use clippy_utils::diagnostics::{multispan_sugg, span_lint_and_then}; use clippy_utils::ptr::get_spans; use clippy_utils::source::{snippet, snippet_opt}; use clippy_utils::ty::{ implements_trait, implements_trait_with_env, is_copy, is_type_diagnostic_item, is_type_lang_item, }; use clippy_utils::{get_trait_def_id, is_self, paths}; use if_chain::if_chain; use rustc_ast::ast::Attribute; use rustc_errors::{Applicability, Diagnostic}; use rustc_hir::intravisit::FnKind; use rustc_hir::{ BindingAnnotation, Body, FnDecl, GenericArg, HirId, Impl, ItemKind, Mutability, Node, PatKind, QPath, TyKind, }; use rustc_hir::{HirIdSet, LangItem}; use rustc_hir_typeck::expr_use_visitor as euv; use rustc_infer::infer::TyCtxtInferExt; use rustc_lint::{LateContext, LateLintPass}; use rustc_middle::mir::FakeReadCause; use rustc_middle::ty::{self, TypeVisitableExt, Ty}; use rustc_session::{declare_lint_pass, declare_tool_lint}; use rustc_span::def_id::LocalDefId; use rustc_span::symbol::kw; use rustc_span::{sym, Span}; use rustc_target::spec::abi::Abi; use rustc_trait_selection::traits; use rustc_trait_selection::traits::misc::type_allowed_to_implement_copy; declare_clippy_lint! { /// ### What it does /// Checks for functions taking arguments by value, but not /// consuming them in its /// body. /// /// ### Why is this bad? /// Taking arguments by reference is more flexible and can /// sometimes avoid /// unnecessary allocations. /// /// ### Known problems /// * This lint suggests taking an argument by reference, /// however sometimes it is better to let users decide the argument type /// (by using `Borrow` trait, for example), depending on how the function is used. /// /// ### Example /// ```rust /// fn foo(v: Vec) { /// assert_eq!(v.len(), 42); /// } /// ``` /// should be /// ```rust /// fn foo(v: &[i32]) { /// assert_eq!(v.len(), 42); /// } /// ``` #[clippy::version = "pre 1.29.0"] pub NEEDLESS_PASS_BY_VALUE, pedantic, "functions taking arguments by value, but not consuming them in its body" } declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]); macro_rules! need { ($e: expr) => { if let Some(x) = $e { x } else { return; } }; } impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue { #[expect(clippy::too_many_lines)] fn check_fn( &mut self, cx: &LateContext<'tcx>, kind: FnKind<'tcx>, decl: &'tcx FnDecl<'_>, body: &'tcx Body<'_>, span: Span, fn_def_id: LocalDefId, ) { if span.from_expansion() { return; } let hir_id = cx.tcx.hir().local_def_id_to_hir_id(fn_def_id); match kind { FnKind::ItemFn(.., header) => { let attrs = cx.tcx.hir().attrs(hir_id); if header.abi != Abi::Rust || requires_exact_signature(attrs) { 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; } } // Allow `Borrow` or functions to be taken by value let allowed_traits = [ need!(cx.tcx.lang_items().fn_trait()), need!(cx.tcx.lang_items().fn_once_trait()), need!(cx.tcx.lang_items().fn_mut_trait()), need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)), ]; let sized_trait = need!(cx.tcx.lang_items().sized_trait()); let preds = traits::elaborate(cx.tcx, cx.param_env.caller_bounds().iter()) .filter(|p| !p.is_global()) .filter_map(|pred| { // Note that we do not want to deal with qualified predicates here. match pred.kind().no_bound_vars() { Some(ty::ClauseKind::Trait(pred)) if pred.def_id() != sized_trait => Some(pred), _ => None, } }) .collect::>(); // Collect moved variables and spans which will need dereferencings from the // function body. let MovedVariablesCtxt { moved_vars } = { let mut ctx = MovedVariablesCtxt::default(); let infcx = cx.tcx.infer_ctxt().build(); euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results()).consume_body(body); ctx }; let fn_sig = cx.tcx.fn_sig(fn_def_id).instantiate_identity(); let fn_sig = cx.tcx.liberate_late_bound_regions(fn_def_id.to_def_id(), fn_sig); for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() { // All spans generated from a proc-macro invocation are the same... if span == input.span { return; } // Ignore `self`s. if