rust/src/librustc_ty/needs_drop.rs

//! Check whether a type has (potentially) non-trivial drop glue.

use rustc::ty::subst::Subst;
use rustc::ty::util::{needs_drop_components, AlwaysRequiresDrop};
use rustc::ty::{self, Ty, TyCtxt};
use rustc_data_structures::fx::FxHashSet;
use rustc_hir::def_id::DefId;
use rustc_span::DUMMY_SP;

type NeedsDropResult<T> = Result<T, AlwaysRequiresDrop>;

fn needs_drop_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
    let adt_fields =
        move |adt_def: &ty::AdtDef| tcx.adt_drop_tys(adt_def.did).map(|tys| tys.iter().copied());
    // If we don't know a type doesn't need drop, say it's a type parameter
    // without a `Copy` bound, then we conservatively return that it needs
    // drop.
    let res = NeedsDropTypes::new(tcx, query.param_env, query.value, adt_fields).next().is_some();
    debug!("needs_drop_raw({:?}) = {:?}", query, res);
    res
}

struct NeedsDropTypes<'tcx, F> {
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    query_ty: Ty<'tcx>,
    seen_tys: FxHashSet<Ty<'tcx>>,
    /// A stack of types left to process. Each round, we pop something from the
    /// stack and check if it needs drop. If the result depends on whether some
    /// other types need drop we push them onto the stack.
    unchecked_tys: Vec<(Ty<'tcx>, usize)>,
    recursion_limit: usize,
    adt_components: F,
}

impl<'tcx, F> NeedsDropTypes<'tcx, F> {
    fn new(
        tcx: TyCtxt<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        ty: Ty<'tcx>,
        adt_components: F,
    ) -> Self {
        let mut seen_tys = FxHashSet::default();
        seen_tys.insert(ty);
        let recursion_limit = *tcx.sess.recursion_limit.get();
        Self {
            tcx,
            param_env,
            seen_tys,
            query_ty: ty,
            unchecked_tys: vec![(ty, 0)],
            recursion_limit,
            adt_components,
        }
    }
}

impl<'tcx, F, I> Iterator for NeedsDropTypes<'tcx, F>
where
    F: Fn(&ty::AdtDef) -> NeedsDropResult<I>,
    I: Iterator<Item = Ty<'tcx>>,
{
    type Item = NeedsDropResult<Ty<'tcx>>;

    fn next(&mut self) -> Option<NeedsDropResult<Ty<'tcx>>> {
        let tcx = self.tcx;

        while let Some((ty, level)) = self.unchecked_tys.pop() {
            if level > self.recursion_limit {
                // Not having a `Span` isn't great. But there's hopefully some other
                // recursion limit error as well.
                tcx.sess.span_err(
                    DUMMY_SP,
                    &format!("overflow while checking whether `{}` requires drop", self.query_ty),
                );
                return Some(Err(AlwaysRequiresDrop));
            }

            let components = match needs_drop_components(ty) {
                Err(e) => return Some(Err(e)),
                Ok(components) => components,
            };
            debug!("needs_drop_components({:?}) = {:?}", ty, components);

            for component in components {
                match component.kind {
                    _ if component.is_copy_modulo_regions(tcx, self.param_env, DUMMY_SP) => (),

                    ty::Array(elem_ty, len) => {
                        // Zero-length arrays never contain anything to drop.
                        if len.try_eval_usize(tcx, self.param_env) != Some(0) {
                            if self.seen_tys.insert(elem_ty) {
                                self.unchecked_tys.push((elem_ty, level + 1));
                            }
                        }
                    }

                    ty::Closure(def_id, substs) => {
                        for upvar_ty in substs.as_closure().upvar_tys(def_id, tcx) {
                            if self.seen_tys.insert(upvar_ty) {
                                self.unchecked_tys.push((upvar_ty, level + 1));
                            }
                        }
                    }

                    // Check for a `Drop` impl and whether this is a union or
                    // `ManuallyDrop`. If it's a struct or enum without a `Drop`
                    // impl then check whether the field types need `Drop`.
                    ty::Adt(adt_def, substs) => {
                        let tys = match (self.adt_components)(adt_def) {
                            Err(e) => return Some(Err(e)),
                            Ok(tys) => tys,
                        };
                        for required_ty in tys {
                            let subst_ty = tcx.normalize_erasing_regions(
                                self.param_env,
                                required_ty.subst(tcx, substs),
                            );
                            if self.seen_tys.insert(subst_ty) {
                                self.unchecked_tys.push((subst_ty, level + 1));
                            }
                        }
                    }
                    ty::Opaque(..) | ty::Projection(..) | ty::Param(_) => {
                        if ty == component {
                            // Return the type to the caller so they can decide
                            // what to do with it.
                            return Some(Ok(component));
                        } else if self.seen_tys.insert(component) {
                            // Store the type for later. We can't return here
                            // because we would then lose any other components
                            // of the type.
                            self.unchecked_tys.push((component, level + 1));
                        }
                    }
                    _ => return Some(Err(AlwaysRequiresDrop)),
                }
            }
        }

        return None;
    }
}

fn adt_drop_tys(tcx: TyCtxt<'_>, def_id: DefId) -> Result<&ty::List<Ty<'_>>, AlwaysRequiresDrop> {
    let adt_components = move |adt_def: &ty::AdtDef| {
        if adt_def.is_manually_drop() {
            debug!("adt_drop_tys: `{:?}` is manually drop", adt_def);
            return Ok(Vec::new().into_iter());
        } else if adt_def.destructor(tcx).is_some() {
            debug!("adt_drop_tys: `{:?}` implements `Drop`", adt_def);
            return Err(AlwaysRequiresDrop);
        } else if adt_def.is_union() {
            debug!("adt_drop_tys: `{:?}` is a union", adt_def);
            return Ok(Vec::new().into_iter());
        }
        Ok(adt_def.all_fields().map(|field| tcx.type_of(field.did)).collect::<Vec<_>>().into_iter())
    };

    let adt_ty = tcx.type_of(def_id);
    let param_env = tcx.param_env(def_id);
    let res: Result<Vec<_>, _> =
        NeedsDropTypes::new(tcx, param_env, adt_ty, adt_components).collect();

    debug!("adt_drop_tys(`{}`) = `{:?}`", tcx.def_path_str(def_id), res);
    res.map(|components| tcx.intern_type_list(&components))
}

pub(crate) fn provide(providers: &mut ty::query::Providers<'_>) {
    *providers = ty::query::Providers { needs_drop_raw, adt_drop_tys, ..*providers };
}