rust/compiler/rustc_hir_analysis/src/coherence/builtin.rs

//! Check properties that are required by built-in traits and set
//! up data structures required by type-checking/codegen.

use crate::errors::{CopyImplOnNonAdt, CopyImplOnTypeWithDtor, DropImplOnWrongItem};
use rustc_errors::{struct_span_err, MultiSpan};
use rustc_hir as hir;
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_hir::lang_items::LangItem;
use rustc_hir::ItemKind;
use rustc_infer::infer;
use rustc_infer::infer::outlives::env::OutlivesEnvironment;
use rustc_infer::infer::TyCtxtInferExt;
use rustc_middle::ty::adjustment::CoerceUnsizedInfo;
use rustc_middle::ty::{self, suggest_constraining_type_params, Ty, TyCtxt, TypeVisitable};
use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
use rustc_trait_selection::traits::misc::{can_type_implement_copy, CopyImplementationError};
use rustc_trait_selection::traits::predicate_for_trait_def;
use rustc_trait_selection::traits::{self, ObligationCause};
use std::collections::BTreeMap;

pub fn check_trait(tcx: TyCtxt<'_>, trait_def_id: DefId) {
    let lang_items = tcx.lang_items();
    Checker { tcx, trait_def_id }
        .check(lang_items.drop_trait(), visit_implementation_of_drop)
        .check(lang_items.copy_trait(), visit_implementation_of_copy)
        .check(lang_items.coerce_unsized_trait(), visit_implementation_of_coerce_unsized)
        .check(lang_items.dispatch_from_dyn_trait(), visit_implementation_of_dispatch_from_dyn);
}

struct Checker<'tcx> {
    tcx: TyCtxt<'tcx>,
    trait_def_id: DefId,
}

impl<'tcx> Checker<'tcx> {
    fn check<F>(&self, trait_def_id: Option<DefId>, mut f: F) -> &Self
    where
        F: FnMut(TyCtxt<'tcx>, LocalDefId),
    {
        if Some(self.trait_def_id) == trait_def_id {
            for &impl_def_id in self.tcx.hir().trait_impls(self.trait_def_id) {
                f(self.tcx, impl_def_id);
            }
        }
        self
    }
}

fn visit_implementation_of_drop(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
    // Destructors only work on local ADT types.
    match tcx.type_of(impl_did).kind() {
        ty::Adt(def, _) if def.did().is_local() => return,
        ty::Error(_) => return,
        _ => {}
    }

    let ItemKind::Impl(impl_) = tcx.hir().expect_item(impl_did).kind else { bug!("expected Drop impl item") };

    tcx.sess.emit_err(DropImplOnWrongItem { span: impl_.self_ty.span });
}

fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
    debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);

    let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);

    let self_type = tcx.type_of(impl_did);
    debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);

    let param_env = tcx.param_env(impl_did);
    assert!(!self_type.has_escaping_bound_vars());

    debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);

    let span = match tcx.hir().expect_item(impl_did).kind {
        ItemKind::Impl(hir::Impl { polarity: hir::ImplPolarity::Negative(_), .. }) => return,
        ItemKind::Impl(impl_) => impl_.self_ty.span,
        _ => bug!("expected Copy impl item"),
    };

    let cause = traits::ObligationCause::misc(span, impl_hir_id);
    match can_type_implement_copy(tcx, param_env, self_type, cause) {
        Ok(()) => {}
        Err(CopyImplementationError::InfrigingFields(fields)) => {
            let mut err = struct_span_err!(
                tcx.sess,
                span,
                E0204,
                "the trait `Copy` may not be implemented for this type"
            );

            // We'll try to suggest constraining type parameters to fulfill the requirements of
            // their `Copy` implementation.
            let mut errors: BTreeMap<_, Vec<_>> = Default::default();
            let mut bounds = vec![];

            for (field, ty) in fields {
                let field_span = tcx.def_span(field.did);
                let field_ty_span = match tcx.hir().get_if_local(field.did) {
                    Some(hir::Node::Field(field_def)) => field_def.ty.span,
                    _ => field_span,
                };
                err.span_label(field_span, "this field does not implement `Copy`");
                // Spin up a new FulfillmentContext, so we can get the _precise_ reason
                // why this field does not implement Copy. This is useful because sometimes
                // it is not immediately clear why Copy is not implemented for a field, since
                // all we point at is the field itself.
                let infcx = tcx.infer_ctxt().ignoring_regions().build();
                for error in traits::fully_solve_bound(
                    &infcx,
                    traits::ObligationCause::dummy_with_span(field_ty_span),
                    param_env,
                    ty,
                    tcx.require_lang_item(LangItem::Copy, Some(span)),
                ) {
                    let error_predicate = error.obligation.predicate;
                    // Only note if it's not the root obligation, otherwise it's trivial and
                    // should be self-explanatory (i.e. a field literally doesn't implement Copy).

