src/tools/clippy/clippy_lints/src/eta_reduction.rs - third_party/rust - Git at Google

 use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
 use clippy_utils::higher::VecArgs;
 use clippy_utils::source::snippet_opt;
 use clippy_utils::ty::type_diagnostic_name;
 use clippy_utils::usage::{local_used_after_expr, local_used_in};
 use clippy_utils::{get_path_from_caller_to_method_type, is_adjusted, path_to_local, path_to_local_id};
 use rustc_errors::Applicability;
 use rustc_hir::{BindingMode, Expr, ExprKind, FnRetTy, Param, PatKind, QPath, Safety, TyKind};
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty::{
     self, Binder, ClosureArgs, ClosureKind, FnSig, GenericArg, GenericArgKind, List, Region, RegionKind, Ty, TyCtxt,
     TypeVisitableExt, TypeckResults,
 };
 use rustc_session::declare_lint_pass;
 use rustc_span::symbol::sym;
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt as _;

 declare_clippy_lint! {
     /// ### What it does
     /// Checks for closures which just call another function where
     /// the function can be called directly. `unsafe` functions, calls where types
     /// get adjusted or where the callee is marked `#[track_caller]` are ignored.
     ///
     /// ### Why is this bad?
     /// Needlessly creating a closure adds code for no benefit
     /// and gives the optimizer more work.
     ///
     /// ### Known problems
     /// If creating the closure inside the closure has a side-
     /// effect then moving the closure creation out will change when that side-
     /// effect runs.
     /// See [#1439](https://github.com/rust-lang/rust-clippy/issues/1439) for more details.
     ///
     /// ### Example
     /// ```rust,ignore
     /// xs.map(|x| foo(x))
     /// ```
     ///
     /// Use instead:
     /// ```rust,ignore
     /// // where `foo(_)` is a plain function that takes the exact argument type of `x`.
     /// xs.map(foo)
     /// ```
     #[clippy::version = "pre 1.29.0"]
     pub REDUNDANT_CLOSURE,
     style,
     "redundant closures, i.e., `|a| foo(a)` (which can be written as just `foo`)"
 }

 declare_clippy_lint! {
     /// ### What it does
     /// Checks for closures which only invoke a method on the closure
     /// argument and can be replaced by referencing the method directly.
     ///
     /// ### Why is this bad?
     /// It's unnecessary to create the closure.
     ///
     /// ### Example
     /// ```rust,ignore
     /// Some('a').map(|s| s.to_uppercase());
     /// ```
     /// may be rewritten as
     /// ```rust,ignore
     /// Some('a').map(char::to_uppercase);
     /// ```
     #[clippy::version = "1.35.0"]
     pub REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
     pedantic,
     "redundant closures for method calls"
 }

 declare_lint_pass!(EtaReduction => [REDUNDANT_CLOSURE, REDUNDANT_CLOSURE_FOR_METHOD_CALLS]);

 impl<'tcx> LateLintPass<'tcx> for EtaReduction {
     #[allow(clippy::too_many_lines)]
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
         let body = if let ExprKind::Closure(c) = expr.kind
             && c.fn_decl.inputs.iter().all(|ty| matches!(ty.kind, TyKind::Infer))
             && matches!(c.fn_decl.output, FnRetTy::DefaultReturn(_))
             && !expr.span.from_expansion()
         {
             cx.tcx.hir().body(c.body)
         } else {
             return;
         };

         if body.value.span.from_expansion() {
             if body.params.is_empty() {
                 if let Some(VecArgs::Vec(&[])) = VecArgs::hir(cx, body.value) {
                     // replace `|| vec![]` with `Vec::new`
                     span_lint_and_sugg(
                         cx,
                         REDUNDANT_CLOSURE,
                         expr.span,
                         "redundant closure",
                         "replace the closure with `Vec::new`",
                         "std::vec::Vec::new".into(),
                         Applicability::MachineApplicable,
                     );
                 }
             }
             // skip `foo(|| macro!())`
             return;
         }

         let typeck = cx.typeck_results();
         let closure = if let ty::Closure(_, closure_subs) = typeck.expr_ty(expr).kind() {
             closure_subs.as_closure()
         } else {
             return;
         };

