[rustc.git] / src / tools / clippy / clippy_lints / src / pass_by_ref_or_value.rs

use std::cmp;
use std::iter;

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::snippet;
use clippy_utils::ty::is_copy;
use clippy_utils::{is_self, is_self_ty};
use if_chain::if_chain;
use rustc_ast::attr;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_hir::intravisit::FnKind;
use rustc_hir::{BindingAnnotation, Body, FnDecl, HirId, Impl, ItemKind, MutTy, Mutability, Node, PatKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_middle::ty::layout::LayoutOf;
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 behaviour** introduced with unsafe code.
    ///
    /// ### Example
    ///
    /// ```rust
    /// // Bad
    /// fn foo(v: &u32) {}
    /// ```
    ///
    /// ```rust
    /// // Better
    /// 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]);
    ///
    /// // Bad
    /// fn foo(v: TooLarge) {}
    /// ```
    /// ```rust
    /// #[derive(Clone, Copy)]
    /// struct TooLarge([u8; 2048]);
    ///
    /// // Good
    /// 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);
            #[allow(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.access_levels.is_exported(def_id) {
            return;
        }

        let fn_sig = cx.tcx.fn_sig(def_id);
        let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);

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

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

            match ty.kind() {
                ty::Ref(input_lt, ty, Mutability::Not) => {
                    // Use lifetimes to determine if we're returning a reference to the
                    // argument. In that case we can't switch to pass-by-value as the
                    // argument will not live long enough.
                    let output_lts = match *fn_sig.output().kind() {
                        ty::Ref(output_lt, _, _) => vec![output_lt],
                        ty::Adt(_, substs) => substs.regions().collect(),
                        _ => vec![],
                    };

                    if_chain! {
                        if !output_lts.contains(input_lt);
                        if is_copy(cx, *ty);
                        if let Some(size) = cx.layout_of(*ty).ok().map(|l| l.size.bytes());
                        if size <= self.ref_min_size;
                        if let hir::TyKind::Rptr(_, MutTy { ty: decl_ty, .. }) = input.kind;
                        then {
                            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 ({} byte) is passed by reference, but would be more efficient if passed by value (limit: {} byte)", size, 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::Unannotated, _, _, _) => {},
                            _ => continue,
                        }
                    }

                    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 ({} byte) is passed by value, but might be more efficient if passed by reference (limit: {} byte)", size, 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.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,
        hir_id: HirId,
    ) {
        if span.from_expansion() {
            return;
        }

        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(cx.tcx.hir().get_parent_node(hir_id)) {
            if matches!(
                item.kind,
                ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
            ) {
                return;
            }
        }

