compiler/rustc_traits/src/dropck_outlives.rs

use rustc_data_structures::fx::FxHashSet;
use rustc_hir::def_id::DefId;
use rustc_infer::infer::canonical::{Canonical, QueryResponse};
use rustc_infer::infer::TyCtxtInferExt;
use rustc_middle::query::Providers;
use rustc_middle::traits::query::{DropckConstraint, DropckOutlivesResult};
use rustc_middle::ty::GenericArgs;
use rustc_middle::ty::TyCtxt;
use rustc_trait_selection::infer::InferCtxtBuilderExt;
use rustc_trait_selection::traits::query::dropck_outlives::{
    compute_dropck_outlives_inner, dtorck_constraint_for_ty_inner,
};
use rustc_trait_selection::traits::query::{CanonicalTyGoal, NoSolution};

pub(crate) fn provide(p: &mut Providers) {
    *p = Providers { dropck_outlives, adt_dtorck_constraint, ..*p };
}

fn dropck_outlives<'tcx>(
    tcx: TyCtxt<'tcx>,
    canonical_goal: CanonicalTyGoal<'tcx>,
) -> Result<&'tcx Canonical<'tcx, QueryResponse<'tcx, DropckOutlivesResult<'tcx>>>, NoSolution> {
    debug!("dropck_outlives(goal={:#?})", canonical_goal);

    tcx.infer_ctxt().enter_canonical_trait_query(&canonical_goal, |ocx, goal| {
        compute_dropck_outlives_inner(ocx, goal)
    })
}

/// Calculates the dtorck constraint for a type.
pub(crate) fn adt_dtorck_constraint(
    tcx: TyCtxt<'_>,
    def_id: DefId,
) -> Result<&DropckConstraint<'_>, NoSolution> {
    let def = tcx.adt_def(def_id);
    let span = tcx.def_span(def_id);
    debug!("dtorck_constraint: {:?}", def);

    if def.is_manually_drop() {
        bug!("`ManuallyDrop` should have been handled by `trivial_dropck_outlives`");
    } else if def.is_phantom_data() {
        // The first generic parameter here is guaranteed to be a type because it's
        // `PhantomData`.
        let args = GenericArgs::identity_for_item(tcx, def_id);
        assert_eq!(args.len(), 1);
        let result = DropckConstraint {
            outlives: vec![],
            dtorck_types: vec![args.type_at(0)],
            overflows: vec![],
        };
        debug!("dtorck_constraint: {:?} => {:?}", def, result);
        return Ok(tcx.arena.alloc(result));
    }

    let mut result = DropckConstraint::empty();
    for field in def.all_fields() {
        let fty = tcx.type_of(field.did).instantiate_identity();
        dtorck_constraint_for_ty_inner(tcx, span, fty, 0, fty, &mut result)?;
    }
    result.outlives.extend(tcx.destructor_constraints(def));
    dedup_dtorck_constraint(&mut result);

    debug!("dtorck_constraint: {:?} => {:?}", def, result);

    Ok(tcx.arena.alloc(result))
}

fn dedup_dtorck_constraint(c: &mut DropckConstraint<'_>) {
    let mut outlives = FxHashSet::default();
    let mut dtorck_types = FxHashSet::default();

    c.outlives.retain(|&val| outlives.replace(val).is_none());
    c.dtorck_types.retain(|&val| dtorck_types.replace(val).is_none());
}