src/tools/clippy/clippy_lints/src/pass_by_ref_or_value.rs - toolchain/rustc - Git at Google

 use std::{cmp, iter};

 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::source::snippet;
 use clippy_utils::ty::{for_each_top_level_late_bound_region, is_copy};
 use clippy_utils::{is_self, is_self_ty};
 use core::ops::ControlFlow;
 use if_chain::if_chain;
 use rustc_ast::attr;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::intravisit::FnKind;
 use rustc_hir::{BindingAnnotation, Body, FnDecl, Impl, ItemKind, MutTy, Mutability, Node, PatKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty::adjustment::{Adjust, PointerCoercion};
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::ty::{self, RegionKind};
 use rustc_session::{declare_tool_lint, impl_lint_pass};
 use rustc_span::def_id::LocalDefId;
 use rustc_span::{sym, Span};
 use rustc_target::spec::abi::Abi;
 use rustc_target::spec::Target;

 declare_clippy_lint! {
     /// ### What it does
     /// Checks for functions taking arguments by reference, where
     /// the argument type is `Copy` and small enough to be more efficient to always
     /// pass by value.
     ///
     /// ### Why is this bad?
     /// In many calling conventions instances of structs will
     /// be passed through registers if they fit into two or less general purpose
     /// registers.
     ///
     /// ### Known problems
     /// This lint is target register size dependent, it is
     /// limited to 32-bit to try and reduce portability problems between 32 and
     /// 64-bit, but if you are compiling for 8 or 16-bit targets then the limit
     /// will be different.
     ///
     /// The configuration option `trivial_copy_size_limit` can be set to override
     /// this limit for a project.
     ///
     /// This lint attempts to allow passing arguments by reference if a reference
     /// to that argument is returned. This is implemented by comparing the lifetime
     /// of the argument and return value for equality. However, this can cause
     /// false positives in cases involving multiple lifetimes that are bounded by
     /// each other.
     ///
     /// Also, it does not take account of other similar cases where getting memory addresses
     /// matters; namely, returning the pointer to the argument in question,
     /// and passing the argument, as both references and pointers,
     /// to a function that needs the memory address. For further details, refer to
     /// [this issue](https://github.com/rust-lang/rust-clippy/issues/5953)
     /// that explains a real case in which this false positive
     /// led to an **undefined behavior** introduced with unsafe code.
     ///
     /// ### Example
     ///
     /// ```rust
     /// fn foo(v: &u32) {}
     /// ```
     ///
     /// Use instead:
     /// ```rust
     /// fn foo(v: u32) {}
     /// ```
     #[clippy::version = "pre 1.29.0"]
     pub TRIVIALLY_COPY_PASS_BY_REF,
     pedantic,
     "functions taking small copyable arguments by reference"
 }

 declare_clippy_lint! {
     /// ### What it does
     /// Checks for functions taking arguments by value, where
     /// the argument type is `Copy` and large enough to be worth considering
     /// passing by reference. Does not trigger if the function is being exported,
     /// because that might induce API breakage, if the parameter is declared as mutable,
     /// or if the argument is a `self`.
     ///
     /// ### Why is this bad?
     /// Arguments passed by value might result in an unnecessary
     /// shallow copy, taking up more space in the stack and requiring a call to
     /// `memcpy`, which can be expensive.
     ///
     /// ### Example
     /// ```rust
     /// #[derive(Clone, Copy)]
     /// struct TooLarge([u8; 2048]);
     ///
     /// fn foo(v: TooLarge) {}
     /// ```
     ///
     /// Use instead:
     /// ```rust
     /// # #[derive(Clone, Copy)]
     /// # struct TooLarge([u8; 2048]);
     /// fn foo(v: &TooLarge) {}
     /// ```
     #[clippy::version = "1.49.0"]
     pub LARGE_TYPES_PASSED_BY_VALUE,
     pedantic,
     "functions taking large arguments by value"
 }

 #[derive(Copy, Clone)]
 pub struct PassByRefOrValue {
     ref_min_size: u64,
     value_max_size: u64,
     avoid_breaking_exported_api: bool,
 }

 impl<'tcx> PassByRefOrValue {
     pub fn new(
         ref_min_size: Option<u64>,
         value_max_size: u64,
         avoid_breaking_exported_api: bool,
         target: &Target,
     ) -> Self {
         let ref_min_size = ref_min_size.unwrap_or_else(|| {
             let bit_width = u64::from(target.pointer_width);
             // Cap the calculated bit width at 32-bits to reduce
             // portability problems between 32 and 64-bit targets
             let bit_width = cmp::min(bit_width, 32);
             #[expect(clippy::integer_division)]
             let byte_width = bit_width / 8;
             // Use a limit of 2 times the register byte width
             byte_width * 2
         });

