compiler/rustc_hir_analysis/src/impl_wf_check.rs - rust-lang/rust - Git at Google

 //! 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> {
     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_ok().enforce_impl_non_lifetime_params_are_constrained(impl_def_id);
     res = res.and(enforce_impl_lifetime_params_are_constrained(tcx, impl_def_id));

     if 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,
 ) -> Result<(), ErrorGuaranteed> {
     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 {
         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(&param_lt)
                     && !input_parameters.contains(&param_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);
                     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();
     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,
     );

     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);
             res = Err(diag.emit());
         }
     }
     res
 }
	//! 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> {
	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_ok().enforce_impl_non_lifetime_params_are_constrained(impl_def_id);
	res = res.and(enforce_impl_lifetime_params_are_constrained(tcx, impl_def_id));

	if 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,
	) -> Result<(), ErrorGuaranteed> {
	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 {
	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(&param_lt)
	&& !input_parameters.contains(&param_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);
	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();
	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,
	);

	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);
	res = Err(diag.emit());
	}
	}
	res
	}