        self.check_poly_fn(cx, cx.tcx.hir().local_def_id(hir_id), decl, Some(span));
    }
}
Commit	Line	Data
f20569fa	1	use std::cmp;
cdc7bbd5	2	use std::iter;
f20569fa	3
cdc7bbd5	4	use clippy_utils::diagnostics::span_lint_and_sugg;
cdc7bbd5 XL	5	use clippy_utils::source::snippet;
cdc7bbd5 XL	6	use clippy_utils::ty::is_copy;
94222f64	7	use clippy_utils::{is_self, is_self_ty};
f20569fa XL	8	use if_chain::if_chain;
	9	use rustc_ast::attr;
	10	use rustc_errors::Applicability;
	11	use rustc_hir as hir;
	12	use rustc_hir::intravisit::FnKind;
	13	use rustc_hir::{BindingAnnotation, Body, FnDecl, HirId, Impl, ItemKind, MutTy, Mutability, Node, PatKind};
	14	use rustc_lint::{LateContext, LateLintPass};
	15	use rustc_middle::ty;
c295e0f8	16	use rustc_middle::ty::layout::LayoutOf;
f20569fa	17	use rustc_session::{declare_tool_lint, impl_lint_pass};
94222f64	18	use rustc_span::def_id::LocalDefId;
f20569fa	19	use rustc_span::{sym, Span};
f20569fa XL	20	use rustc_target::spec::abi::Abi;
	21	use rustc_target::spec::Target;
	22
	23	declare_clippy_lint! {
94222f64 XL	24	/// ### What it does
94222f64 XL	25	/// Checks for functions taking arguments by reference, where
f20569fa XL	26	/// the argument type is `Copy` and small enough to be more efficient to always
	27	/// pass by value.
	28	///
94222f64 XL	29	/// ### Why is this bad?
94222f64 XL	30	/// In many calling conventions instances of structs will
f20569fa XL	31	/// be passed through registers if they fit into two or less general purpose
	32	/// registers.
	33	///
94222f64 XL	34	/// ### Known problems
94222f64 XL	35	/// This lint is target register size dependent, it is
f20569fa XL	36	/// limited to 32-bit to try and reduce portability problems between 32 and
	37	/// 64-bit, but if you are compiling for 8 or 16-bit targets then the limit
	38	/// will be different.
	39	///
	40	/// The configuration option `trivial_copy_size_limit` can be set to override
	41	/// this limit for a project.
	42	///
	43	/// This lint attempts to allow passing arguments by reference if a reference
	44	/// to that argument is returned. This is implemented by comparing the lifetime
	45	/// of the argument and return value for equality. However, this can cause
	46	/// false positives in cases involving multiple lifetimes that are bounded by
	47	/// each other.
	48	///
cdc7bbd5 XL	49	/// Also, it does not take account of other similar cases where getting memory addresses
	50	/// matters; namely, returning the pointer to the argument in question,
	51	/// and passing the argument, as both references and pointers,
	52	/// to a function that needs the memory address. For further details, refer to
	53	/// [this issue](https://github.com/rust-lang/rust-clippy/issues/5953)
	54	/// that explains a real case in which this false positive
	55	/// led to an undefined behaviour introduced with unsafe code.
	56	///
94222f64	57	/// ### Example
f20569fa XL	58	///
	59	/// ```rust
	60	/// // Bad
	61	/// fn foo(v: &u32) {}
	62	/// ```
	63	///
	64	/// ```rust
	65	/// // Better
	66	/// fn foo(v: u32) {}
	67	/// ```
a2a8927a	68	#[clippy::version = "pre 1.29.0"]
f20569fa XL	69	pub TRIVIALLY_COPY_PASS_BY_REF,
	70	pedantic,
	71	"functions taking small copyable arguments by reference"
	72	}
	73
	74	declare_clippy_lint! {
94222f64 XL	75	/// ### What it does
94222f64 XL	76	/// Checks for functions taking arguments by value, where
f20569fa XL	77	/// the argument type is `Copy` and large enough to be worth considering
	78	/// passing by reference. Does not trigger if the function is being exported,
	79	/// because that might induce API breakage, if the parameter is declared as mutable,
	80	/// or if the argument is a `self`.
	81	///
94222f64 XL	82	/// ### Why is this bad?
94222f64 XL	83	/// Arguments passed by value might result in an unnecessary
f20569fa XL	84	/// shallow copy, taking up more space in the stack and requiring a call to
	85	/// `memcpy`, which can be expensive.
	86	///
94222f64	87	/// ### Example
f20569fa XL	88	/// ```rust
	89	/// #[derive(Clone, Copy)]
	90	/// struct TooLarge([u8; 2048]);
	91	///
	92	/// // Bad
	93	/// fn foo(v: TooLarge) {}
	94	/// ```
	95	/// ```rust
	96	/// #[derive(Clone, Copy)]
	97	/// struct TooLarge([u8; 2048]);
	98	///
	99	/// // Good
	100	/// fn foo(v: &TooLarge) {}
	101	/// ```
a2a8927a	102	#[clippy::version = "1.49.0"]
f20569fa XL	103	pub LARGE_TYPES_PASSED_BY_VALUE,
	104	pedantic,
	105	"functions taking large arguments by value"
	106	}
	107
	108	#[derive(Copy, Clone)]
	109	pub struct PassByRefOrValue {
	110	ref_min_size: u64,
	111	value_max_size: u64,
17df50a5	112	avoid_breaking_exported_api: bool,
f20569fa XL	113	}
	114
	115	impl<'tcx> PassByRefOrValue {
17df50a5 XL	116	pub fn new(
	117	ref_min_size: Option<u64>,
	118	value_max_size: u64,
	119	avoid_breaking_exported_api: bool,
	120	target: &Target,
	121	) -> Self {
f20569fa XL	122	let ref_min_size = ref_min_size.unwrap_or_else(\|\| {
	123	let bit_width = u64::from(target.pointer_width);
	124	// Cap the calculated bit width at 32-bits to reduce
	125	// portability problems between 32 and 64-bit targets