         Self {
             ref_min_size,
             value_max_size,
             avoid_breaking_exported_api,
         }
     }

     fn check_poly_fn(&mut self, cx: &LateContext<'tcx>, def_id: LocalDefId, decl: &FnDecl<'_>, span: Option<Span>) {
         if self.avoid_breaking_exported_api && cx.effective_visibilities.is_exported(def_id) {
             return;
         }

         let fn_sig = cx.tcx.fn_sig(def_id).instantiate_identity();
         let fn_body = cx.enclosing_body.map(|id| cx.tcx.hir().body(id));

         // Gather all the lifetimes found in the output type which may affect whether
         // `TRIVIALLY_COPY_PASS_BY_REF` should be linted.
         let mut output_regions = FxHashSet::default();
         for_each_top_level_late_bound_region(fn_sig.skip_binder().output(), |region| -> ControlFlow<!> {
             output_regions.insert(region);
             ControlFlow::Continue(())
         });

         for (index, (input, ty)) in iter::zip(
             decl.inputs,
             fn_sig.skip_binder().inputs().iter().map(|&ty| fn_sig.rebind(ty)),
         )
         .enumerate()
         {
             // All spans generated from a proc-macro invocation are the same...
             match span {
                 Some(s) if s == input.span => continue,
                 _ => (),
             }

             match *ty.skip_binder().kind() {
                 ty::Ref(lt, ty, Mutability::Not) => {
                     match lt.kind() {
                         RegionKind::ReLateBound(index, region)
                             if index.as_u32() == 0 && output_regions.contains(&region) =>
                         {
                             continue;
                         },
                         // Early bound regions on functions are either from the containing item, are bounded by another
                         // lifetime, or are used as a bound for a type or lifetime.
                         RegionKind::ReEarlyBound(..) => continue,
                         _ => (),
                     }

                     let ty = cx.tcx.erase_late_bound_regions(fn_sig.rebind(ty));
                     if is_copy(cx, ty)
                         && let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes())
                         && size <= self.ref_min_size
                         && let hir::TyKind::Ref(_, MutTy { ty: decl_ty, .. }) = input.kind
                     {
                         if let Some(typeck) = cx.maybe_typeck_results() {
                             // Don't lint if an unsafe pointer is created.
                             // TODO: Limit the check only to unsafe pointers to the argument (or part of the argument)
                             //       which escape the current function.
                             if typeck.node_types().items().any(|(_, &ty)| ty.is_unsafe_ptr())
                                 || typeck
                                     .adjustments()
                                     .items()
                                     .flat_map(|(_, a)| a)
                                     .any(|a| matches!(a.kind, Adjust::Pointer(PointerCoercion::UnsafeFnPointer)))
                             {
                                 continue;
                             }
                         }
                         let value_type = if fn_body.and_then(|body| body.params.get(index)).map_or(false, is_self) {
                             "self".into()
                         } else {
                             snippet(cx, decl_ty.span, "_").into()
                         };
                         span_lint_and_sugg(
                             cx,
                             TRIVIALLY_COPY_PASS_BY_REF,
                             input.span,
                             &format!("this argument ({size} byte) is passed by reference, but would be more efficient if passed by value (limit: {} byte)", self.ref_min_size),
                             "consider passing by value instead",
                             value_type,
                             Applicability::Unspecified,
                         );
                     }
                 },

                 ty::Adt(_, _) | ty::Array(_, _) | ty::Tuple(_) => {
                     // if function has a body and parameter is annotated with mut, ignore
                     if let Some(param) = fn_body.and_then(|body| body.params.get(index)) {
                         match param.pat.kind {
                             PatKind::Binding(BindingAnnotation::NONE, _, _, _) => {},
                             _ => continue,
                         }
                     }
                     let ty = cx.tcx.erase_late_bound_regions(ty);

                     if_chain! {
                         if is_copy(cx, ty);
                         if !is_self_ty(input);
                         if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
                         if size > self.value_max_size;
                         then {
                             span_lint_and_sugg(
                                 cx,
                                 LARGE_TYPES_PASSED_BY_VALUE,
                                 input.span,
                                 &format!("this argument ({size} byte) is passed by value, but might be more efficient if passed by reference (limit: {} byte)", self.value_max_size),
                                 "consider passing by reference instead",
                                 format!("&{}", snippet(cx, input.span, "_")),
                                 Applicability::MaybeIncorrect,
                             );
                         }
                     }
                 },