                    // FIXME: This error could be more descriptive, especially if the error_predicate
                    // contains a foreign type or if it's a deeply nested type...
                    if error_predicate != error.root_obligation.predicate {
                        errors
                            .entry((ty.to_string(), error_predicate.to_string()))
                            .or_default()
                            .push(error.obligation.cause.span);
                    }
                    if let ty::PredicateKind::Clause(ty::Clause::Trait(ty::TraitPredicate {
                        trait_ref,
                        polarity: ty::ImplPolarity::Positive,
                        ..
                    })) = error_predicate.kind().skip_binder()
                    {
                        let ty = trait_ref.self_ty();
                        if let ty::Param(_) = ty.kind() {
                            bounds.push((
                                format!("{ty}"),
                                trait_ref.print_only_trait_path().to_string(),
                                Some(trait_ref.def_id),
                            ));
                        }
                    }
                }
            }
            for ((ty, error_predicate), spans) in errors {
                let span: MultiSpan = spans.into();
                err.span_note(
                    span,
                    &format!("the `Copy` impl for `{}` requires that `{}`", ty, error_predicate),
                );
            }
            suggest_constraining_type_params(
                tcx,
                tcx.hir().get_generics(impl_did).expect("impls always have generics"),
                &mut err,
                bounds.iter().map(|(param, constraint, def_id)| {
                    (param.as_str(), constraint.as_str(), *def_id)
                }),
            );
            err.emit();
        }
        Err(CopyImplementationError::NotAnAdt) => {
            tcx.sess.emit_err(CopyImplOnNonAdt { span });
        }
        Err(CopyImplementationError::HasDestructor) => {
            tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
        }
    }
}

fn visit_implementation_of_coerce_unsized(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
    debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);

    // Just compute this for the side-effects, in particular reporting
    // errors; other parts of the code may demand it for the info of
    // course.
    let span = tcx.def_span(impl_did);
    tcx.at(span).coerce_unsized_info(impl_did);
}

fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
    debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);

    let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
    let span = tcx.hir().span(impl_hir_id);

    let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));

    let source = tcx.type_of(impl_did);
    assert!(!source.has_escaping_bound_vars());
    let target = {
        let trait_ref = tcx.impl_trait_ref(impl_did).unwrap().subst_identity();
        assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);

        trait_ref.substs.type_at(1)
    };

    debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);

    let param_env = tcx.param_env(impl_did);

    let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);

    let infcx = tcx.infer_ctxt().build();
    let cause = ObligationCause::misc(span, impl_hir_id);

    use rustc_type_ir::sty::TyKind::*;
    match (source.kind(), target.kind()) {
        (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
            if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
        (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
        (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
            if def_a.is_struct() && def_b.is_struct() =>
        {
            if def_a != def_b {
                let source_path = tcx.def_path_str(def_a.did());
                let target_path = tcx.def_path_str(def_b.did());

                create_err(&format!(
                    "the trait `DispatchFromDyn` may only be implemented \
                            for a coercion between structures with the same \
                            definition; expected `{}`, found `{}`",
                    source_path, target_path,
                ))
                .emit();

                return;
            }

            if def_a.repr().c() || def_a.repr().packed() {
                create_err(
                    "structs implementing `DispatchFromDyn` may not have \
                         `#[repr(packed)]` or `#[repr(C)]`",
                )
                .emit();
            }

            let fields = &def_a.non_enum_variant().fields;

            let coerced_fields = fields
                .iter()
                .filter(|field| {
                    let ty_a = field.ty(tcx, substs_a);
                    let ty_b = field.ty(tcx, substs_b);

                    if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
                        if layout.is_zst() && layout.align.abi.bytes() == 1 {
                            // ignore ZST fields with alignment of 1 byte
                            return false;
                        }
                    }

                    if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
                        if ok.obligations.is_empty() {
                            create_err(
                                "the trait `DispatchFromDyn` may only be implemented \
                                 for structs containing the field being coerced, \
                                 ZST fields with 1 byte alignment, and nothing else",
                            )
                            .note(&format!(
                                "extra field `{}` of type `{}` is not allowed",
                                field.name, ty_a,
                            ))
                            .emit();