         if is_adjusted(cx, body.value) {
             return;
         }

         match body.value.kind {
             ExprKind::Call(callee, args)
                 if matches!(
                     callee.kind,
                     ExprKind::Path(QPath::Resolved(..) | QPath::TypeRelative(..))
                 ) =>
             {
                 let callee_ty_raw = typeck.expr_ty(callee);
                 let callee_ty = callee_ty_raw.peel_refs();
                 if matches!(type_diagnostic_name(cx, callee_ty), Some(sym::Arc | sym::Rc))
                     || !check_inputs(typeck, body.params, None, args)
                 {
                     return;
                 }
                 let callee_ty_adjusted = typeck
                     .expr_adjustments(callee)
                     .last()
                     .map_or(callee_ty, |a| a.target.peel_refs());

                 let sig = match callee_ty_adjusted.kind() {
                     ty::FnDef(def, _) => {
                         // Rewriting `x(|| f())` to `x(f)` where f is marked `#[track_caller]` moves the `Location`
                         if cx.tcx.has_attr(*def, sym::track_caller) {
                             return;
                         }

                         cx.tcx.fn_sig(def).skip_binder().skip_binder()
                     },
                     ty::FnPtr(sig) => sig.skip_binder(),
                     ty::Closure(_, subs) => cx
                         .tcx
                         .signature_unclosure(subs.as_closure().sig(), Safety::Safe)
                         .skip_binder(),
                     _ => {
                         if typeck.type_dependent_def_id(body.value.hir_id).is_some()
                             && let subs = typeck.node_args(body.value.hir_id)
                             && let output = typeck.expr_ty(body.value)
                             && let ty::Tuple(tys) = *subs.type_at(1).kind()
                         {
                             cx.tcx.mk_fn_sig(tys, output, false, Safety::Safe, Abi::Rust)
                         } else {
                             return;
                         }
                     },
                 };
                 if check_sig(cx, closure, sig)
                     && let generic_args = typeck.node_args(callee.hir_id)
                     // Given some trait fn `fn f() -> ()` and some type `T: Trait`, `T::f` is not
                     // `'static` unless `T: 'static`. The cast `T::f as fn()` will, however, result
                     // in a type which is `'static`.
                     // For now ignore all callee types which reference a type parameter.
                     && !generic_args.types().any(|t| matches!(t.kind(), ty::Param(_)))
                 {
                     span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure", |diag| {
                         if let Some(mut snippet) = snippet_opt(cx, callee.span) {
                             if path_to_local(callee).map_or(false, |l| {
                                 // FIXME: Do we really need this `local_used_in` check?
                                 // Isn't it checking something like... `callee(callee)`?
                                 // If somehow this check is needed, add some test for it,
                                 // 'cuz currently nothing changes after deleting this check.
                                 local_used_in(cx, l, args) || local_used_after_expr(cx, l, expr)
                             }) {
                                 match cx.tcx.infer_ctxt().build().err_ctxt().type_implements_fn_trait(
                                     cx.param_env,
                                     Binder::bind_with_vars(callee_ty_adjusted, List::empty()),
                                     ty::PredicatePolarity::Positive,
                                 ) {
                                     // Mutable closure is used after current expr; we cannot consume it.
                                     Ok((ClosureKind::FnMut, _)) => snippet = format!("&mut {snippet}"),
                                     Ok((ClosureKind::Fn, _)) if !callee_ty_raw.is_ref() => {
                                         snippet = format!("&{snippet}");
                                     },
                                     _ => (),
                                 }
                             }
                             diag.span_suggestion(
                                 expr.span,
                                 "replace the closure with the function itself",
                                 snippet,
                                 Applicability::MachineApplicable,
                             );
                         }
                     });
                 }
             },
             ExprKind::MethodCall(path, self_, args, _) if check_inputs(typeck, body.params, Some(self_), args) => {
                 if let Some(method_def_id) = typeck.type_dependent_def_id(body.value.hir_id)
                     && !cx.tcx.has_attr(method_def_id, sym::track_caller)
                     && check_sig(cx, closure, cx.tcx.fn_sig(method_def_id).skip_binder().skip_binder())
                 {
                     span_lint_and_then(
                         cx,
                         REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
                         expr.span,
                         "redundant closure",
                         |diag| {
                             let args = typeck.node_args(body.value.hir_id);
                             let caller = self_.hir_id.owner.def_id;
                             let type_name = get_path_from_caller_to_method_type(cx.tcx, caller, method_def_id, args);
                             diag.span_suggestion(
                                 expr.span,
                                 "replace the closure with the method itself",
                                 format!("{}::{}", type_name, path.ident.name),
                                 Applicability::MachineApplicable,
                             );
                         },
                     );
                 }
             },
             _ => (),
         }
     }
 }