                 _ => {},
             }
         }
     }
 }

 impl_lint_pass!(PassByRefOrValue => [TRIVIALLY_COPY_PASS_BY_REF, LARGE_TYPES_PASSED_BY_VALUE]);

 impl<'tcx> LateLintPass<'tcx> for PassByRefOrValue {
     fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::TraitItem<'_>) {
         if item.span.from_expansion() {
             return;
         }

         if let hir::TraitItemKind::Fn(method_sig, _) = &item.kind {
             self.check_poly_fn(cx, item.owner_id.def_id, method_sig.decl, None);
         }
     }

     fn check_fn(
         &mut self,
         cx: &LateContext<'tcx>,
         kind: FnKind<'tcx>,
         decl: &'tcx FnDecl<'_>,
         _body: &'tcx Body<'_>,
         span: Span,
         def_id: LocalDefId,
     ) {
         if span.from_expansion() {
             return;
         }

         let hir_id = cx.tcx.hir().local_def_id_to_hir_id(def_id);
         match kind {
             FnKind::ItemFn(.., header) => {
                 if header.abi != Abi::Rust {
                     return;
                 }
                 let attrs = cx.tcx.hir().attrs(hir_id);
                 for a in attrs {
                     if let Some(meta_items) = a.meta_item_list() {
                         if a.has_name(sym::proc_macro_derive)
                             || (a.has_name(sym::inline) && attr::list_contains_name(&meta_items, sym::always))
                         {
                             return;
                         }
                     }
                 }
             },
             FnKind::Method(..) => (),
             FnKind::Closure => return,
         }

         // Exclude non-inherent impls
         if let Some(Node::Item(item)) = cx.tcx.hir().find_parent(hir_id) {
             if matches!(
                 item.kind,
                 ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
             ) {
                 return;
             }
         }

         self.check_poly_fn(cx, def_id, decl, Some(span));
     }
 }
	use std::{cmp, iter};

	use clippy_utils::diagnostics::span_lint_and_sugg;
	use clippy_utils::source::snippet;
	use clippy_utils::ty::{for_each_top_level_late_bound_region, is_copy};
	use clippy_utils::{is_self, is_self_ty};
	use core::ops::ControlFlow;
	use if_chain::if_chain;
	use rustc_ast::attr;
	use rustc_data_structures::fx::FxHashSet;
	use rustc_errors::Applicability;
	use rustc_hir as hir;
	use rustc_hir::intravisit::FnKind;
	use rustc_hir::{BindingAnnotation, Body, FnDecl, Impl, ItemKind, MutTy, Mutability, Node, PatKind};
	use rustc_lint::{LateContext, LateLintPass};
	use rustc_middle::ty::adjustment::{Adjust, PointerCoercion};
	use rustc_middle::ty::layout::LayoutOf;
	use rustc_middle::ty::{self, RegionKind};
	use rustc_session::{declare_tool_lint, impl_lint_pass};
	use rustc_span::def_id::LocalDefId;
	use rustc_span::{sym, Span};
	use rustc_target::spec::abi::Abi;
	use rustc_target::spec::Target;

	declare_clippy_lint! {
	/// ### What it does
	/// Checks for functions taking arguments by reference, where
	/// the argument type is `Copy` and small enough to be more efficient to always
	/// pass by value.
	///
	/// ### Why is this bad?
	/// In many calling conventions instances of structs will
	/// be passed through registers if they fit into two or less general purpose
	/// registers.
	///
	/// ### Known problems
	/// This lint is target register size dependent, it is
	/// limited to 32-bit to try and reduce portability problems between 32 and
	/// 64-bit, but if you are compiling for 8 or 16-bit targets then the limit
	/// will be different.
	///
	/// The configuration option `trivial_copy_size_limit` can be set to override
	/// this limit for a project.
	///
	/// This lint attempts to allow passing arguments by reference if a reference
	/// to that argument is returned. This is implemented by comparing the lifetime
	/// of the argument and return value for equality. However, this can cause
	/// false positives in cases involving multiple lifetimes that are bounded by
	/// each other.
	///
	/// Also, it does not take account of other similar cases where getting memory addresses
	/// matters; namely, returning the pointer to the argument in question,
	/// and passing the argument, as both references and pointers,
	/// to a function that needs the memory address. For further details, refer to
	/// [this issue](https://github.com/rust-lang/rust-clippy/issues/5953)
	/// that explains a real case in which this false positive
	/// led to an undefined behavior introduced with unsafe code.
	///
	/// ### Example
	///
	/// ```rust
	/// fn foo(v: &u32) {}
	/// ```
	///
	/// Use instead:
	/// ```rust
	/// fn foo(v: u32) {}
	/// ```
	#[clippy::version = "pre 1.29.0"]
	pub TRIVIALLY_COPY_PASS_BY_REF,
	pedantic,
	"functions taking small copyable arguments by reference"
	}

