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Diffstat (limited to 'compiler/rustc_typeck/src/impl_wf_check.rs')
| -rw-r--r-- | compiler/rustc_typeck/src/impl_wf_check.rs | 228 |
1 files changed, 228 insertions, 0 deletions
diff --git a/compiler/rustc_typeck/src/impl_wf_check.rs b/compiler/rustc_typeck/src/impl_wf_check.rs new file mode 100644 index 000000000..9fee1eaae --- /dev/null +++ b/compiler/rustc_typeck/src/impl_wf_check.rs @@ -0,0 +1,228 @@ +//! 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 crate::constrained_generic_params as cgp; +use min_specialization::check_min_specialization; + +use rustc_data_structures::fx::{FxHashMap, FxHashSet}; +use rustc_errors::struct_span_err; +use rustc_hir::def::DefKind; +use rustc_hir::def_id::LocalDefId; +use rustc_middle::ty::query::Providers; +use rustc_middle::ty::{self, TyCtxt, TypeVisitable}; +use rustc_span::{Span, Symbol}; + +use std::collections::hash_map::Entry::{Occupied, Vacant}; + +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 +/// ``` +fn check_mod_impl_wf(tcx: TyCtxt<'_>, module_def_id: LocalDefId) { + let min_specialization = tcx.features().min_specialization; + let module = tcx.hir_module_items(module_def_id); + for id in module.items() { + if matches!(tcx.def_kind(id.def_id), DefKind::Impl) { + enforce_impl_params_are_constrained(tcx, id.def_id); + enforce_impl_items_are_distinct(tcx, id.def_id); + if min_specialization { + check_min_specialization(tcx, id.def_id); + } + } + } +} + +pub fn provide(providers: &mut Providers) { + *providers = Providers { check_mod_impl_wf, ..*providers }; +} + +fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) { + // Every lifetime used in an associated type must be constrained. + let impl_self_ty = tcx.type_of(impl_def_id); + if impl_self_ty.references_error() { + // Don't complain about unconstrained type params when self ty isn't known due to errors. + // (#36836) + tcx.sess.delay_span_bug( + tcx.def_span(impl_def_id), + &format!( + "potentially unconstrained type parameters weren't evaluated: {:?}", + impl_self_ty, + ), + ); + return; + } + let impl_generics = tcx.generics_of(impl_def_id); + let impl_predicates = tcx.predicates_of(impl_def_id); + let impl_trait_ref = tcx.impl_trait_ref(impl_def_id); + + let mut input_parameters = cgp::parameters_for_impl(impl_self_ty, impl_trait_ref); + cgp::identify_constrained_generic_params( + tcx, + impl_predicates, + 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.type_of(def_id), true) + } else { + Vec::new() + } + } + ty::AssocKind::Fn | ty::AssocKind::Const => Vec::new(), + } + }) + .collect(); + + for param in &impl_generics.params { + match param.kind { + // Disallow ANY unconstrained type parameters. + ty::GenericParamDefKind::Type { .. } => { + let param_ty = ty::ParamTy::for_def(param); + if !input_parameters.contains(&cgp::Parameter::from(param_ty)) { + report_unused_parameter(tcx, tcx.def_span(param.def_id), "type", param_ty.name); + } + } + ty::GenericParamDefKind::Lifetime => { + 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) + { + report_unused_parameter( + tcx, + tcx.def_span(param.def_id), + "lifetime", + param.name, + ); + } + } + ty::GenericParamDefKind::Const { .. } => { + let param_ct = ty::ParamConst::for_def(param); + if !input_parameters.contains(&cgp::Parameter::from(param_ct)) { + report_unused_parameter( + tcx, + tcx.def_span(param.def_id), + "const", + param_ct.name, + ); + } + } + } + } + + // (*) 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. +} + +fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: Symbol) { + let mut err = struct_span_err!( + tcx.sess, + span, + E0207, + "the {} parameter `{}` is not constrained by the \ + impl trait, self type, or predicates", + kind, + name + ); + err.span_label(span, format!("unconstrained {} parameter", kind)); + if kind == "const" { + err.note( + "expressions using a const parameter must map each value to a distinct output value", + ); + err.note( + "proving the result of expressions other than the parameter are unique is not supported", + ); + } + err.emit(); +} + +/// Enforce that we do not have two items in an impl with the same name. +fn enforce_impl_items_are_distinct(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) { + let mut seen_type_items = FxHashMap::default(); + let mut seen_value_items = FxHashMap::default(); + for &impl_item_ref in tcx.associated_item_def_ids(impl_def_id) { + let impl_item = tcx.associated_item(impl_item_ref); + let seen_items = match impl_item.kind { + ty::AssocKind::Type => &mut seen_type_items, + _ => &mut seen_value_items, + }; + let span = tcx.def_span(impl_item_ref); + let ident = impl_item.ident(tcx); + match seen_items.entry(ident.normalize_to_macros_2_0()) { + Occupied(entry) => { + let mut err = struct_span_err!( + tcx.sess, + span, + E0201, + "duplicate definitions with name `{}`:", + ident + ); + err.span_label(*entry.get(), format!("previous definition of `{}` here", ident)); + err.span_label(span, "duplicate definition"); + err.emit(); + } + Vacant(entry) => { + entry.insert(span); + } + } + } +} |