 fn check_inputs(
     typeck: &TypeckResults<'_>,
     params: &[Param<'_>],
     self_arg: Option<&Expr<'_>>,
     args: &[Expr<'_>],
 ) -> bool {
     params.len() == self_arg.map_or(0, |_| 1) + args.len()
         && params.iter().zip(self_arg.into_iter().chain(args)).all(|(p, arg)| {
             matches!(
                 p.pat.kind,
                 PatKind::Binding(BindingMode::NONE, id, _, None)
                 if path_to_local_id(arg, id)
             )
             // Only allow adjustments which change regions (i.e. re-borrowing).
             && typeck
                 .expr_adjustments(arg)
                 .last()
                 .map_or(true, |a| a.target == typeck.expr_ty(arg))
         })
 }

 fn check_sig<'tcx>(cx: &LateContext<'tcx>, closure: ClosureArgs<TyCtxt<'tcx>>, call_sig: FnSig<'_>) -> bool {
     call_sig.safety == Safety::Safe
         && !has_late_bound_to_non_late_bound_regions(
             cx.tcx.signature_unclosure(closure.sig(), Safety::Safe).skip_binder(),
             call_sig,
         )
 }

 /// This walks through both signatures and checks for any time a late-bound region is expected by an
 /// `impl Fn` type, but the target signature does not have a late-bound region in the same position.
 ///
 /// This is needed because rustc is unable to late bind early-bound regions in a function signature.
 fn has_late_bound_to_non_late_bound_regions(from_sig: FnSig<'_>, to_sig: FnSig<'_>) -> bool {
     fn check_region(from_region: Region<'_>, to_region: Region<'_>) -> bool {
         matches!(from_region.kind(), RegionKind::ReBound(..)) && !matches!(to_region.kind(), RegionKind::ReBound(..))
     }

     fn check_subs(from_subs: &[GenericArg<'_>], to_subs: &[GenericArg<'_>]) -> bool {
         if from_subs.len() != to_subs.len() {
             return true;
         }
         for (from_arg, to_arg) in to_subs.iter().zip(from_subs) {
             match (from_arg.unpack(), to_arg.unpack()) {
                 (GenericArgKind::Lifetime(from_region), GenericArgKind::Lifetime(to_region)) => {
                     if check_region(from_region, to_region) {
                         return true;
                     }
                 },
                 (GenericArgKind::Type(from_ty), GenericArgKind::Type(to_ty)) => {
                     if check_ty(from_ty, to_ty) {
                         return true;
                     }
                 },
                 (GenericArgKind::Const(_), GenericArgKind::Const(_)) => (),
                 _ => return true,
             }
         }
         false
     }

     fn check_ty(from_ty: Ty<'_>, to_ty: Ty<'_>) -> bool {
         match (from_ty.kind(), to_ty.kind()) {
             (&ty::Adt(_, from_subs), &ty::Adt(_, to_subs)) => check_subs(from_subs, to_subs),
             (&ty::Array(from_ty, _), &ty::Array(to_ty, _)) | (&ty::Slice(from_ty), &ty::Slice(to_ty)) => {
                 check_ty(from_ty, to_ty)
             },
             (&ty::Ref(from_region, from_ty, _), &ty::Ref(to_region, to_ty, _)) => {
                 check_region(from_region, to_region) || check_ty(from_ty, to_ty)
             },
             (&ty::Tuple(from_tys), &ty::Tuple(to_tys)) => {
                 from_tys.len() != to_tys.len()
                     || from_tys
                         .iter()
                         .zip(to_tys)
                         .any(|(from_ty, to_ty)| check_ty(from_ty, to_ty))
             },
             _ => from_ty.has_bound_regions(),
         }
     }