	declare_clippy_lint! {
	/// ### What it does
	/// Checks for functions taking arguments by value, where
	/// the argument type is `Copy` and large enough to be worth considering
	/// passing by reference. Does not trigger if the function is being exported,
	/// because that might induce API breakage, if the parameter is declared as mutable,
	/// or if the argument is a `self`.
	///
	/// ### Why is this bad?
	/// Arguments passed by value might result in an unnecessary
	/// shallow copy, taking up more space in the stack and requiring a call to
	/// `memcpy`, which can be expensive.
	///
	/// ### Example
	/// ```rust
	/// #[derive(Clone, Copy)]
	/// struct TooLarge([u8; 2048]);
	///
	/// fn foo(v: TooLarge) {}
	/// ```
	///
	/// Use instead:
	/// ```rust
	/// # #[derive(Clone, Copy)]
	/// # struct TooLarge([u8; 2048]);
	/// fn foo(v: &TooLarge) {}
	/// ```
	#[clippy::version = "1.49.0"]
	pub LARGE_TYPES_PASSED_BY_VALUE,
	pedantic,
	"functions taking large arguments by value"
	}

	#[derive(Copy, Clone)]
	pub struct PassByRefOrValue {
	ref_min_size: u64,
	value_max_size: u64,
	avoid_breaking_exported_api: bool,
	}

	impl<'tcx> PassByRefOrValue {
	pub fn new(
	ref_min_size: Option<u64>,
	value_max_size: u64,
	avoid_breaking_exported_api: bool,
	target: &Target,
	) -> Self {
	let ref_min_size = ref_min_size.unwrap_or_else(\|\| {
	let bit_width = u64::from(target.pointer_width);
	// Cap the calculated bit width at 32-bits to reduce
	// portability problems between 32 and 64-bit targets
	let bit_width = cmp::min(bit_width, 32);
	#[expect(clippy::integer_division)]
	let byte_width = bit_width / 8;
	// Use a limit of 2 times the register byte width
	byte_width * 2
	});

	Self {
	ref_min_size,
	value_max_size,
	avoid_breaking_exported_api,
	}
	}

	fn check_poly_fn(&mut self, cx: &LateContext<'tcx>, def_id: LocalDefId, decl: &FnDecl<'_>, span: Option<Span>) {
	if self.avoid_breaking_exported_api && cx.effective_visibilities.is_exported(def_id) {
	return;
	}

	let fn_sig = cx.tcx.fn_sig(def_id).instantiate_identity();
	let fn_body = cx.enclosing_body.map(\|id\| cx.tcx.hir().body(id));

	// Gather all the lifetimes found in the output type which may affect whether
	// `TRIVIALLY_COPY_PASS_BY_REF` should be linted.
	let mut output_regions = FxHashSet::default();
	for_each_top_level_late_bound_region(fn_sig.skip_binder().output(), \|region\| -> ControlFlow<!> {
	output_regions.insert(region);
	ControlFlow::Continue(())
	});

	for (index, (input, ty)) in iter::zip(
	decl.inputs,
	fn_sig.skip_binder().inputs().iter().map(\|&ty\| fn_sig.rebind(ty)),
	)
	.enumerate()
	{
	// All spans generated from a proc-macro invocation are the same...
	match span {
	Some(s) if s == input.span => continue,
	_ => (),
	}

	match *ty.skip_binder().kind() {
	ty::Ref(lt, ty, Mutability::Not) => {
	match lt.kind() {
	RegionKind::ReLateBound(index, region)
	if index.as_u32() == 0 && output_regions.contains(&region) =>
	{
	continue;
	},
	// Early bound regions on functions are either from the containing item, are bounded by another
	// lifetime, or are used as a bound for a type or lifetime.
	RegionKind::ReEarlyBound(..) => continue,
	_ => (),
	}

