rustc_mir_build/thir/pattern/

const_to_pat.rs

use core::ops::ControlFlow;

use rustc_abi::{FieldIdx, VariantIdx};
use rustc_apfloat::Float;
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::{Diag, msg};
use rustc_hir as hir;
use rustc_hir::find_attr;
use rustc_index::Idx;
use rustc_infer::infer::TyCtxtInferExt;
use rustc_infer::traits::Obligation;
use rustc_middle::mir::interpret::ErrorHandled;
use rustc_middle::span_bug;
use rustc_middle::thir::{FieldPat, Pat, PatKind};
use rustc_middle::ty::{
    self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitableExt, TypeVisitor, Unnormalized,
};
use rustc_span::def_id::DefId;
use rustc_span::{DUMMY_SP, Span};
use rustc_trait_selection::traits::ObligationCause;
use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
use tracing::{debug, instrument, trace};

use super::PatCtxt;
use crate::diagnostics::{
    ConstPatternDependsOnGenericParameter, CouldNotEvalConstPattern, InvalidPattern, NaNPattern,
    PointerPattern, SuggestEq, TypeNotPartialEq, TypeNotStructural, UnionPattern, UnsizedPattern,
};

impl<'tcx, 'ptcx> PatCtxt<'tcx, 'ptcx> {
    /// Converts a constant to a pattern (if possible).
    /// This means aggregate values (like structs and enums) are converted
    /// to a pattern that matches the value (as if you'd compared via structural equality).
    ///
    /// Only type system constants are supported, as we are using valtrees
    /// as an intermediate step. Unfortunately those don't carry a type
    /// so we have to carry one ourselves.
    #[instrument(level = "debug", skip(self), ret)]
    pub(super) fn const_to_pat(
        &self,
        c: ty::Const<'tcx>,
        ty: Ty<'tcx>,
        id: hir::HirId,
        span: Span,
    ) -> Box<Pat<'tcx>> {
        let mut convert = ConstToPat::new(self, id, span, c);

        match c.kind() {
            ty::ConstKind::Unevaluated(uv) => convert.unevaluated_to_pat(uv, ty),
            ty::ConstKind::Value(value) => convert.valtree_to_pat(value),
            _ => span_bug!(span, "Invalid `ConstKind` for `const_to_pat`: {:?}", c),
        }
    }
}

struct ConstToPat<'tcx> {
    tcx: TyCtxt<'tcx>,
    typing_env: ty::TypingEnv<'tcx>,
    span: Span,
    id: hir::HirId,

    c: ty::Const<'tcx>,
}

impl<'tcx> ConstToPat<'tcx> {
    fn new(pat_ctxt: &PatCtxt<'tcx, '_>, id: hir::HirId, span: Span, c: ty::Const<'tcx>) -> Self {
        trace!(?pat_ctxt.typeck_results.hir_owner);
        ConstToPat { tcx: pat_ctxt.tcx, typing_env: pat_ctxt.typing_env, span, id, c }
    }

    fn type_marked_structural(&self, ty: Ty<'tcx>) -> bool {
        ty.is_structural_eq_shallow(self.tcx)
    }

    /// We errored. Signal that in the pattern, so that follow up errors can be silenced.
    fn mk_err(&self, mut err: Diag<'_>, ty: Ty<'tcx>) -> Box<Pat<'tcx>> {
        if let ty::ConstKind::Unevaluated(uv) = self.c.kind() {
            if let ty::UnevaluatedConstKind::Projection { def_id }
            | ty::UnevaluatedConstKind::Inherent { def_id } = uv.kind
                && let Some(def_id) = def_id.as_local()
            {
                // Include the container item in the output.
                err.span_label(self.tcx.def_span(self.tcx.local_parent(def_id)), "");
            }
            if let ty::UnevaluatedConstKind::Projection { def_id }
            | ty::UnevaluatedConstKind::Inherent { def_id }
            | ty::UnevaluatedConstKind::Free { def_id } = uv.kind
            {
                err.span_label(self.tcx.def_span(def_id), msg!("constant defined here"));
            }
        }
        Box::new(Pat { span: self.span, ty, kind: PatKind::Error(err.emit()), extra: None })
    }

