dropck_outlives.rs

use crate::infer::at::At;
use crate::infer::InferOk;
use crate::infer::canonical::OriginalQueryValues;
use std::iter::FromIterator;
use syntax::source_map::Span;
use crate::ty::subst::Kind;
use crate::ty::{self, Ty, TyCtxt};

impl<'cx, 'tcx> At<'cx, 'tcx> {
    /// Given a type `ty` of some value being dropped, computes a set
    /// of "kinds" (types, regions) that must be outlive the execution
    /// of the destructor. These basically correspond to data that the
    /// destructor might access. This is used during regionck to
    /// impose "outlives" constraints on any lifetimes referenced
    /// within.
    ///
    /// The rules here are given by the "dropck" RFCs, notably [#1238]
    /// and [#1327]. This is a fixed-point computation, where we
    /// explore all the data that will be dropped (transitively) when
    /// a value of type `ty` is dropped. For each type T that will be
    /// dropped and which has a destructor, we must assume that all
    /// the types/regions of T are live during the destructor, unless
    /// they are marked with a special attribute (`#[may_dangle]`).
    ///
    /// [#1238]: https://github.com/rust-lang/rfcs/blob/master/text/1238-nonparametric-dropck.md
    /// [#1327]: https://github.com/rust-lang/rfcs/blob/master/text/1327-dropck-param-eyepatch.md
    pub fn dropck_outlives(&self, ty: Ty<'tcx>) -> InferOk<'tcx, Vec<Kind<'tcx>>> {
        debug!(
            "dropck_outlives(ty={:?}, param_env={:?})",
            ty, self.param_env,
        );

        // Quick check: there are a number of cases that we know do not require
        // any destructor.
        let tcx = self.infcx.tcx;
        if trivial_dropck_outlives(tcx, ty) {
            return InferOk {
                value: vec![],
                obligations: vec![],
            };
        }

        let gcx = tcx.global_tcx();
        let mut orig_values = OriginalQueryValues::default();
        let c_ty = self.infcx.canonicalize_query(&self.param_env.and(ty), &mut orig_values);
        let span = self.cause.span;
        debug!("c_ty = {:?}", c_ty);
        if let Ok(result) = &gcx.dropck_outlives(c_ty) {
            if result.is_proven() {
                if let Ok(InferOk { value, obligations }) =
                    self.infcx.instantiate_query_response_and_region_obligations(
                    self.cause,
                    self.param_env,
                    &orig_values,
                    result)
                {
                    let ty = self.infcx.resolve_vars_if_possible(&ty);
                    let kinds = value.into_kinds_reporting_overflows(tcx, span, ty);
                    return InferOk {
                        value: kinds,
                        obligations,
                    };
                }
            }
        }

        // Errors and ambiuity in dropck occur in two cases:
        // - unresolved inference variables at the end of typeck
        // - non well-formed types where projections cannot be resolved
        // Either of these should have created an error before.
        tcx.sess
            .delay_span_bug(span, "dtorck encountered internal error");

        InferOk {
            value: vec![],
            obligations: vec![],
        }
    }
}

#[derive(Clone, Debug, Default)]
pub struct DropckOutlivesResult<'tcx> {
    pub kinds: Vec<Kind<'tcx>>,
    pub overflows: Vec<Ty<'tcx>>,
}

impl<'tcx> DropckOutlivesResult<'tcx> {
    pub fn report_overflows(&self, tcx: TyCtxt<'tcx>, span: Span, ty: Ty<'tcx>) {
        if let Some(overflow_ty) = self.overflows.iter().next() {
            let mut err = struct_span_err!(
                tcx.sess,
                span,
                E0320,
                "overflow while adding drop-check rules for {}",
                ty,
            );
            err.note(&format!("overflowed on {}", overflow_ty));
            err.emit();
        }
    }

    pub fn into_kinds_reporting_overflows(
        self,
        tcx: TyCtxt<'tcx>,
        span: Span,
        ty: Ty<'tcx>,
    ) -> Vec<Kind<'tcx>> {
        self.report_overflows(tcx, span, ty);
        let DropckOutlivesResult { kinds, overflows: _ } = self;
        kinds
    }
}

/// A set of constraints that need to be satisfied in order for
/// a type to be valid for destruction.
#[derive(Clone, Debug)]
pub struct DtorckConstraint<'tcx> {
    /// Types that are required to be alive in order for this
    /// type to be valid for destruction.
    pub outlives: Vec<ty::subst::Kind<'tcx>>,

