structural_match.rs

use crate::infer::{InferCtxt, TyCtxtInferExt};
use crate::traits::ObligationCause;
use crate::traits::{self, ConstPatternStructural, TraitEngine};

use rustc::ty::{self, AdtDef, Ty, TyCtxt, TypeFoldable, TypeVisitor};
use rustc_data_structures::fx::FxHashSet;
use rustc_hir as hir;
use rustc_span::Span;

#[derive(Debug)]
pub enum NonStructuralMatchTy<'tcx> {
    Adt(&'tcx AdtDef),
    Param,
}

/// This method traverses the structure of `ty`, trying to find an
/// instance of an ADT (i.e. struct or enum) that doesn't implement
/// the structural-match traits, or a generic type parameter
/// (which cannot be determined to be structural-match).
///
/// The "structure of a type" includes all components that would be
/// considered when doing a pattern match on a constant of that
/// type.
///
///  * This means this method descends into fields of structs/enums,
///    and also descends into the inner type `T` of `&T` and `&mut T`
///
///  * The traversal doesn't dereference unsafe pointers (`*const T`,
///    `*mut T`), and it does not visit the type arguments of an
///    instantiated generic like `PhantomData<T>`.
///
/// The reason we do this search is Rust currently require all ADTs
/// reachable from a constant's type to implement the
/// structural-match traits, which essentially say that
/// the implementation of `PartialEq::eq` behaves *equivalently* to a
/// comparison against the unfolded structure.
///
/// For more background on why Rust has this requirement, and issues
/// that arose when the requirement was not enforced completely, see
/// Rust RFC 1445, rust-lang/rust#61188, and rust-lang/rust#62307.
pub fn search_for_structural_match_violation<'tcx>(
    id: hir::HirId,
    span: Span,
    tcx: TyCtxt<'tcx>,
    ty: Ty<'tcx>,
) -> Option<NonStructuralMatchTy<'tcx>> {
    // FIXME: we should instead pass in an `infcx` from the outside.
    tcx.infer_ctxt().enter(|infcx| {
        let mut search = Search { id, span, infcx, found: None, seen: FxHashSet::default() };
        ty.visit_with(&mut search);
        search.found
    })
}

/// This method returns true if and only if `adt_ty` itself has been marked as
/// eligible for structural-match: namely, if it implements both
/// `StructuralPartialEq` and `StructuralEq` (which are respectively injected by
/// `#[derive(PartialEq)]` and `#[derive(Eq)]`).
///
/// Note that this does *not* recursively check if the substructure of `adt_ty`
/// implements the traits.
pub fn type_marked_structural(
    id: hir::HirId,
    span: Span,
    infcx: &InferCtxt<'_, 'tcx>,
    adt_ty: Ty<'tcx>,
) -> bool {
    let mut fulfillment_cx = traits::FulfillmentContext::new();
    let cause = ObligationCause::new(span, id, ConstPatternStructural);
    // require `#[derive(PartialEq)]`
    let structural_peq_def_id = infcx.tcx.lang_items().structural_peq_trait().unwrap();
    fulfillment_cx.register_bound(
        infcx,
        ty::ParamEnv::empty(),
        adt_ty,
        structural_peq_def_id,
        cause,
    );
    // for now, require `#[derive(Eq)]`. (Doing so is a hack to work around
    // the type `for<'a> fn(&'a ())` failing to implement `Eq` itself.)
    let cause = ObligationCause::new(span, id, ConstPatternStructural);
    let structural_teq_def_id = infcx.tcx.lang_items().structural_teq_trait().unwrap();
    fulfillment_cx.register_bound(
        infcx,
        ty::ParamEnv::empty(),
        adt_ty,
        structural_teq_def_id,
        cause,
    );

    // We deliberately skip *reporting* fulfillment errors (via
    // `report_fulfillment_errors`), for two reasons:
    //
    // 1. The error messages would mention `std::marker::StructuralPartialEq`
    //    (a trait which is solely meant as an implementation detail
    //    for now), and
    //
    // 2. We are sometimes doing future-incompatibility lints for
    //    now, so we do not want unconditional errors here.
    fulfillment_cx.select_all_or_error(infcx).is_ok()
}