    fn unevaluated_to_pat(
        &mut self,
        uv: ty::UnevaluatedConst<'tcx>,
        ty: Ty<'tcx>,
    ) -> Box<Pat<'tcx>> {
        // It's not *technically* correct to be revealing opaque types here as borrowcheck has
        // not run yet. However, CTFE itself uses `TypingMode::PostAnalysis` unconditionally even
        // during typeck and not doing so has a lot of (undesirable) fallout (#101478, #119821).
        // As a result we always use a revealed env when resolving the instance to evaluate.
        //
        // FIXME: `const_eval_resolve_for_typeck` should probably just modify the env itself
        // instead of having this logic here
        let typing_env =
            self.tcx.erase_and_anonymize_regions(self.typing_env).with_codegen_normalized(self.tcx);
        let uv = self.tcx.erase_and_anonymize_regions(uv);

        // FIXME(gca): This will become insufficient once associated constants can be
        // implemented as `type` consts (project-const-generics#76). At that point it'll
        // become necessary to just use type system normalization for all const patterns
        // but that's not yet possible.
        let mut thir_pat = if uv.kind.is_type_const(self.tcx) {
            let Ok(normalize) = self
                .tcx
                .try_normalize_erasing_regions(self.typing_env, Unnormalized::new_wip(self.c))
            else {
                let err = self.tcx.dcx().create_err(CouldNotEvalConstPattern { span: self.span });
                return self.mk_err(err, ty);
            };

            let ty::ConstKind::Value(value) = normalize.kind() else {
                let err = self.tcx.dcx().create_err(CouldNotEvalConstPattern { span: self.span });
                return self.mk_err(err, ty);
            };
            self.valtree_to_pat(value)
        } else {
            // try to resolve e.g. associated constants to their definition on an impl, and then
            // evaluate the const.
            let valtree = match self.tcx.const_eval_resolve_for_typeck(typing_env, uv, self.span) {
                Ok(Ok(c)) => c,
                Err(ErrorHandled::Reported(_, _)) => {
                    // Let's tell the use where this failing const occurs.
                    let mut err =
                        self.tcx.dcx().create_err(CouldNotEvalConstPattern { span: self.span });
                    // We've emitted an error on the original const, it would be redundant to complain
                    // on its use as well.
                    if let ty::ConstKind::Unevaluated(uv) = self.c.kind()
                        && let ty::UnevaluatedConstKind::Projection { .. }
                        | ty::UnevaluatedConstKind::Inherent { .. }
                        | ty::UnevaluatedConstKind::Free { .. } = uv.kind
                    {
                        err.downgrade_to_delayed_bug();
                    }
                    return self.mk_err(err, ty);
                }
                Err(ErrorHandled::TooGeneric(_)) => {
                    let mut e = self
                        .tcx
                        .dcx()
                        .create_err(ConstPatternDependsOnGenericParameter { span: self.span });
                    for arg in uv.args {
                        if let ty::GenericArgKind::Type(ty) = arg.kind()
                            && let ty::Param(param_ty) = ty.kind()
                        {
                            let def_id = self.tcx.hir_enclosing_body_owner(self.id);
                            let generics = self.tcx.generics_of(def_id);
                            let param = generics.type_param(*param_ty, self.tcx);
                            let span = self.tcx.def_span(param.def_id);
                            e.span_label(span, "constant depends on this generic parameter");
                            if let Some(ident) = self.tcx.def_ident_span(def_id)
                                && self.tcx.sess.source_map().is_multiline(ident.between(span))
                            {
                                // Display the `fn` name as well in the diagnostic, as the generic isn't
                                // in the same line and it could be confusing otherwise.
                                e.span_label(ident, "");
                            }
                        }
                    }
                    return self.mk_err(e, ty);
                }
                Ok(Err(bad_ty)) => {
                    // The pattern cannot be turned into a valtree.
                    let e = match bad_ty.kind() {
                        ty::Adt(def, ..) => {
                            assert!(def.is_union());
                            self.tcx.dcx().create_err(UnionPattern { span: self.span })
                        }
                        ty::FnPtr(..) | ty::RawPtr(..) => {
                            self.tcx.dcx().create_err(PointerPattern { span: self.span })
                        }
                        _ => self.tcx.dcx().create_err(InvalidPattern {
                            span: self.span,
                            non_sm_ty: bad_ty,
                            prefix: bad_ty.prefix_string(self.tcx).to_string(),
                        }),
                    };
                    return self.mk_err(e, ty);
                }
            };