     assert!(from_sig.inputs_and_output.len() == to_sig.inputs_and_output.len());
     from_sig
         .inputs_and_output
         .iter()
         .zip(to_sig.inputs_and_output)
         .any(|(from_ty, to_ty)| check_ty(from_ty, to_ty))
 }
	use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
	use clippy_utils::higher::VecArgs;
	use clippy_utils::source::snippet_opt;
	use clippy_utils::ty::type_diagnostic_name;
	use clippy_utils::usage::{local_used_after_expr, local_used_in};
	use clippy_utils::{get_path_from_caller_to_method_type, is_adjusted, path_to_local, path_to_local_id};
	use rustc_errors::Applicability;
	use rustc_hir::{BindingMode, Expr, ExprKind, FnRetTy, Param, PatKind, QPath, Safety, TyKind};
	use rustc_infer::infer::TyCtxtInferExt;
	use rustc_lint::{LateContext, LateLintPass};
	use rustc_middle::ty::{
	self, Binder, ClosureArgs, ClosureKind, FnSig, GenericArg, GenericArgKind, List, Region, RegionKind, Ty, TyCtxt,
	TypeVisitableExt, TypeckResults,
	};
	use rustc_session::declare_lint_pass;
	use rustc_span::symbol::sym;
	use rustc_target::spec::abi::Abi;
	use rustc_trait_selection::error_reporting::InferCtxtErrorExt as _;

	declare_clippy_lint! {
	/// ### What it does
	/// Checks for closures which just call another function where
	/// the function can be called directly. `unsafe` functions, calls where types
	/// get adjusted or where the callee is marked `#[track_caller]` are ignored.
	///
	/// ### Why is this bad?
	/// Needlessly creating a closure adds code for no benefit
	/// and gives the optimizer more work.
	///
	/// ### Known problems
	/// If creating the closure inside the closure has a side-
	/// effect then moving the closure creation out will change when that side-
	/// effect runs.
	/// See [#1439](https://github.com/rust-lang/rust-clippy/issues/1439) for more details.
	///
	/// ### Example
	/// ```rust,ignore
	/// xs.map(\|x\| foo(x))
	/// ```
	///
	/// Use instead:
	/// ```rust,ignore
	/// // where `foo(_)` is a plain function that takes the exact argument type of `x`.
	/// xs.map(foo)
	/// ```
	#[clippy::version = "pre 1.29.0"]
	pub REDUNDANT_CLOSURE,
	style,
	"redundant closures, i.e., `\|a\| foo(a)` (which can be written as just `foo`)"
	}

	declare_clippy_lint! {
	/// ### What it does
	/// Checks for closures which only invoke a method on the closure
	/// argument and can be replaced by referencing the method directly.
	///
	/// ### Why is this bad?
	/// It's unnecessary to create the closure.
	///
	/// ### Example
	/// ```rust,ignore
	/// Some('a').map(\|s\| s.to_uppercase());
	/// ```
	/// may be rewritten as
	/// ```rust,ignore
	/// Some('a').map(char::to_uppercase);
	/// ```
	#[clippy::version = "1.35.0"]
	pub REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
	pedantic,
	"redundant closures for method calls"
	}

	declare_lint_pass!(EtaReduction => [REDUNDANT_CLOSURE, REDUNDANT_CLOSURE_FOR_METHOD_CALLS]);

	impl<'tcx> LateLintPass<'tcx> for EtaReduction {
	#[allow(clippy::too_many_lines)]
	fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
	let body = if let ExprKind::Closure(c) = expr.kind
	&& c.fn_decl.inputs.iter().all(\|ty\| matches!(ty.kind, TyKind::Infer))
	&& matches!(c.fn_decl.output, FnRetTy::DefaultReturn(_))
	&& !expr.span.from_expansion()
	{
	cx.tcx.hir().body(c.body)
	} else {
	return;
	};

	if body.value.span.from_expansion() {
	if body.params.is_empty() {
	if let Some(VecArgs::Vec(&[])) = VecArgs::hir(cx, body.value) {
	// replace `\|\| vec![]` with `Vec::new`
	span_lint_and_sugg(
	cx,
	REDUNDANT_CLOSURE,
	expr.span,
	"redundant closure",
	"replace the closure with `Vec::new`",
	"std::vec::Vec::new".into(),
	Applicability::MachineApplicable,
	);
	}
	}
	// skip `foo(\|\| macro!())`
	return;
	}

