_for(place) {
                Ok(path) | Err(MoveError::UnionMove { path }) => self.record_move(place, path),
                Err(error @ MoveError::IllegalMove { .. }) => {
                    self.builder.errors.push((*place, error));
                }
            };
        }
    }

    fn record_move(&mut self, place: &Place<'tcx>, path: MovePathIndex) {
        let move_out = self.builder.data.moves.push(MoveOut { path, source: self.loc });
        debug!(
            "gather_move({:?}, {:?}): adding move {:?} of {:?}",
            self.loc, place, move_out, path
        );
        self.builder.data.path_map[path].push(move_out);
        self.builder.data.loc_map[self.loc].push(move_out);
    }

    fn gather_init(&mut self, place: PlaceRef<'tcx>, kind: InitKind) {
        debug!("gather_init({:?}, {:?})", self.loc, place);

        let mut place = place;

        // Check if we are assigning into a field of a union, if so, lookup the place
        // of the union so it is marked as initialized again.
        if let [proj_base @ .., ProjectionElem::Field(_, _)] = place.projection {
            if let ty::Adt(def, _) =
                Place::ty_from(place.local, proj_base, self.builder.body, self.builder.tcx).ty.kind
            {
                if def.is_union() {
                    place = PlaceRef { local: place.local, projection: proj_base }
                }
            }
        }

        if let LookupResult::Exact(path) = self.builder.data.rev_lookup.find(place) {
            let init = self.builder.data.inits.push(Init {
                location: InitLocation::Statement(self.loc),
                path,
                kind,
            });

            debug!(
                "gather_init({:?}, {:?}): adding init {:?} of {:?}",
                self.loc, place, init, path
            );

            self.builder.data.init_path_map[path].push(init);
            self.builder.data.init_loc_map[self.loc].push(init);
        }
    }
}