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//! 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::assert_matches::debug_assert_matches;
use min_specialization::check_min_specialization;
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::codes::*;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::LocalDefId;
use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
use rustc_span::ErrorGuaranteed;
use crate::constrained_generic_params as cgp;
use crate::errors::UnconstrainedGenericParameter;
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,
) -> Result<(), ErrorGuaranteed> {
let min_specialization = tcx.features().min_specialization;
let mut res = Ok(());
debug_assert_matches!(tcx.def_kind(impl_def_id), DefKind::Impl { .. });
res = res.and(enforce_impl_params_are_constrained(tcx, impl_def_id));
if min_specialization {
res = res.and(check_min_specialization(tcx, impl_def_id));
}
res
}
fn enforce_impl_params_are_constrained(
tcx: TyCtxt<'_>,
impl_def_id: LocalDefId,
) -> Result<(), ErrorGuaranteed> {
// Every lifetime used in an associated type must be constrained.
let impl_self_ty = tcx.type_of(impl_def_id).instantiate_identity();
if impl_self_ty.references_error() {
// Don't complain about unconstrained type params when self ty isn't known due to errors.
// (#36836)
tcx.dcx().span_delayed_bug(
tcx.def_span(impl_def_id),
format!(
"potentially unconstrained type parameters weren't evaluated: {impl_self_ty:?}",
),
);
// This is super fishy, but our current `rustc_hir_analysis::check_crate` pipeline depends on
// `type_of` having been called much earlier, and thus this value being read from cache.
// Compilation must continue in order for other important diagnostics to keep showing up.
return Ok(());
}
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).map(ty::EarlyBinder::instantiate_identity);
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_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, tcx.type_of(def_id).instantiate_identity(), true)
} else {
vec![]
}
}
ty::AssocKind::Fn | ty::AssocKind::Const => vec![],
}
})
.collect();
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::Lifetime => {
let param_lt = cgp::Parameter::from(param.to_early_bound_region_data());
lifetimes_in_associated_types.contains(¶m_lt) && // (*)
!input_parameters.contains(¶m_lt)
}
ty::GenericParamDefKind::Const { .. } => {
let param_ct = ty::ParamConst::for_def(param);
!input_parameters.contains(&cgp::Parameter::from(param_ct))
}
};
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: param.name,
param_def_kind: tcx.def_descr(param.def_id),
const_param_note,
const_param_note2: const_param_note,
});
diag.code(E0207);
res = Err(diag.emit());
}
}
res
// (*) 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.
}