| //! This pass enforces various "well-formedness constraints" on impls. | |
| //! Logically, it is part of wfcheck -- but we do it early so that we | |
| //! can stop compilation afterwards, since part of the trait matching | |
| //! infrastructure gets very grumpy if these conditions don't hold. In | |
| //! particular, if there are type parameters that are not part of the | |
| //! impl, then coherence will report strange inference ambiguity | |
| //! errors; if impls have duplicate items, we get misleading | |
| //! specialization errors. These things can (and probably should) be | |
| //! fixed, but for the moment it's easier to do these checks early. | |
| use std::debug_assert_matches; | |
| use min_specialization::check_min_specialization; | |
| use rustc_data_structures::fx::FxHashSet; | |
| use rustc_errors::Applicability; | |
| use rustc_errors::codes::*; | |
| use rustc_hir::def::DefKind; | |
| use rustc_hir::def_id::LocalDefId; | |
| use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt, Unnormalized}; | |
| use rustc_span::{ErrorGuaranteed, kw}; | |
| use crate::constrained_generic_params as cgp; | |
| use crate::diagnostics::UnconstrainedGenericParameter; | |
| use crate::diagnostics::remove_or_use_generic::suggest_to_remove_or_use_generic; | |
| mod min_specialization; | |
| /// Checks that all the type/lifetime parameters on an impl also | |
| /// appear in the trait ref or self type (or are constrained by a | |
| /// where-clause). These rules are needed to ensure that, given a | |
| /// trait ref like `<T as Trait<U>>`, we can derive the values of all | |
| /// parameters on the impl (which is needed to make specialization | |
| /// possible). | |
| /// | |
| /// However, in the case of lifetimes, we only enforce these rules if | |
| /// the lifetime parameter is used in an associated type. This is a | |
| /// concession to backwards compatibility; see comment at the end of | |
| /// the fn for details. | |
| /// | |
| /// Example: | |
| /// | |
| /// ```rust,ignore (pseudo-Rust) | |
| /// impl<T> Trait<Foo> for Bar { ... } | |
| /// // ^ T does not appear in `Foo` or `Bar`, error! | |
| /// | |
| /// impl<T> Trait<Foo<T>> for Bar { ... } | |
| /// // ^ T appears in `Foo<T>`, ok. | |
| /// | |
| /// impl<T> Trait<Foo> for Bar where Bar: Iterator<Item = T> { ... } | |
| /// // ^ T is bound to `<Bar as Iterator>::Item`, ok. | |
| /// | |
| /// impl<'a> Trait<Foo> for Bar { } | |
| /// // ^ 'a is unused, but for back-compat we allow it | |
| /// | |
| /// impl<'a> Trait<Foo> for Bar { type X = &'a i32; } | |
| /// // ^ 'a is unused and appears in assoc type, error | |
| /// ``` | |
| pub(crate) fn check_impl_wf( | |
| tcx: TyCtxt<'_>, | |
| impl_def_id: LocalDefId, | |
| of_trait: bool, | |
| ) -> Result<(), ErrorGuaranteed> { | |
| debug_assert_matches!(tcx.def_kind(impl_def_id), DefKind::Impl { .. }); | |
| // Check that the args are constrained. We queryfied the check for ty/const params | |
| // since unconstrained type/const params cause ICEs in projection, so we want to | |
| // detect those specifically and project those to `TyKind::Error`. | |
| let mut res = tcx.ensure_result().enforce_impl_non_lifetime_params_are_constrained(impl_def_id); | |
| res = res.and(enforce_impl_lifetime_params_are_constrained(tcx, impl_def_id, of_trait)); | |
| if of_trait && tcx.features().min_specialization() { | |
| res = res.and(check_min_specialization(tcx, impl_def_id)); | |
| } | |
| res | |
| } | |
| pub(crate) fn enforce_impl_lifetime_params_are_constrained( | |
| tcx: TyCtxt<'_>, | |
| impl_def_id: LocalDefId, | |
| of_trait: bool, | |
| ) -> Result<(), ErrorGuaranteed> { | |
| let impl_self_ty = tcx.type_of(impl_def_id).instantiate_identity().skip_norm_wip(); | |
| // Don't complain about unconstrained type params when self ty isn't known due to errors. | |
| // (#36836) | |
| impl_self_ty.error_reported()?; | |
| let impl_generics = tcx.generics_of(impl_def_id); | |
| let impl_clauses = tcx.clauses_of(impl_def_id); | |
| let impl_trait_ref = | |
| of_trait.then(|| tcx.impl_trait_ref(impl_def_id).instantiate_identity().skip_norm_wip()); | |
| impl_trait_ref.error_reported()?; | |
| let mut input_parameters = cgp::parameters_for_impl(tcx, impl_self_ty, impl_trait_ref); | |
| cgp::identify_constrained_generic_params( | |
| tcx, | |
| impl_clauses, | |
| impl_trait_ref, | |
| &mut input_parameters, | |
| ); | |
| // Disallow unconstrained lifetimes, but only if they appear in assoc types. | |
| let lifetimes_in_associated_types: FxHashSet<_> = tcx | |
| .associated_item_def_ids(impl_def_id) | |
| .iter() | |
| .flat_map(|&def_id| { | |
| let item = tcx.associated_item(def_id); | |
| match item.kind { | |
| ty::AssocKind::Type { .. } => { | |