/// This implements the traversal over the structure of a given type to try to
/// find instances of ADTs (specifically structs or enums) that do not implement
/// the structural-match traits (`StructuralPartialEq` and `StructuralEq`).
struct Search<'a, 'tcx> {
    id: hir::HirId,
    span: Span,

    infcx: InferCtxt<'a, 'tcx>,

    /// Records first ADT that does not implement a structural-match trait.
    found: Option<NonStructuralMatchTy<'tcx>>,

    /// Tracks ADTs previously encountered during search, so that
    /// we will not recur on them again.
    seen: FxHashSet<hir::def_id::DefId>,
}

impl Search<'a, 'tcx> {
    fn tcx(&self) -> TyCtxt<'tcx> {
        self.infcx.tcx
    }

    fn type_marked_structural(&self, adt_ty: Ty<'tcx>) -> bool {
        type_marked_structural(self.id, self.span, &self.infcx, adt_ty)
    }
}

impl<'a, 'tcx> TypeVisitor<'tcx> for Search<'a, 'tcx> {
    fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
        debug!("Search visiting ty: {:?}", ty);

        let (adt_def, substs) = match ty.kind {
            ty::Adt(adt_def, substs) => (adt_def, substs),
            ty::Param(_) => {
                self.found = Some(NonStructuralMatchTy::Param);
                return true; // Stop visiting.
            }
            ty::RawPtr(..) => {
                // structural-match ignores substructure of
                // `*const _`/`*mut _`, so skip `super_visit_with`.
                //
                // For example, if you have:
                // ```
                // struct NonStructural;
                // #[derive(PartialEq, Eq)]
                // struct T(*const NonStructural);
                // const C: T = T(std::ptr::null());
                // ```
                //
                // Even though `NonStructural` does not implement `PartialEq`,
                // structural equality on `T` does not recur into the raw
                // pointer. Therefore, one can still use `C` in a pattern.

                // (But still tell caller to continue search.)
                return false;
            }
            ty::FnDef(..) | ty::FnPtr(..) => {
                // types of formals and return in `fn(_) -> _` are also irrelevant;
                // so we do not recur into them via `super_visit_with`
                //
                // (But still tell caller to continue search.)
                return false;
            }
            ty::Array(_, n)
                if { n.try_eval_usize(self.tcx(), ty::ParamEnv::reveal_all()) == Some(0) } =>
            {
                // rust-lang/rust#62336: ignore type of contents
                // for empty array.
                return false;
            }
            _ => {
                ty.super_visit_with(self);
                return false;
            }
        };

        if !self.seen.insert(adt_def.did) {
            debug!("Search already seen adt_def: {:?}", adt_def);
            // let caller continue its search
            return false;
        }

        if !self.type_marked_structural(ty) {
            debug!("Search found ty: {:?}", ty);
            self.found = Some(NonStructuralMatchTy::Adt(&adt_def));
            return true; // Halt visiting!
        }

        // structural-match does not care about the
        // instantiation of the generics in an ADT (it
        // instead looks directly at its fields outside
        // this match), so we skip super_visit_with.
        //
        // (Must not recur on substs for `PhantomData<T>` cf
        // rust-lang/rust#55028 and rust-lang/rust#55837; but also
        // want to skip substs when only uses of generic are
        // behind unsafe pointers `*const T`/`*mut T`.)

        // even though we skip super_visit_with, we must recur on
        // fields of ADT.
        let tcx = self.tcx();
        for field_ty in adt_def.all_fields().map(|field| field.ty(tcx, substs)) {
            if field_ty.visit_with(self) {
                // found an ADT without structural-match; halt visiting!
                assert!(self.found.is_some());
                return true;
            }
        }

        // Even though we do not want to recur on substs, we do
        // want our caller to continue its own search.
        false
    }
}