use crate::utils::ptr::get_spans; use crate::utils::{ get_trait_def_id, implements_trait, is_copy, is_self, is_type_diagnostic_item, multispan_sugg, paths, snippet, snippet_opt, span_lint_and_then, }; use if_chain::if_chain; use rustc_ast::ast::Attribute; use rustc_data_structures::fx::{FxHashMap, FxHashSet}; use rustc_errors::{Applicability, DiagnosticBuilder}; use rustc_hir::intravisit::FnKind; use rustc_hir::{BindingAnnotation, Body, FnDecl, GenericArg, HirId, ItemKind, Node, PatKind, QPath, TyKind}; use rustc_infer::infer::TyCtxtInferExt; use rustc_lint::{LateContext, LateLintPass}; use rustc_middle::ty::{self, TypeFoldable}; use rustc_session::{declare_lint_pass, declare_tool_lint}; use rustc_span::Span; use rustc_target::spec::abi::Abi; use rustc_trait_selection::traits; use rustc_trait_selection::traits::misc::can_type_implement_copy; use rustc_typeck::expr_use_visitor as euv; use std::borrow::Cow; 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); /// } /// ``` /// /// ```rust /// // should be /// fn foo(v: &[i32]) { /// assert_eq!(v.len(), 42); /// } /// ``` 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<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue { #[allow(clippy::too_many_lines)] fn check_fn( &mut self, cx: &LateContext<'a, '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, _, attrs) => { 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(cx.tcx.hir().get_parent_node(hir_id)) { if matches!(item.kind, ItemKind::Impl{ of_trait: Some(_), .. } | ItemKind::Trait(..)) { return; } } // Allow `Borrow` or functions to be taken by value let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT)); let whitelisted_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 fn_def_id = cx.tcx.hir().local_def_id(hir_id); let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.iter()) .filter(|p| !p.is_global()) .filter_map(|obligation| { if let ty::PredicateKind::Trait(poly_trait_ref, _) = obligation.predicate.kind() { if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars() { return None; } Some(poly_trait_ref) } else { None } }) .collect::>(); // Collect moved variables and spans which will need dereferencings from the // function body. let MovedVariablesCtxt { moved_vars, spans_need_deref, .. } = { let mut ctx = MovedVariablesCtxt::default(); cx.tcx.infer_ctxt().enter(|infcx| { euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.tables).consume_body(body); }); ctx }; let fn_sig = cx.tcx.fn_sig(fn_def_id); let fn_sig = cx.tcx.erase_late_bound_regions(&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.as_str() == "self" { 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.skip_binder().self_ty() == ty) .collect::>(); ( preds.iter().any(|t| t.def_id() == borrow_trait), !preds.is_empty() && { let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_root_empty, ty); preds.iter().all(|t| { let ty_params = &t .skip_binder() .trait_ref .substs .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 !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[])); if !implements_borrow_trait; if !all_borrowable_trait; if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.kind; if !moved_vars.contains(&canonical_id); then { if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut { continue; } // Dereference suggestion let sugg = |diag: &mut DiagnosticBuilder<'_>| { if let ty::Adt(def, ..) = ty.kind { if let Some(span) = cx.tcx.hir().span_if_local(def.did) { if can_type_implement_copy(cx.tcx, cx.param_env, ty).is_ok() { diag.span_help(span, "consider marking this type as `Copy`"); } } } let deref_span = spans_need_deref.get(&canonical_id); if_chain! { if is_type_diagnostic_item(cx, ty, sym!(vec_type)); if let Some(clone_spans) = get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]); if let TyKind::Path(QPath::Resolved(_, ref 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| Cow::from(format!("change `{}` to", x)), ), suggestion.into(), Applicability::Unspecified, ); } // cannot be destructured, no need for `*` suggestion assert!(deref_span.is_none()); return; } } if is_type_diagnostic_item(cx, ty, sym!(string_type)) { 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".to_string(), Applicability::Unspecified, ); for (span, suggestion) in clone_spans { diag.span_suggestion( span, &snippet_opt(cx, span) .map_or( "change the call to".into(), |x| Cow::from(format!("change `{}` to", x)) ), suggestion.into(), Applicability::Unspecified, ); } assert!(deref_span.is_none()); return; } } let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))]; // Suggests adding `*` to dereference the added reference. if let Some(deref_span) = deref_span { spans.extend( deref_span .iter() .cloned() .map(|span| (span, format!("*{}", snippet(cx, span, "")))), ); spans.sort_by_key(|&(span, _)| 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.check_name(allow)) }) } #[derive(Default)] struct MovedVariablesCtxt { moved_vars: FxHashSet, /// Spans which need to be prefixed with `*` for dereferencing the /// suggested additional reference. spans_need_deref: FxHashMap>, } impl MovedVariablesCtxt { fn move_common(&mut self, cmt: &euv::Place<'_>) { if let euv::PlaceBase::Local(vid) = cmt.base { self.moved_vars.insert(vid); } } } impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt { fn consume(&mut self, cmt: &euv::Place<'tcx>, mode: euv::ConsumeMode) { if let euv::ConsumeMode::Move = mode { self.move_common(cmt); } } fn borrow(&mut self, _: &euv::Place<'tcx>, _: ty::BorrowKind) {} fn mutate(&mut self, _: &euv::Place<'tcx>) {} }