            // Lower the valtree to a THIR pattern.
            self.valtree_to_pat(ty::Value { ty, valtree })
        };

        if !thir_pat.references_error() {
            // Always check for `PartialEq` if we had no other errors yet.
            if !type_has_partial_eq_impl(self.tcx, typing_env, ty).has_impl {
                let mut err = self.tcx.dcx().create_err(TypeNotPartialEq { span: self.span, ty });
                extend_type_not_partial_eq(self.tcx, typing_env, ty, &mut err);
                return self.mk_err(err, ty);
            }
        }

        // Mark the pattern to indicate that it is the result of lowering a named
        // constant. This is used for diagnostics.
        thir_pat.extra.get_or_insert_default().expanded_const = uv.kind.opt_def_id();
        thir_pat
    }

    fn lower_field_values_to_fieldpats(
        &self,
        values: impl Iterator<Item = ty::Value<'tcx>>,
    ) -> Vec<FieldPat<'tcx>> {
        values
            .enumerate()
            .map(|(index, value)| FieldPat {
                field: FieldIdx::new(index),
                pattern: *self.valtree_to_pat(value),
            })
            .collect()
    }

    // Recursive helper for `to_pat`; invoke that (instead of calling this directly).
    #[instrument(skip(self), level = "debug")]
    fn valtree_to_pat(&self, value: ty::Value<'tcx>) -> Box<Pat<'tcx>> {
        let span = self.span;
        let tcx = self.tcx;
        let ty::Value { ty, valtree } = value;