	let typeck = cx.typeck_results();
	let closure = if let ty::Closure(_, closure_subs) = typeck.expr_ty(expr).kind() {
	closure_subs.as_closure()
	} else {
	return;
	};

	if is_adjusted(cx, body.value) {
	return;
	}

	match body.value.kind {
	ExprKind::Call(callee, args)
	if matches!(
	callee.kind,
	ExprKind::Path(QPath::Resolved(..) \| QPath::TypeRelative(..))
	) =>
	{
	let callee_ty_raw = typeck.expr_ty(callee);
	let callee_ty = callee_ty_raw.peel_refs();
	if matches!(type_diagnostic_name(cx, callee_ty), Some(sym::Arc \| sym::Rc))
	\|\| !check_inputs(typeck, body.params, None, args)
	{
	return;
	}
	let callee_ty_adjusted = typeck
	.expr_adjustments(callee)
	.last()
	.map_or(callee_ty, \|a\| a.target.peel_refs());

	let sig = match callee_ty_adjusted.kind() {
	ty::FnDef(def, _) => {
	// Rewriting `x(\|\| f())` to `x(f)` where f is marked `#[track_caller]` moves the `Location`
	if cx.tcx.has_attr(*def, sym::track_caller) {
	return;
	}

	cx.tcx.fn_sig(def).skip_binder().skip_binder()
	},
	ty::FnPtr(sig) => sig.skip_binder(),
	ty::Closure(_, subs) => cx
	.tcx
	.signature_unclosure(subs.as_closure().sig(), Safety::Safe)
	.skip_binder(),
	_ => {
	if typeck.type_dependent_def_id(body.value.hir_id).is_some()
	&& let subs = typeck.node_args(body.value.hir_id)
	&& let output = typeck.expr_ty(body.value)
	&& let ty::Tuple(tys) = *subs.type_at(1).kind()
	{
	cx.tcx.mk_fn_sig(tys, output, false, Safety::Safe, Abi::Rust)
	} else {
	return;
	}
	},
	};
	if check_sig(cx, closure, sig)
	&& let generic_args = typeck.node_args(callee.hir_id)
	// Given some trait fn `fn f() -> ()` and some type `T: Trait`, `T::f` is not
	// `'static` unless `T: 'static`. The cast `T::f as fn()` will, however, result
	// in a type which is `'static`.
	// For now ignore all callee types which reference a type parameter.
	&& !generic_args.types().any(\|t\| matches!(t.kind(), ty::Param(_)))
	{
	span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure", \|diag\| {
	if let Some(mut snippet) = snippet_opt(cx, callee.span) {
	if path_to_local(callee).map_or(false, \|l\| {
	// FIXME: Do we really need this `local_used_in` check?
	// Isn't it checking something like... `callee(callee)`?
	// If somehow this check is needed, add some test for it,
	// 'cuz currently nothing changes after deleting this check.
	local_used_in(cx, l, args) \|\| local_used_after_expr(cx, l, expr)
	}) {
	match cx.tcx.infer_ctxt().build().err_ctxt().type_implements_fn_trait(
	cx.param_env,
	Binder::bind_with_vars(callee_ty_adjusted, List::empty()),
	ty::PredicatePolarity::Positive,
	) {
	// Mutable closure is used after current expr; we cannot consume it.
	Ok((ClosureKind::FnMut, _)) => snippet = format!("&mut {snippet}"),
	Ok((ClosureKind::Fn, _)) if !callee_ty_raw.is_ref() => {
	snippet = format!("&{snippet}");
	},
	_ => (),
	}
	}
	diag.span_suggestion(
	expr.span,
	"replace the closure with the function itself",
	snippet,
	Applicability::MachineApplicable,
	);
	}
	});
	}
	},
	ExprKind::MethodCall(path, self_, args, _) if check_inputs(typeck, body.params, Some(self_), args) => {
	if let Some(method_def_id) = typeck.type_dependent_def_id(body.value.hir_id)
	&& !cx.tcx.has_attr(method_def_id, sym::track_caller)
	&& check_sig(cx, closure, cx.tcx.fn_sig(method_def_id).skip_binder().skip_binder())
	{
	span_lint_and_then(
	cx,
	REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
	expr.span,
	"redundant closure",
	\|diag\| {
	let args = typeck.node_args(body.value.hir_id);
	let caller = self_.hir_id.owner.def_id;
	let type_name = get_path_from_caller_to_method_type(cx.tcx, caller, method_def_id, args);
	diag.span_suggestion(
	expr.span,
	"replace the closure with the method itself",
	format!("{}::{}", type_name, path.ident.name),
	Applicability::MachineApplicable,
	);
	},
	);
	}
	},
	_ => (),
	}
	}
	}

