193 lines
7.3 KiB
Rust
193 lines
7.3 KiB
Rust
//! This pass enforces various "well-formedness constraints" on impls.
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//! Logically, it is part of wfcheck -- but we do it early so that we
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//! can stop compilation afterwards, since part of the trait matching
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//! infrastructure gets very grumpy if these conditions don't hold. In
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//! particular, if there are type parameters that are not part of the
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//! impl, then coherence will report strange inference ambiguity
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//! errors; if impls have duplicate items, we get misleading
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//! specialization errors. These things can (and probably should) be
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//! fixed, but for the moment it's easier to do these checks early.
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use crate::constrained_generic_params as cgp;
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use min_specialization::check_min_specialization;
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use rustc_data_structures::fx::FxHashSet;
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use rustc_errors::struct_span_err;
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use rustc_hir::def::DefKind;
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use rustc_hir::def_id::LocalDefId;
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use rustc_middle::query::Providers;
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use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
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use rustc_span::{Span, Symbol};
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mod min_specialization;
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/// Checks that all the type/lifetime parameters on an impl also
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/// appear in the trait ref or self type (or are constrained by a
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/// where-clause). These rules are needed to ensure that, given a
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/// trait ref like `<T as Trait<U>>`, we can derive the values of all
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/// parameters on the impl (which is needed to make specialization
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/// possible).
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///
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/// However, in the case of lifetimes, we only enforce these rules if
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/// the lifetime parameter is used in an associated type. This is a
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/// concession to backwards compatibility; see comment at the end of
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/// the fn for details.
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///
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/// Example:
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///
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/// ```rust,ignore (pseudo-Rust)
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/// impl<T> Trait<Foo> for Bar { ... }
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/// // ^ T does not appear in `Foo` or `Bar`, error!
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///
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/// impl<T> Trait<Foo<T>> for Bar { ... }
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/// // ^ T appears in `Foo<T>`, ok.
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///
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/// impl<T> Trait<Foo> for Bar where Bar: Iterator<Item = T> { ... }
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/// // ^ T is bound to `<Bar as Iterator>::Item`, ok.
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///
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/// impl<'a> Trait<Foo> for Bar { }
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/// // ^ 'a is unused, but for back-compat we allow it
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///
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/// impl<'a> Trait<Foo> for Bar { type X = &'a i32; }
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/// // ^ 'a is unused and appears in assoc type, error
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/// ```
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fn check_mod_impl_wf(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
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let min_specialization = tcx.features().min_specialization;
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let module = tcx.hir_module_items(module_def_id);
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for id in module.items() {
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if matches!(tcx.def_kind(id.owner_id), DefKind::Impl { .. }) {
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enforce_impl_params_are_constrained(tcx, id.owner_id.def_id);
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if min_specialization {
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check_min_specialization(tcx, id.owner_id.def_id);
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}
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}
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}
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}
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pub fn provide(providers: &mut Providers) {
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*providers = Providers { check_mod_impl_wf, ..*providers };
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}
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fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) {
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// Every lifetime used in an associated type must be constrained.
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let impl_self_ty = tcx.type_of(impl_def_id).instantiate_identity();
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if impl_self_ty.references_error() {
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// Don't complain about unconstrained type params when self ty isn't known due to errors.
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// (#36836)
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tcx.sess.delay_span_bug(
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tcx.def_span(impl_def_id),
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format!(
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"potentially unconstrained type parameters weren't evaluated: {impl_self_ty:?}",
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),
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);
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return;
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}
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let impl_generics = tcx.generics_of(impl_def_id);
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let impl_predicates = tcx.predicates_of(impl_def_id);
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let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).map(ty::EarlyBinder::instantiate_identity);
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let mut input_parameters = cgp::parameters_for_impl(impl_self_ty, impl_trait_ref);
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cgp::identify_constrained_generic_params(
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tcx,
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impl_predicates,
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impl_trait_ref,
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&mut input_parameters,
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);
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// Disallow unconstrained lifetimes, but only if they appear in assoc types.
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let lifetimes_in_associated_types: FxHashSet<_> = tcx
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.associated_item_def_ids(impl_def_id)
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.iter()
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.flat_map(|def_id| {
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let item = tcx.associated_item(def_id);
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match item.kind {
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ty::AssocKind::Type => {
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if item.defaultness(tcx).has_value() {
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cgp::parameters_for(&tcx.type_of(def_id).instantiate_identity(), true)
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} else {
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vec![]
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}
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}
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ty::AssocKind::Fn | ty::AssocKind::Const => vec![],
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}
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})
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.collect();
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for param in &impl_generics.params {
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match param.kind {
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// Disallow ANY unconstrained type parameters.
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ty::GenericParamDefKind::Type { .. } => {
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let param_ty = ty::ParamTy::for_def(param);
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if !input_parameters.contains(&cgp::Parameter::from(param_ty)) {
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report_unused_parameter(tcx, tcx.def_span(param.def_id), "type", param_ty.name);
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}
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}
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ty::GenericParamDefKind::Lifetime => {
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let param_lt = cgp::Parameter::from(param.to_early_bound_region_data());
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if lifetimes_in_associated_types.contains(¶m_lt) && // (*)
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!input_parameters.contains(¶m_lt)
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{
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report_unused_parameter(
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tcx,
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tcx.def_span(param.def_id),
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"lifetime",
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param.name,
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);
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}
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}
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ty::GenericParamDefKind::Const { .. } => {
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let param_ct = ty::ParamConst::for_def(param);
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if !input_parameters.contains(&cgp::Parameter::from(param_ct)) {
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report_unused_parameter(
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tcx,
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tcx.def_span(param.def_id),
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"const",
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param_ct.name,
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);
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}
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}
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}
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}
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// (*) This is a horrible concession to reality. I think it'd be
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// better to just ban unconstrained lifetimes outright, but in
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// practice people do non-hygienic macros like:
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//
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// ```
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// macro_rules! __impl_slice_eq1 {
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// ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
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// impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
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// ....
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// }
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// }
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// }
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// ```
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//
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// In a concession to backwards compatibility, we continue to
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// permit those, so long as the lifetimes aren't used in
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// associated types. I believe this is sound, because lifetimes
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// used elsewhere are not projected back out.
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}
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fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: Symbol) {
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let mut err = struct_span_err!(
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tcx.sess,
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span,
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E0207,
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"the {} parameter `{}` is not constrained by the \
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impl trait, self type, or predicates",
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kind,
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name
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);
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err.span_label(span, format!("unconstrained {kind} parameter"));
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if kind == "const" {
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err.note(
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"expressions using a const parameter must map each value to a distinct output value",
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);
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err.note(
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"proving the result of expressions other than the parameter are unique is not supported",
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);
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}
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err.emit();
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}
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