compiler/rustc_hir_analysis/src/impl_wf_check.rs - third_party/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 crate::constrained_generic_params as cgp;
 use min_specialization::check_min_specialization;

 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{codes::*, struct_span_code_err};
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{LocalDefId, LocalModDefId};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
 use rustc_span::{ErrorGuaranteed, Span, Symbol};

 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: LocalModDefId) -> Result<(), ErrorGuaranteed> {
     let min_specialization = tcx.features().min_specialization;
     let module = tcx.hir_module_items(module_def_id);
     let mut res = Ok(());
     for id in module.items() {
         if matches!(tcx.def_kind(id.owner_id), DefKind::Impl { .. }) {
             res = res.and(enforce_impl_params_are_constrained(tcx, id.owner_id.def_id));
             if min_specialization {
                 res = res.and(check_min_specialization(tcx, id.owner_id.def_id));
             }
         }
     }
     res
 }

 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,
 ) -> 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);

     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).instantiate_identity(), true)
                     } else {
                         vec![]
                     }
                 }
                 ty::AssocKind::Fn | ty::AssocKind::Const => vec![],
             }
         })
         .collect();

     let mut res = Ok(());
     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)) {
                     res = Err(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(&param_lt) && // (*)
                     !input_parameters.contains(&param_lt)
                 {
                     res = Err(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)) {
                     res = Err(report_unused_parameter(
                         tcx,
                         tcx.def_span(param.def_id),
                         "const",
                         param_ct.name,
                     ));
                 }
             }
         }
     }
     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.
 }

 fn report_unused_parameter(
     tcx: TyCtxt<'_>,
     span: Span,
     kind: &str,
     name: Symbol,
 ) -> ErrorGuaranteed {
     let mut err = struct_span_code_err!(
         tcx.dcx(),
         span,
         E0207,
         "the {} parameter `{}` is not constrained by the \
         impl trait, self type, or predicates",
         kind,
         name
     );
     err.span_label(span, format!("unconstrained {kind} parameter"));
     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()
 }
	//! 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::FxHashSet;
	use rustc_errors::{codes::*, struct_span_code_err};
	use rustc_hir::def::DefKind;
	use rustc_hir::def_id::{LocalDefId, LocalModDefId};
	use rustc_middle::query::Providers;
	use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
	use rustc_span::{ErrorGuaranteed, Span, Symbol};

	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: LocalModDefId) -> Result<(), ErrorGuaranteed> {
	let min_specialization = tcx.features().min_specialization;
	let module = tcx.hir_module_items(module_def_id);
	let mut res = Ok(());
	for id in module.items() {
	if matches!(tcx.def_kind(id.owner_id), DefKind::Impl { .. }) {
	res = res.and(enforce_impl_params_are_constrained(tcx, id.owner_id.def_id));
	if min_specialization {
	res = res.and(check_min_specialization(tcx, id.owner_id.def_id));
	}
	}
	}
	res
	}

	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,
	) -> 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);

	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).instantiate_identity(), true)
	} else {
	vec![]
	}
	}
	ty::AssocKind::Fn \| ty::AssocKind::Const => vec![],
	}
	})
	.collect();

	let mut res = Ok(());
	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)) {
	res = Err(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(&param_lt) && // (*)
	!input_parameters.contains(&param_lt)
	{
	res = Err(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)) {
	res = Err(report_unused_parameter(
	tcx,
	tcx.def_span(param.def_id),
	"const",
	param_ct.name,
	));
	}
	}
	}
	}
	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.
	}

	fn report_unused_parameter(
	tcx: TyCtxt<'_>,
	span: Span,
	kind: &str,
	name: Symbol,
	) -> ErrorGuaranteed {
	let mut err = struct_span_code_err!(
	tcx.dcx(),
	span,
	E0207,
	"the {} parameter `{}` is not constrained by the \
	impl trait, self type, or predicates",
	kind,
	name
	);
	err.span_label(span, format!("unconstrained {kind} parameter"));
	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()
	}