	fn check_inputs(
	typeck: &TypeckResults<'_>,
	params: &[Param<'_>],
	self_arg: Option<&Expr<'_>>,
	args: &[Expr<'_>],
	) -> bool {
	params.len() == self_arg.map_or(0, \|_\| 1) + args.len()
	&& params.iter().zip(self_arg.into_iter().chain(args)).all(\|(p, arg)\| {
	matches!(
	p.pat.kind,
	PatKind::Binding(BindingMode::NONE, id, _, None)
	if path_to_local_id(arg, id)
	)
	// Only allow adjustments which change regions (i.e. re-borrowing).
	&& typeck
	.expr_adjustments(arg)
	.last()
	.map_or(true, \|a\| a.target == typeck.expr_ty(arg))
	})
	}

	fn check_sig<'tcx>(cx: &LateContext<'tcx>, closure: ClosureArgs<TyCtxt<'tcx>>, call_sig: FnSig<'_>) -> bool {
	call_sig.safety == Safety::Safe
	&& !has_late_bound_to_non_late_bound_regions(
	cx.tcx.signature_unclosure(closure.sig(), Safety::Safe).skip_binder(),
	call_sig,
	)
	}

	/// This walks through both signatures and checks for any time a late-bound region is expected by an
	/// `impl Fn` type, but the target signature does not have a late-bound region in the same position.
	///
	/// This is needed because rustc is unable to late bind early-bound regions in a function signature.
	fn has_late_bound_to_non_late_bound_regions(from_sig: FnSig<'_>, to_sig: FnSig<'_>) -> bool {
	fn check_region(from_region: Region<'_>, to_region: Region<'_>) -> bool {
	matches!(from_region.kind(), RegionKind::ReBound(..)) && !matches!(to_region.kind(), RegionKind::ReBound(..))
	}

	fn check_subs(from_subs: &[GenericArg<'_>], to_subs: &[GenericArg<'_>]) -> bool {
	if from_subs.len() != to_subs.len() {
	return true;
	}
	for (from_arg, to_arg) in to_subs.iter().zip(from_subs) {
	match (from_arg.unpack(), to_arg.unpack()) {
	(GenericArgKind::Lifetime(from_region), GenericArgKind::Lifetime(to_region)) => {
	if check_region(from_region, to_region) {
	return true;
	}
	},
	(GenericArgKind::Type(from_ty), GenericArgKind::Type(to_ty)) => {
	if check_ty(from_ty, to_ty) {
	return true;
	}
	},
	(GenericArgKind::Const(_), GenericArgKind::Const(_)) => (),
	_ => return true,
	}
	}
	false
	}

	fn check_ty(from_ty: Ty<'_>, to_ty: Ty<'_>) -> bool {
	match (from_ty.kind(), to_ty.kind()) {
	(&ty::Adt(_, from_subs), &ty::Adt(_, to_subs)) => check_subs(from_subs, to_subs),
	(&ty::Array(from_ty, _), &ty::Array(to_ty, _)) \| (&ty::Slice(from_ty), &ty::Slice(to_ty)) => {
	check_ty(from_ty, to_ty)
	},
	(&ty::Ref(from_region, from_ty, _), &ty::Ref(to_region, to_ty, _)) => {
	check_region(from_region, to_region) \|\| check_ty(from_ty, to_ty)
	},
	(&ty::Tuple(from_tys), &ty::Tuple(to_tys)) => {
	from_tys.len() != to_tys.len()
	\|\| from_tys
	.iter()
	.zip(to_tys)
	.any(\|(from_ty, to_ty)\| check_ty(from_ty, to_ty))
	},
	_ => from_ty.has_bound_regions(),
	}
	}

	assert!(from_sig.inputs_and_output.len() == to_sig.inputs_and_output.len());
	from_sig
	.inputs_and_output
	.iter()
	.zip(to_sig.inputs_and_output)
	.any(\|(from_ty, to_ty)\| check_ty(from_ty, to_ty))
	}