    /// Types that could not be resolved: projections and params.
    pub dtorck_types: Vec<Ty<'tcx>>,

    /// If, during the computation of the dtorck constraint, we
    /// overflow, that gets recorded here. The caller is expected to
    /// report an error.
    pub overflows: Vec<Ty<'tcx>>,
}

impl<'tcx> DtorckConstraint<'tcx> {
    pub fn empty() -> DtorckConstraint<'tcx> {
        DtorckConstraint {
            outlives: vec![],
            dtorck_types: vec![],
            overflows: vec![],
        }
    }
}

impl<'tcx> FromIterator<DtorckConstraint<'tcx>> for DtorckConstraint<'tcx> {
    fn from_iter<I: IntoIterator<Item = DtorckConstraint<'tcx>>>(iter: I) -> Self {
        let mut result = Self::empty();

        for DtorckConstraint { outlives, dtorck_types, overflows } in iter {
            result.outlives.extend(outlives);
            result.dtorck_types.extend(dtorck_types);
            result.overflows.extend(overflows);
        }

        result
    }
}
BraceStructTypeFoldableImpl! {
    impl<'tcx> TypeFoldable<'tcx> for DropckOutlivesResult<'tcx> {
        kinds, overflows
    }
}

BraceStructLiftImpl! {
    impl<'a, 'tcx> Lift<'tcx> for DropckOutlivesResult<'a> {
        type Lifted = DropckOutlivesResult<'tcx>;
        kinds, overflows
    }
}

impl_stable_hash_for!(struct DropckOutlivesResult<'tcx> {
    kinds, overflows
});

impl_stable_hash_for!(struct DtorckConstraint<'tcx> {
    outlives,
    dtorck_types,
    overflows
});

/// This returns true if the type `ty` is "trivial" for
/// dropck-outlives -- that is, if it doesn't require any types to
/// outlive. This is similar but not *quite* the same as the
/// `needs_drop` test in the compiler already -- that is, for every
/// type T for which this function return true, needs-drop would
/// return `false`. But the reverse does not hold: in particular,
/// `needs_drop` returns false for `PhantomData`, but it is not
/// trivial for dropck-outlives.
///
/// Note also that `needs_drop` requires a "global" type (i.e., one
/// with erased regions), but this function does not.
pub fn trivial_dropck_outlives<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
    match ty.sty {
        // None of these types have a destructor and hence they do not
        // require anything in particular to outlive the dtor's
        // execution.
        ty::Infer(ty::FreshIntTy(_))
        | ty::Infer(ty::FreshFloatTy(_))
        | ty::Bool
        | ty::Int(_)
        | ty::Uint(_)
        | ty::Float(_)
        | ty::Never
        | ty::FnDef(..)
        | ty::FnPtr(_)
        | ty::Char
        | ty::GeneratorWitness(..)
        | ty::RawPtr(_)
        | ty::Ref(..)
        | ty::Str
        | ty::Foreign(..)
        | ty::Error => true,

        // [T; N] and [T] have same properties as T.
        ty::Array(ty, _) | ty::Slice(ty) => trivial_dropck_outlives(tcx, ty),

        // (T1..Tn) and closures have same properties as T1..Tn --
        // check if *any* of those are trivial.
        ty::Tuple(ref tys) => tys.iter().all(|t| trivial_dropck_outlives(tcx, t.expect_ty())),
        ty::Closure(def_id, ref substs) => substs
            .upvar_tys(def_id, tcx)
            .all(|t| trivial_dropck_outlives(tcx, t)),

        ty::Adt(def, _) => {
            if Some(def.did) == tcx.lang_items().manually_drop() {
                // `ManuallyDrop` never has a dtor.
                true
            } else {
                // Other types might. Moreover, PhantomData doesn't
                // have a dtor, but it is considered to own its
                // content, so it is non-trivial. Unions can have `impl Drop`,
                // and hence are non-trivial as well.
                false
            }
        }

        // The following *might* require a destructor: needs deeper inspection.
        ty::Dynamic(..)
        | ty::Projection(..)
        | ty::Param(_)
        | ty::Opaque(..)
        | ty::Placeholder(..)
        | ty::Infer(_)
        | ty::Bound(..)
        | ty::Generator(..) => false,

        ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
    }
}