idx == 0 { if let PatKind::Binding(.., ident, _) = arg.pat.kind { if ident.name == kw::SelfLower { continue; } } } // // * Exclude a type that is specifically bounded by `Borrow`. // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`, // `serde::Serialize`) let (implements_borrow_trait, all_borrowable_trait) = { let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::>(); ( preds.iter().any(|t| cx.tcx.is_diagnostic_item(sym::Borrow, t.def_id())), !preds.is_empty() && { let ty_empty_region = Ty::new_imm_ref(cx.tcx,cx.tcx.lifetimes.re_erased, ty); preds.iter().all(|t| { let ty_params = t.trait_ref.args.iter().skip(1).collect::>(); implements_trait(cx, ty_empty_region, t.def_id(), &ty_params) }) }, ) }; if_chain! { if !is_self(arg); if !ty.is_mutable_ptr(); if !is_copy(cx, ty); if ty.is_sized(cx.tcx, cx.param_env); if !allowed_traits.iter().any(|&t| implements_trait_with_env(cx.tcx, cx.param_env, ty, t, [None])); if !implements_borrow_trait; if !all_borrowable_trait; if let PatKind::Binding(BindingAnnotation(_, Mutability::Not), canonical_id, ..) = arg.pat.kind; if !moved_vars.contains(&canonical_id); then { // Dereference suggestion let sugg = |diag: &mut Diagnostic| { if let ty::Adt(def, ..) = ty.kind() { if let Some(span) = cx.tcx.hir().span_if_local(def.did()) { if type_allowed_to_implement_copy( cx.tcx, cx.param_env, ty, traits::ObligationCause::dummy_with_span(span), ).is_ok() { diag.span_help(span, "consider marking this type as `Copy`"); } } } if_chain! { if is_type_diagnostic_item(cx, ty, sym::Vec); if let Some(clone_spans) = get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]); if let TyKind::Path(QPath::Resolved(_, path)) = input.kind; if let Some(elem_ty) = path.segments.iter() .find(|seg| seg.ident.name == sym::Vec) .and_then(|ps| ps.args.as_ref()) .map(|params| params.args.iter().find_map(|arg| match arg { GenericArg::Type(ty) => Some(ty), _ => None, }).unwrap()); then { let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_")); diag.span_suggestion( input.span, "consider changing the type to", slice_ty, Applicability::Unspecified, ); for (span, suggestion) in clone_spans { diag.span_suggestion( span, snippet_opt(cx, span) .map_or( "change the call to".into(), |x| format!("change `{x}` to"), ), suggestion, Applicability::Unspecified, ); } // cannot be destructured, no need for `*` suggestion return; } } if is_type_lang_item(cx, ty, LangItem::String) { if let Some(clone_spans) = get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) { diag.span_suggestion( input.span, "consider changing the type to", "&str", Applicability::Unspecified, ); for (span, suggestion) in clone_spans { diag.span_suggestion( span, snippet_opt(cx, span) .map_or( "change the call to".into(), |x| format!("change `{x}` to") ), suggestion, Applicability::Unspecified, ); } return; } } let spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))]; multispan_sugg(diag, "consider taking a reference instead", spans); }; span_lint_and_then( cx, NEEDLESS_PASS_BY_VALUE, input.span, "this argument is passed by value, but not consumed in the function body", sugg, ); } } } } } /// Functions marked with these attributes must have the exact signature. fn requires_exact_signature(attrs: &[Attribute]) -> bool { attrs.iter().any(|attr| { [sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive] .iter() .any(|&allow| attr.has_name(allow)) }) } #[derive(Default)] struct MovedVariablesCtxt { moved_vars: HirIdSet, } impl MovedVariablesCtxt { fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) { if let euv::PlaceBase::Local(vid) = cmt.place.base { self.moved_vars.insert(vid); } } } impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt { fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId) { self.move_common(cmt); } fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {} fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {} fn fake_read(&mut self, _: &rustc_hir_typeck::expr_use_visitor::PlaceWithHirId<'tcx>, _: FakeReadCause, _: HirId) {} }