                            return false;
                        }
                    }

                    return true;
                })
                .collect::<Vec<_>>();

            if coerced_fields.is_empty() {
                create_err(
                    "the trait `DispatchFromDyn` may only be implemented \
                        for a coercion between structures with a single field \
                        being coerced, none found",
                )
                .emit();
            } else if coerced_fields.len() > 1 {
                create_err("implementing the `DispatchFromDyn` trait requires multiple coercions")
                    .note(
                        "the trait `DispatchFromDyn` may only be implemented \
                            for a coercion between structures with a single field \
                            being coerced",
                    )
                    .note(&format!(
                        "currently, {} fields need coercions: {}",
                        coerced_fields.len(),
                        coerced_fields
                            .iter()
                            .map(|field| {
                                format!(
                                    "`{}` (`{}` to `{}`)",
                                    field.name,
                                    field.ty(tcx, substs_a),
                                    field.ty(tcx, substs_b),
                                )
                            })
                            .collect::<Vec<_>>()
                            .join(", ")
                    ))
                    .emit();
            } else {
                let errors = traits::fully_solve_obligations(
                    &infcx,
                    coerced_fields.into_iter().map(|field| {
                        predicate_for_trait_def(
                            tcx,
                            param_env,
                            cause.clone(),
                            dispatch_from_dyn_trait,
                            0,
                            [field.ty(tcx, substs_a), field.ty(tcx, substs_b)],
                        )
                    }),
                );
                if !errors.is_empty() {
                    infcx.err_ctxt().report_fulfillment_errors(&errors, None);
                }

                // Finally, resolve all regions.
                let outlives_env = OutlivesEnvironment::new(param_env);
                let _ = infcx
                    .err_ctxt()
                    .check_region_obligations_and_report_errors(impl_did, &outlives_env);
            }
        }
        _ => {
            create_err(
                "the trait `DispatchFromDyn` may only be implemented \
                    for a coercion between structures",
            )
            .emit();
        }
    }
}

pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
    debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);

    // this provider should only get invoked for local def-ids
    let impl_did = impl_did.expect_local();
    let span = tcx.def_span(impl_did);

    let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));

    let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
        tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err.to_string()));
    });

    let source = tcx.type_of(impl_did);
    let trait_ref = tcx.impl_trait_ref(impl_did).unwrap().subst_identity();
    assert_eq!(trait_ref.def_id, coerce_unsized_trait);
    let target = trait_ref.substs.type_at(1);
    debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);

    let param_env = tcx.param_env(impl_did);
    assert!(!source.has_escaping_bound_vars());

    let err_info = CoerceUnsizedInfo { custom_kind: None };

    debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);

    let infcx = tcx.infer_ctxt().build();
    let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
    let cause = ObligationCause::misc(span, impl_hir_id);
    let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
                       mt_b: ty::TypeAndMut<'tcx>,
                       mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
        if mt_a.mutbl < mt_b.mutbl {
            infcx
                .err_ctxt()
                .report_mismatched_types(
                    &cause,
                    mk_ptr(mt_b.ty),
                    target,
                    ty::error::TypeError::Mutability,
                )
                .emit();
        }
        (mt_a.ty, mt_b.ty, unsize_trait, None)
    };
    let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
        (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
            infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
            let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
            let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
            check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
        }

        (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
            let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
            check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
        }

        (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty)),

        (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
            if def_a.is_struct() && def_b.is_struct() =>
        {
            if def_a != def_b {
                let source_path = tcx.def_path_str(def_a.did());
                let target_path = tcx.def_path_str(def_b.did());
                struct_span_err!(
                    tcx.sess,
                    span,
                    E0377,
                    "the trait `CoerceUnsized` may only be implemented \
                           for a coercion between structures with the same \
                           definition; expected `{}`, found `{}`",
                    source_path,
                    target_path
                )
                .emit();
                return err_info;
            }