	let ty = cx.tcx.erase_late_bound_regions(fn_sig.rebind(ty));
	if is_copy(cx, ty)
	&& let Some(size) = cx.layout_of(ty).ok().map(\|l\| l.size.bytes())
	&& size <= self.ref_min_size
	&& let hir::TyKind::Ref(_, MutTy { ty: decl_ty, .. }) = input.kind
	{
	if let Some(typeck) = cx.maybe_typeck_results() {
	// Don't lint if an unsafe pointer is created.
	// TODO: Limit the check only to unsafe pointers to the argument (or part of the argument)
	// which escape the current function.
	if typeck.node_types().items().any(\|(_, &ty)\| ty.is_unsafe_ptr())
	\|\| typeck
	.adjustments()
	.items()
	.flat_map(\|(_, a)\| a)
	.any(\|a\| matches!(a.kind, Adjust::Pointer(PointerCoercion::UnsafeFnPointer)))
	{
	continue;
	}
	}
	let value_type = if fn_body.and_then(\|body\| body.params.get(index)).map_or(false, is_self) {
	"self".into()
	} else {
	snippet(cx, decl_ty.span, "_").into()
	};
	span_lint_and_sugg(
	cx,
	TRIVIALLY_COPY_PASS_BY_REF,
	input.span,
	&format!("this argument ({size} byte) is passed by reference, but would be more efficient if passed by value (limit: {} byte)", self.ref_min_size),
	"consider passing by value instead",
	value_type,
	Applicability::Unspecified,
	);
	}
	},

	ty::Adt(_, _) \| ty::Array(_, _) \| ty::Tuple(_) => {
	// if function has a body and parameter is annotated with mut, ignore
	if let Some(param) = fn_body.and_then(\|body\| body.params.get(index)) {
	match param.pat.kind {
	PatKind::Binding(BindingAnnotation::NONE, _, _, _) => {},
	_ => continue,
	}
	}
	let ty = cx.tcx.erase_late_bound_regions(ty);

	if_chain! {
	if is_copy(cx, ty);
	if !is_self_ty(input);
	if let Some(size) = cx.layout_of(ty).ok().map(\|l\| l.size.bytes());
	if size > self.value_max_size;
	then {
	span_lint_and_sugg(
	cx,
	LARGE_TYPES_PASSED_BY_VALUE,
	input.span,
	&format!("this argument ({size} byte) is passed by value, but might be more efficient if passed by reference (limit: {} byte)", self.value_max_size),
	"consider passing by reference instead",
	format!("&{}", snippet(cx, input.span, "_")),
	Applicability::MaybeIncorrect,
	);
	}
	}
	},

	_ => {},
	}
	}
	}
	}

	impl_lint_pass!(PassByRefOrValue => [TRIVIALLY_COPY_PASS_BY_REF, LARGE_TYPES_PASSED_BY_VALUE]);

	impl<'tcx> LateLintPass<'tcx> for PassByRefOrValue {
	fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::TraitItem<'_>) {
	if item.span.from_expansion() {
	return;
	}

	if let hir::TraitItemKind::Fn(method_sig, _) = &item.kind {
	self.check_poly_fn(cx, item.owner_id.def_id, method_sig.decl, None);
	}
	}

	fn check_fn(
	&mut self,
	cx: &LateContext<'tcx>,
	kind: FnKind<'tcx>,
	decl: &'tcx FnDecl<'_>,
	_body: &'tcx Body<'_>,
	span: Span,
	def_id: LocalDefId,
	) {
	if span.from_expansion() {
	return;
	}

	let hir_id = cx.tcx.hir().local_def_id_to_hir_id(def_id);
	match kind {
	FnKind::ItemFn(.., header) => {
	if header.abi != Abi::Rust {
	return;
	}
	let attrs = cx.tcx.hir().attrs(hir_id);
	for a in attrs {
	if let Some(meta_items) = a.meta_item_list() {
	if a.has_name(sym::proc_macro_derive)
	\|\| (a.has_name(sym::inline) && attr::list_contains_name(&meta_items, sym::always))
	{
	return;
	}
	}
	}
	},
	FnKind::Method(..) => (),
	FnKind::Closure => return,
	}

	// Exclude non-inherent impls
	if let Some(Node::Item(item)) = cx.tcx.hir().find_parent(hir_id) {
	if matches!(
	item.kind,
	ItemKind::Impl(Impl { of_trait: Some(_), .. }) \| ItemKind::Trait(..)
	) {
	return;
	}
	}

	self.check_poly_fn(cx, def_id, decl, Some(span));
	}
	}