| if item.defaultness(tcx).has_value() { | |
| cgp::parameters_for( | |
| tcx, | |
| tcx.type_of(def_id).instantiate_identity().skip_norm_wip(), | |
| true, | |
| ) | |
| } else { | |
| vec![] | |
| } | |
| } | |
| ty::AssocKind::Fn { .. } | ty::AssocKind::Const { .. } => vec![], | |
| } | |
| }) | |
| .collect(); | |
| let mut res = Ok(()); | |
| for param in &impl_generics.own_params { | |
| match param.kind { | |
| ty::GenericParamDefKind::Lifetime => { | |
| // This is a horrible concession to reality. I think it'd be | |
| // better to just ban unconstrained lifetimes outright, but in | |
| // practice people do non-hygienic macros like: | |
| // | |
| // ``` | |
| // macro_rules! __impl_slice_eq1 { | |
| // ($Lhs: ty, $Rhs: ty, $Bound: ident) => { | |
| // impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> { | |
| // .... | |
| // } | |
| // } | |
| // } | |
| // ``` | |
| // | |
| // In a concession to backwards compatibility, we continue to | |
| // permit those, so long as the lifetimes aren't used in | |
| // associated types. I believe this is sound, because lifetimes | |
| // used elsewhere are not projected back out. | |
| let param_lt = cgp::Parameter::from(param.to_early_bound_region_data()); | |
| if lifetimes_in_associated_types.contains(¶m_lt) | |
| && !input_parameters.contains(¶m_lt) | |
| { | |
| let mut diag = tcx.dcx().create_err(UnconstrainedGenericParameter { | |
| span: tcx.def_span(param.def_id), | |
| param_name: tcx.item_ident(param.def_id), | |
| param_def_kind: tcx.def_descr(param.def_id), | |
| const_param_note: false, | |
| const_param_note2: false, | |
| }); | |
| diag.code(E0207); | |
| for p in &impl_generics.own_params { | |
| if p.name == kw::UnderscoreLifetime { | |
| let span = tcx.def_span(p.def_id); | |
| let Ok(snippet) = tcx.sess.source_map().span_to_snippet(span) else { | |
| continue; | |
| }; | |
| let (span, sugg) = if &snippet == "'_" { | |
| (span, param.name.to_string()) | |
| } else { | |
| (span.shrink_to_hi(), format!("{} ", param.name)) | |
| }; | |
| diag.span_suggestion_verbose( | |
| span, | |
| "consider using the named lifetime here instead of an implicit \ | |
| lifetime", | |
| sugg, | |
| Applicability::MaybeIncorrect, | |
| ); | |
| } | |
| } | |
| suggest_to_remove_or_use_generic(tcx, &mut diag, impl_def_id, param, true); | |
| res = Err(diag.emit()); | |
| } | |
| } | |
| ty::GenericParamDefKind::Type { .. } | ty::GenericParamDefKind::Const { .. } => { | |
| // Enforced in `enforce_impl_non_lifetime_params_are_constrained`. | |
| } | |
| } | |
| } | |
| res | |
| } | |
| pub(crate) fn enforce_impl_non_lifetime_params_are_constrained( | |
| tcx: TyCtxt<'_>, | |
| impl_def_id: LocalDefId, | |
| ) -> Result<(), ErrorGuaranteed> { | |
| let impl_self_ty = tcx.type_of(impl_def_id).instantiate_identity().skip_norm_wip(); | |
| // Don't complain about unconstrained type params when self ty isn't known due to errors. | |
| // (#36836) | |
| impl_self_ty.error_reported()?; | |
| let impl_generics = tcx.generics_of(impl_def_id); | |
| let impl_clauses = tcx.clauses_of(impl_def_id); | |
| let impl_trait_ref = tcx | |
| .impl_opt_trait_ref(impl_def_id) | |
| .map(ty::EarlyBinder::instantiate_identity) | |
| .map(Unnormalized::skip_norm_wip); | |
| impl_trait_ref.error_reported()?; | |
| let mut input_parameters = cgp::parameters_for_impl(tcx, impl_self_ty, impl_trait_ref); | |
| cgp::identify_constrained_generic_params( | |
| tcx, | |
| impl_clauses, | |
| impl_trait_ref, | |
| &mut input_parameters, | |
| ); | |
| let mut res = Ok(()); | |
| for param in &impl_generics.own_params { | |
| let err = match param.kind { | |
| // Disallow ANY unconstrained type parameters. | |
| ty::GenericParamDefKind::Type { .. } => { | |
| let param_ty = ty::ParamTy::for_def(param); | |
| !input_parameters.contains(&cgp::Parameter::from(param_ty)) | |
| } | |
| ty::GenericParamDefKind::Const { .. } => { | |
| let param_ct = ty::ParamConst::for_def(param); | |
| !input_parameters.contains(&cgp::Parameter::from(param_ct)) | |
| } | |
| ty::GenericParamDefKind::Lifetime => { | |
| // Enforced in `enforce_impl_type_params_are_constrained`. | |
| false | |
| } | |
| }; | |
| if err { | |
| let const_param_note = matches!(param.kind, ty::GenericParamDefKind::Const { .. }); | |
| let mut diag = tcx.dcx().create_err(UnconstrainedGenericParameter { | |
| span: tcx.def_span(param.def_id), | |
| param_name: tcx.item_ident(param.def_id), | |
| param_def_kind: tcx.def_descr(param.def_id), | |
| const_param_note, | |
| const_param_note2: const_param_note, | |
| }); | |
| diag.code(E0207); | |
| suggest_to_remove_or_use_generic(tcx, &mut diag, impl_def_id, ¶m, false); | |
| res = Err(diag.emit()); | |
| } | |
| } | |
| res | |
| } |