	126	let bit_width = cmp::min(bit_width, 32);
	127	#[allow(clippy::integer_division)]
	128	let byte_width = bit_width / 8;
	129	// Use a limit of 2 times the register byte width
	130	byte_width * 2
	131	});
	132
	133	Self {
	134	ref_min_size,
	135	value_max_size,
17df50a5	136	avoid_breaking_exported_api,
f20569fa XL	137	}
	138	}
	139
94222f64 XL	140	fn check_poly_fn(&mut self, cx: &LateContext<'tcx>, def_id: LocalDefId, decl: &FnDecl<'_>, span: Option<Span>) {
94222f64 XL	141	if self.avoid_breaking_exported_api && cx.access_levels.is_exported(def_id) {
17df50a5 XL	142	return;
17df50a5 XL	143	}
f20569fa	144
94222f64	145	let fn_sig = cx.tcx.fn_sig(def_id);
f20569fa XL	146	let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
	147
	148	let fn_body = cx.enclosing_body.map(\|id\| cx.tcx.hir().body(id));
	149
cdc7bbd5	150	for (index, (input, &ty)) in iter::zip(decl.inputs, fn_sig.inputs()).enumerate() {
f20569fa XL	151	// All spans generated from a proc-macro invocation are the same...
	152	match span {
	153	Some(s) if s == input.span => return,
	154	_ => (),
	155	}
	156
	157	match ty.kind() {
	158	ty::Ref(input_lt, ty, Mutability::Not) => {
	159	// Use lifetimes to determine if we're returning a reference to the
	160	// argument. In that case we can't switch to pass-by-value as the
	161	// argument will not live long enough.
	162	let output_lts = match *fn_sig.output().kind() {
	163	ty::Ref(output_lt, _, _) => vec![output_lt],
	164	ty::Adt(_, substs) => substs.regions().collect(),
	165	_ => vec![],
	166	};
	167
	168	if_chain! {
cdc7bbd5	169	if !output_lts.contains(input_lt);
5099ac24 FG	170	if is_copy(cx, *ty);
5099ac24 FG	171	if let Some(size) = cx.layout_of(*ty).ok().map(\|l\| l.size.bytes());
f20569fa	172	if size <= self.ref_min_size;
cdc7bbd5	173	if let hir::TyKind::Rptr(_, MutTy { ty: decl_ty, .. }) = input.kind;
f20569fa	174	then {
94222f64	175	let value_type = if fn_body.and_then(\|body\| body.params.get(index)).map_or(false, is_self) {
f20569fa XL	176	"self".into()
	177	} else {
	178	snippet(cx, decl_ty.span, "_").into()
	179	};
	180	span_lint_and_sugg(
	181	cx,
	182	TRIVIALLY_COPY_PASS_BY_REF,
	183	input.span,
	184	&format!("this argument ({} byte) is passed by reference, but would be more efficient if passed by value (limit: {} byte)", size, self.ref_min_size),
	185	"consider passing by value instead",
	186	value_type,
	187	Applicability::Unspecified,
	188	);
	189	}
	190	}
	191	},
	192
	193	ty::Adt(_, _) \| ty::Array(_, _) \| ty::Tuple(_) => {
	194	// if function has a body and parameter is annotated with mut, ignore
	195	if let Some(param) = fn_body.and_then(\|body\| body.params.get(index)) {
	196	match param.pat.kind {
	197	PatKind::Binding(BindingAnnotation::Unannotated, _, _, _) => {},
	198	_ => continue,
	199	}
	200	}
	201
	202	if_chain! {
f20569fa XL	203	if is_copy(cx, ty);
	204	if !is_self_ty(input);
	205	if let Some(size) = cx.layout_of(ty).ok().map(\|l\| l.size.bytes());
	206	if size > self.value_max_size;
	207	then {
	208	span_lint_and_sugg(
	209	cx,
	210	LARGE_TYPES_PASSED_BY_VALUE,
	211	input.span,
	212	&format!("this argument ({} byte) is passed by value, but might be more efficient if passed by reference (limit: {} byte)", size, self.value_max_size),
	213	"consider passing by reference instead",
	214	format!("&{}", snippet(cx, input.span, "_")),
	215	Applicability::MaybeIncorrect,
	216	);
	217	}
	218	}
	219	},
	220
	221	_ => {},
	222	}
	223	}
	224	}
	225	}
	226
	227	impl_lint_pass!(PassByRefOrValue => [TRIVIALLY_COPY_PASS_BY_REF, LARGE_TYPES_PASSED_BY_VALUE]);
	228
	229	impl<'tcx> LateLintPass<'tcx> for PassByRefOrValue {
	230	fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::TraitItem<'_>) {
	231	if item.span.from_expansion() {
	232	return;
	233	}
	234
	235	if let hir::TraitItemKind::Fn(method_sig, _) = &item.kind {
94222f64	236	self.check_poly_fn(cx, item.def_id, &*method_sig.decl, None);
f20569fa XL	237	}
	238	}
	239
	240	fn check_fn(
	241	&mut self,
	242	cx: &LateContext<'tcx>,
	243	kind: FnKind<'tcx>,
	244	decl: &'tcx FnDecl<'_>,
	245	_body: &'tcx Body<'_>,
	246	span: Span,
	247	hir_id: HirId,
	248	) {
	249	if span.from_expansion() {
	250	return;
	251	}
	252
	253	match kind {
04454e1e	254	FnKind::ItemFn(.., header) => {
f20569fa XL	255	if header.abi != Abi::Rust {
	256	return;
	257	}
	258	let attrs = cx.tcx.hir().attrs(hir_id);
	259	for a in attrs {
	260	if let Some(meta_items) = a.meta_item_list() {
	261	if a.has_name(sym::proc_macro_derive)
	262	\|\| (a.has_name(sym::inline) && attr::list_contains_name(&meta_items, sym::always))
	263	{
	264	return;
	265	}
	266	}
	267	}
	268	},
	269	FnKind::Method(..) => (),
	270	FnKind::Closure => return,
	271	}
	272
	273	// Exclude non-inherent impls
	274	if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
	275	if matches!(
	276	item.kind,
	277	ItemKind::Impl(Impl { of_trait: Some(_), .. }) \| ItemKind::Trait(..)
	278	) {
	279	return;
	280	}
	281	}
	282
94222f64	283	self.check_poly_fn(cx, cx.tcx.hir().local_def_id(hir_id), decl, Some(span));
f20569fa XL	284	}
f20569fa XL	285	}