        let kind = match ty.kind() {
            // Extremely important check for all ADTs!
            // Make sure they are eligible to be used in patterns, and if not, emit an error.
            ty::Adt(adt_def, _) if !self.type_marked_structural(ty) => {
                // This ADT cannot be used as a constant in patterns.
                debug!(?adt_def, ?value.ty, "ADT type in pattern is not `type_marked_structural`");
                let PartialEqImplStatus {
                    is_derived, structural_partial_eq, non_blanket_impl, ..
                } = type_has_partial_eq_impl(self.tcx, self.typing_env, ty);
                let (manual_partialeq_impl_span, manual_partialeq_impl_note) =
                    match (structural_partial_eq, non_blanket_impl) {
                        (true, _) => (None, false),
                        (_, Some(def_id)) if def_id.is_local() && !is_derived => {
                            (Some(tcx.def_span(def_id)), false)
                        }
                        _ => (None, true),
                    };
                let manual_partialeq_impl =
                    manual_partialeq_impl_note || manual_partialeq_impl_span.is_some();
                let is_local = adt_def.did().is_local();
                let ty_def_span = tcx.def_span(adt_def.did());
                let suggestion = if let Ok(name) = tcx.sess.source_map().span_to_snippet(self.span)
                    && (is_local || manual_partialeq_impl)
                {
                    let mut hir_id = self.id;
                    while let hir::Node::Pat(pat) = tcx.parent_hir_node(hir_id) {
                        hir_id = pat.hir_id;
                    }
                    match tcx.parent_hir_node(hir_id) {
                        hir::Node::Arm(hir::Arm { pat, guard: None, .. }) => {
                            // Add an if condition to the match arm.
                            Some(SuggestEq::AddIf {
                                if_span: pat.span.shrink_to_hi(),
                                pat_span: self.span,
                                name,
                                ty,
                                manual_partialeq_impl,
                            })
                        }
                        hir::Node::Arm(hir::Arm { guard: Some(guard), .. }) => {
                            // Modify the the match arm if condition and add a check for equality.
                            Some(SuggestEq::AddToIf {
                                span: guard.span.shrink_to_hi(),
                                pat_span: self.span,
                                name,
                                ty,
                                manual_partialeq_impl,
                            })
                        }
                        hir::Node::Expr(hir::Expr {
                            kind: hir::ExprKind::Let(let_expr),
                            span,
                            ..
                        }) => {
                            if let_expr.pat.span == self.span {
                                // `if let CONST = expr` -> `if CONST == expr`.
                                Some(SuggestEq::ReplaceWithEq {
                                    removal: span.until(self.span),
                                    eq: self.span.between(let_expr.init.span),
                                    ty,
                                    manual_partialeq_impl,
                                })
                            } else if tcx.sess.edition().at_least_rust_2024() {
                                // `if let Some(CONST) = expr` ->
                                // `if let Some(binding) = expr && binding == CONST`.
                                Some(SuggestEq::AddToLetChain {
                                    span: span.shrink_to_hi(),
                                    pat_span: self.span,
                                    name,
                                    ty,
                                    manual_partialeq_impl,
                                })
                            } else {
                                None
                            }
                        }
                        hir::Node::LetStmt(let_stmt)
                            if let Some(init) = let_stmt.init
                                && let Some(els) = let_stmt.els
                                && init.span.ctxt().is_root()
                                && els.span.ctxt().is_root() =>
                        {
                            // `let PAT = expr else {` -> `if PAT == expr {`.
                            Some(SuggestEq::ReplaceLetElseWithIf {
                                if_span: let_stmt.span.until(let_stmt.pat.span),
                                eq: let_stmt.pat.span.between(init.span),
                                else_span: init.span.between(els.span),
                                ty,
                                manual_partialeq_impl,
                            })
                        }
                        _ => None,
                    }
                } else {
                    None
                };
                let err = TypeNotStructural {
                    span,
                    ty,
                    ty_def_span,
                    manual_partialeq_impl_span,
                    manual_partialeq_impl_note,
                    is_local,
                    suggestion,
                };
                return self.mk_err(tcx.dcx().create_err(err), ty);
            }
            ty::Adt(adt_def, args) if adt_def.is_enum() => {
                let (&variant_index, fields) = valtree.to_branch().split_first().unwrap();
                let variant_index = VariantIdx::from_u32(variant_index.to_leaf().to_u32());
                PatKind::Variant {
                    adt_def: *adt_def,
                    args,
                    variant_index,
                    subpatterns: self
                        .lower_field_values_to_fieldpats(fields.iter().map(|ct| ct.to_value())),
                }
            }
            ty::Adt(def, _) => {
                assert!(!def.is_union()); // Valtree construction would never succeed for unions.
                PatKind::Leaf {
                    subpatterns: self.lower_field_values_to_fieldpats(
                        valtree.to_branch().iter().map(|ct| ct.to_value()),
                    ),
                }
            }
            ty::Tuple(_) => PatKind::Leaf {
                subpatterns: self.lower_field_values_to_fieldpats(
                    valtree.to_branch().iter().map(|ct| ct.to_value()),
                ),
            },
            ty::Slice(_) => PatKind::Slice {
                prefix: valtree
                    .to_branch()
                    .iter()
                    .map(|val| *self.valtree_to_pat(val.to_value()))
                    .collect(),
                slice: None,
                suffix: Box::new([]),
            },
            ty::Array(_, _) => PatKind::Array {
                prefix: valtree
                    .to_branch()
                    .iter()
                    .map(|val| *self.valtree_to_pat(val.to_value()))
                    .collect(),
                slice: None,
                suffix: Box::new([]),
            },
            ty::Str => {
                // Constant/literal patterns of type `&str` are lowered to a
                // `PatKind::Deref` wrapping a `PatKind::Constant` of type `str`.
                // This pattern node is the `str` constant part.
                //
                // Under `feature(deref_patterns)`, string literal patterns can also
                // have type `str` directly, without the `&`, in order to allow things
                // like `deref!("...")` to work when the scrutinee is `String`.
                PatKind::Constant { value }
            }
            ty::Ref(_, pointee_ty, ..) => {
                if pointee_ty.is_str()
                    || pointee_ty.is_slice()
                    || pointee_ty.is_sized(tcx, self.typing_env)
                {
                    PatKind::Deref {
                        // This node has type `ty::Ref`, so it's not a pin-deref.
                        pin: hir::Pinnedness::Not,
                        // Lower the valtree to a pattern as the pointee type.
                        // This works because references have the same valtree
                        // representation as their pointee.
                        subpattern: self.valtree_to_pat(ty::Value { ty: *pointee_ty, valtree }),
                    }
                } else {
                    return self.mk_err(
                        tcx.dcx().create_err(UnsizedPattern { span, non_sm_ty: *pointee_ty }),
                        ty,
                    );
                }
            }
            ty::Float(flt) => {
                let v = valtree.to_leaf();
                let is_nan = match flt {
                    ty::FloatTy::F16 => v.to_f16().is_nan(),
                    ty::FloatTy::F32 => v.to_f32().is_nan(),
                    ty::FloatTy::F64 => v.to_f64().is_nan(),
                    ty::FloatTy::F128 => v.to_f128().is_nan(),
                };
                if is_nan {
                    // NaNs are not ever equal to anything so they make no sense as patterns.
                    // Also see <https://github.com/rust-lang/rfcs/pull/3535>.
                    return self.mk_err(tcx.dcx().create_err(NaNPattern { span }), ty);
                } else {
                    PatKind::Constant { value }
                }
            }
            ty::Pat(..) | ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::RawPtr(..) => {
                // The raw pointers we see here have been "vetted" by valtree construction to be
                // just integers, so we simply allow them.
                PatKind::Constant { value }
            }
            ty::FnPtr(..) => {
                unreachable!(
                    "Valtree construction would never succeed for FnPtr, so this is unreachable."
                )
            }
            _ => {
                let err = InvalidPattern {
                    span,
                    non_sm_ty: ty,
                    prefix: ty.prefix_string(tcx).to_string(),
                };
                return self.mk_err(tcx.dcx().create_err(err), ty);
            }
        };