            // Here we are considering a case of converting
            // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
            // which acts like a pointer to `U`, but carries along some extra data of type `T`:
            //
            //     struct Foo<T, U> {
            //         extra: T,
            //         ptr: *mut U,
            //     }
            //
            // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
            // to `Foo<T, [i32]>`. That impl would look like:
            //
            //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
            //
            // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
            // when this coercion occurs, we would be changing the
            // field `ptr` from a thin pointer of type `*mut [i32;
            // 3]` to a fat pointer of type `*mut [i32]` (with
            // extra data `3`). **The purpose of this check is to
            // make sure that we know how to do this conversion.**
            //
            // To check if this impl is legal, we would walk down
            // the fields of `Foo` and consider their types with
            // both substitutes. We are looking to find that
            // exactly one (non-phantom) field has changed its
            // type, which we will expect to be the pointer that
            // is becoming fat (we could probably generalize this
            // to multiple thin pointers of the same type becoming
            // fat, but we don't). In this case:
            //
            // - `extra` has type `T` before and type `T` after
            // - `ptr` has type `*mut U` before and type `*mut V` after
            //
            // Since just one field changed, we would then check
            // that `*mut U: CoerceUnsized<*mut V>` is implemented
            // (in other words, that we know how to do this
            // conversion). This will work out because `U:
            // Unsize<V>`, and we have a builtin rule that `*mut
            // U` can be coerced to `*mut V` if `U: Unsize<V>`.
            let fields = &def_a.non_enum_variant().fields;
            let diff_fields = fields
                .iter()
                .enumerate()
                .filter_map(|(i, f)| {
                    let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));

                    if tcx.type_of(f.did).is_phantom_data() {
                        // Ignore PhantomData fields
                        return None;
                    }

                    // Ignore fields that aren't changed; it may
                    // be that we could get away with subtyping or
                    // something more accepting, but we use
                    // equality because we want to be able to
                    // perform this check without computing
                    // variance where possible. (This is because
                    // we may have to evaluate constraint
                    // expressions in the course of execution.)
                    // See e.g., #41936.
                    if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
                        if ok.obligations.is_empty() {
                            return None;
                        }
                    }

                    // Collect up all fields that were significantly changed
                    // i.e., those that contain T in coerce_unsized T -> U
                    Some((i, a, b))
                })
                .collect::<Vec<_>>();

            if diff_fields.is_empty() {
                struct_span_err!(
                    tcx.sess,
                    span,
                    E0374,
                    "the trait `CoerceUnsized` may only be implemented \
                           for a coercion between structures with one field \
                           being coerced, none found"
                )
                .emit();
                return err_info;
            } else if diff_fields.len() > 1 {
                let item = tcx.hir().expect_item(impl_did);
                let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(t), .. }) = &item.kind {
                    t.path.span
                } else {
                    tcx.def_span(impl_did)
                };

                struct_span_err!(
                    tcx.sess,
                    span,
                    E0375,
                    "implementing the trait \
                                                `CoerceUnsized` requires multiple \
                                                coercions"
                )
                .note(
                    "`CoerceUnsized` may only be implemented for \
                          a coercion between structures with one field being coerced",
                )
                .note(&format!(
                    "currently, {} fields need coercions: {}",
                    diff_fields.len(),
                    diff_fields
                        .iter()
                        .map(|&(i, a, b)| { format!("`{}` (`{}` to `{}`)", fields[i].name, a, b) })
                        .collect::<Vec<_>>()
                        .join(", ")
                ))
                .span_label(span, "requires multiple coercions")
                .emit();
                return err_info;
            }

            let (i, a, b) = diff_fields[0];
            let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
            (a, b, coerce_unsized_trait, Some(kind))
        }

        _ => {
            struct_span_err!(
                tcx.sess,
                span,
                E0376,
                "the trait `CoerceUnsized` may only be implemented \
                       for a coercion between structures"
            )
            .emit();
            return err_info;
        }
    };

    // Register an obligation for `A: Trait<B>`.
    let cause = traits::ObligationCause::misc(span, impl_hir_id);
    let predicate =
        predicate_for_trait_def(tcx, param_env, cause, trait_def_id, 0, [source, target]);
    let errors = traits::fully_solve_obligation(&infcx, predicate);
    if !errors.is_empty() {
        infcx.err_ctxt().report_fulfillment_errors(&errors, None);
    }

    // Finally, resolve all regions.
    let outlives_env = OutlivesEnvironment::new(param_env);
    let _ = infcx.err_ctxt().check_region_obligations_and_report_errors(impl_did, &outlives_env);

    CoerceUnsizedInfo { custom_kind: kind }
}