        Box::new(Pat { span, ty, kind, extra: None })
    }
}

/// Given a type with type parameters, visit every ADT looking for types that need to
/// `#[derive(PartialEq)]` for it to be a structural type.
fn extend_type_not_partial_eq<'tcx>(
    tcx: TyCtxt<'tcx>,
    typing_env: ty::TypingEnv<'tcx>,
    ty: Ty<'tcx>,
    err: &mut Diag<'_>,
) {
    /// Collect all types that need to be `StructuralPartialEq`.
    struct UsedParamsNeedInstantiationVisitor<'tcx> {
        tcx: TyCtxt<'tcx>,
        typing_env: ty::TypingEnv<'tcx>,
        /// The user has written `impl PartialEq for Ty` which means it's non-structural.
        adts_with_manual_partialeq: FxHashSet<Span>,
        /// The type has no `PartialEq` implementation, neither manual or derived.
        adts_without_partialeq: FxHashSet<Span>,
        /// The user has written `impl PartialEq for Ty` which means it's non-structural,
        /// but we don't have a span to point at, so we'll just add them as a `note`.
        manual: FxHashSet<Ty<'tcx>>,
        /// The type has no `PartialEq` implementation, neither manual or derived, but
        /// we don't have a span to point at, so we'll just add them as a `note`.
        without: FxHashSet<Ty<'tcx>>,
    }

    impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for UsedParamsNeedInstantiationVisitor<'tcx> {
        type Result = ControlFlow<()>;
        fn visit_ty(&mut self, ty: Ty<'tcx>) -> Self::Result {
            match ty.kind() {
                ty::Dynamic(..) => return ControlFlow::Break(()),
                // Unsafe binders never implement `PartialEq`, so avoid walking into them
                // which would require instantiating its binder with placeholders too.
                ty::UnsafeBinder(..) => return ControlFlow::Break(()),
                ty::FnPtr(..) => return ControlFlow::Continue(()),
                ty::Adt(def, _args) => {
                    let ty_def_id = def.did();
                    let ty_def_span = self.tcx.def_span(ty_def_id);
                    let PartialEqImplStatus {
                        has_impl,
                        is_derived,
                        structural_partial_eq,
                        non_blanket_impl,
                    } = type_has_partial_eq_impl(self.tcx, self.typing_env, ty);
                    match (has_impl, is_derived, structural_partial_eq, non_blanket_impl) {
                        (_, _, true, _) => {}
                        (true, false, _, Some(def_id)) if def_id.is_local() => {
                            self.adts_with_manual_partialeq.insert(self.tcx.def_span(def_id));
                        }
                        (true, false, _, _) if ty_def_id.is_local() => {
                            self.adts_with_manual_partialeq.insert(ty_def_span);
                        }
                        (false, _, _, _) if ty_def_id.is_local() => {
                            self.adts_without_partialeq.insert(ty_def_span);
                        }
                        (true, false, _, _) => {
                            self.manual.insert(ty);
                        }
                        (false, _, _, _) => {
                            self.without.insert(ty);
                        }
                        _ => {}
                    };
                    ty.super_visit_with(self)
                }
                _ => ty.super_visit_with(self),
            }
        }
    }
    let mut v = UsedParamsNeedInstantiationVisitor {
        tcx,
        typing_env,
        adts_with_manual_partialeq: FxHashSet::default(),
        adts_without_partialeq: FxHashSet::default(),
        manual: FxHashSet::default(),
        without: FxHashSet::default(),
    };
    if v.visit_ty(ty).is_break() {
        return;
    }
    #[allow(rustc::potential_query_instability)] // Span labels will be sorted by the rendering
    for span in v.adts_with_manual_partialeq {
        err.span_note(span, "the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details");
    }
    #[allow(rustc::potential_query_instability)] // Span labels will be sorted by the rendering
    for span in v.adts_without_partialeq {
        err.span_label(
            span,
            "must be annotated with `#[derive(PartialEq)]` to be usable in patterns",
        );
    }
    #[allow(rustc::potential_query_instability)]
    let mut manual: Vec<_> = v.manual.into_iter().map(|t| t.to_string()).collect();
    manual.sort();
    for ty in manual {
        err.note(format!(
            "`{ty}` must be annotated with `#[derive(PartialEq)]` to be usable in patterns, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details"
        ));
    }
    #[allow(rustc::potential_query_instability)]
    let mut without: Vec<_> = v.without.into_iter().map(|t| t.to_string()).collect();
    without.sort();
    for ty in without {
        err.note(format!(
            "`{ty}` must be annotated with `#[derive(PartialEq)]` to be usable in patterns"
        ));
    }
}

#[derive(Debug)]
struct PartialEqImplStatus {
    has_impl: bool,
    is_derived: bool,
    structural_partial_eq: bool,
    non_blanket_impl: Option<DefId>,
}

#[instrument(level = "trace", skip(tcx), ret)]
fn type_has_partial_eq_impl<'tcx>(
    tcx: TyCtxt<'tcx>,
    typing_env: ty::TypingEnv<'tcx>,
    ty: Ty<'tcx>,
) -> PartialEqImplStatus {
    let (infcx, param_env) = tcx.infer_ctxt().build_with_typing_env(typing_env);
    // double-check there even *is* a semantic `PartialEq` to dispatch to.
    //
    // (If there isn't, then we can safely issue a hard
    // error, because that's never worked, due to compiler
    // using `PartialEq::eq` in this scenario in the past.)
    let partial_eq_trait_id = tcx.require_lang_item(hir::LangItem::PartialEq, DUMMY_SP);
    let structural_partial_eq_trait_id =
        tcx.require_lang_item(hir::LangItem::StructuralPeq, DUMMY_SP);

    let partial_eq_obligation = Obligation::new(
        tcx,
        ObligationCause::dummy(),
        param_env,
        ty::TraitRef::new(tcx, partial_eq_trait_id, [ty, ty]),
    );

    let mut automatically_derived = false;
    let mut structural_peq = false;
    let mut impl_def_id = None;
    for def_id in tcx.non_blanket_impls_for_ty(partial_eq_trait_id, ty) {
        automatically_derived = find_attr!(tcx, def_id, AutomaticallyDerived);
        impl_def_id = Some(def_id);
    }
    for _ in tcx.non_blanket_impls_for_ty(structural_partial_eq_trait_id, ty) {
        structural_peq = true;
    }
    // This *could* accept a type that isn't actually `PartialEq`, because region bounds get
    // ignored. However that should be pretty much impossible since consts that do not depend on
    // generics can only mention the `'static` lifetime, and how would one have a type that's
    // `PartialEq` for some lifetime but *not* for `'static`? If this ever becomes a problem
    // we'll need to leave some sort of trace of this requirement in the MIR so that borrowck
    // can ensure that the type really implements `PartialEq`.
    // We also do *not* require `const PartialEq`, not even in `const fn`. This violates the model
    // that patterns can only do things that the code could also do without patterns, but it is
    // needed for backwards compatibility. The actual pattern matching compares primitive values,
    // `PartialEq::eq` never gets invoked, so there's no risk of us running non-const code.
    PartialEqImplStatus {
        has_impl: infcx.predicate_must_hold_modulo_regions(&partial_eq_obligation),
        is_derived: automatically_derived,
        structural_partial_eq: structural_peq,
        non_blanket_impl: impl_def_id,
    }
}