clippy_lints/src/needless_pass_by_value.rs

   1 use crate::utils::ptr::get_spans;
   2 use crate::utils::{
   3     get_trait_def_id, implements_trait, is_copy, is_self, is_type_diagnostic_item, multispan_sugg, paths, snippet,
   4     snippet_opt, span_lint_and_then,
   5 };
   6 use if_chain::if_chain;
   7 use rustc_ast::ast::Attribute;
   8 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
   9 use rustc_errors::{Applicability, DiagnosticBuilder};
  10 use rustc_hir::intravisit::FnKind;
  11 use rustc_hir::{BindingAnnotation, Body, FnDecl, GenericArg, HirId, Impl, ItemKind, Node, PatKind, QPath, TyKind};
  12 use rustc_infer::infer::TyCtxtInferExt;
  13 use rustc_lint::{LateContext, LateLintPass};
  14 use rustc_middle::ty::{self, TypeFoldable};
  15 use rustc_session::{declare_lint_pass, declare_tool_lint};
  16 use rustc_span::symbol::kw;
  17 use rustc_span::{sym, Span};
  18 use rustc_target::spec::abi::Abi;
  19 use rustc_trait_selection::traits;
  20 use rustc_trait_selection::traits::misc::can_type_implement_copy;
  21 use rustc_typeck::expr_use_visitor as euv;
  22 use std::borrow::Cow;
  23
  24 declare_clippy_lint! {
  25     /// **What it does:** Checks for functions taking arguments by value, but not
  26     /// consuming them in its
  27     /// body.
  28     ///
  29     /// **Why is this bad?** Taking arguments by reference is more flexible and can
  30     /// sometimes avoid
  31     /// unnecessary allocations.
  32     ///
  33     /// **Known problems:**
  34     /// * This lint suggests taking an argument by reference,
  35     /// however sometimes it is better to let users decide the argument type
  36     /// (by using `Borrow` trait, for example), depending on how the function is used.
  37     ///
  38     /// **Example:**
  39     /// ```rust
  40     /// fn foo(v: Vec<i32>) {
  41     ///     assert_eq!(v.len(), 42);
  42     /// }
  43     /// ```
  44     /// should be
  45     /// ```rust
  46     /// fn foo(v: &[i32]) {
  47     ///     assert_eq!(v.len(), 42);
  48     /// }
  49     /// ```
  50     pub NEEDLESS_PASS_BY_VALUE,
  51     pedantic,
  52     "functions taking arguments by value, but not consuming them in its body"
  53 }
  54
  55 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
  56
  57 macro_rules! need {
  58     ($e: expr) => {
  59         if let Some(x) = $e {
  60             x
  61         } else {
  62             return;
  63         }
  64     };
  65 }
  66
  67 impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
  68     #[allow(clippy::too_many_lines)]
  69     fn check_fn(
  70         &mut self,
  71         cx: &LateContext<'tcx>,
  72         kind: FnKind<'tcx>,
  73         decl: &'tcx FnDecl<'_>,
  74         body: &'tcx Body<'_>,
  75         span: Span,
  76         hir_id: HirId,
  77     ) {
  78         if span.from_expansion() {
  79             return;
  80         }
  81
  82         match kind {
  83             FnKind::ItemFn(.., header, _, attrs) => {
  84                 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
  85                     return;
  86                 }
  87             },
  88             FnKind::Method(..) => (),
  89             FnKind::Closure(..) => return,
  90         }
  91
  92         // Exclude non-inherent impls
  93         if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
  94             if matches!(
  95                 item.kind,
  96                 ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
  97             ) {
  98                 return;
  99             }
 100         }
 101
 102         // Allow `Borrow` or functions to be taken by value
 103         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 104         let allowed_traits = [
 105             need!(cx.tcx.lang_items().fn_trait()),
 106             need!(cx.tcx.lang_items().fn_once_trait()),
 107             need!(cx.tcx.lang_items().fn_mut_trait()),
 108             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
 109         ];
 110
 111         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 112
 113         let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
 114
 115         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
 116             .filter(|p| !p.is_global())
 117             .filter_map(|obligation| {
 118                 // Note that we do not want to deal with qualified predicates here.
 119                 match obligation.predicate.kind().no_bound_vars() {
 120                     Some(ty::PredicateKind::Trait(pred, _)) if pred.def_id() != sized_trait => Some(pred),
 121                     _ => None,
 122                 }
 123             })
 124             .collect::<Vec<_>>();
 125
 126         // Collect moved variables and spans which will need dereferencings from the
 127         // function body.
 128         let MovedVariablesCtxt {
 129             moved_vars,
 130             spans_need_deref,
 131             ..
 132         } = {
 133             let mut ctx = MovedVariablesCtxt::default();
 134             cx.tcx.infer_ctxt().enter(|infcx| {
 135                 euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results())
 136                     .consume_body(body);
 137             });
 138             ctx
 139         };
 140
 141         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 142         let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
 143
 144         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
 145             // All spans generated from a proc-macro invocation are the same...
 146             if span == input.span {
 147                 return;
 148             }
 149
 150             // Ignore `self`s.
 151             if idx == 0 {
 152                 if let PatKind::Binding(.., ident, _) = arg.pat.kind {
 153                     if ident.name == kw::SelfLower {
 154                         continue;
 155                     }
 156                 }
 157             }
 158
 159             //
 160             // * Exclude a type that is specifically bounded by `Borrow`.
 161             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 162             //   `serde::Serialize`)
 163             let (implements_borrow_trait, all_borrowable_trait) = {
 164                 let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::<Vec<_>>();
 165
 166                 (
 167                     preds.iter().any(|t| t.def_id() == borrow_trait),
 168                     !preds.is_empty() && {
 169                         let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_root_empty, ty);
 170                         preds.iter().all(|t| {
 171                             let ty_params = t.trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
 172                             implements_trait(cx, ty_empty_region, t.def_id(), &ty_params)
 173                         })
 174                     },
 175                 )
 176             };
 177
 178             if_chain! {
 179                 if !is_self(arg);
 180                 if !ty.is_mutable_ptr();
 181                 if !is_copy(cx, ty);
 182                 if !allowed_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 183                 if !implements_borrow_trait;
 184                 if !all_borrowable_trait;
 185
 186                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.kind;
 187                 if !moved_vars.contains(&canonical_id);
 188                 then {
 189                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 190                         continue;
 191                     }
 192
 193                     // Dereference suggestion
 194                     let sugg = |diag: &mut DiagnosticBuilder<'_>| {
 195                         if let ty::Adt(def, ..) = ty.kind() {
 196                             if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
 197                                 if can_type_implement_copy(cx.tcx, cx.param_env, ty).is_ok() {
 198                                     diag.span_help(span, "consider marking this type as `Copy`");
 199                                 }
 200                             }
 201                         }
 202
 203                         let deref_span = spans_need_deref.get(&canonical_id);
 204                         if_chain! {
 205                             if is_type_diagnostic_item(cx, ty, sym::vec_type);
 206                             if let Some(clone_spans) =
 207                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 208                             if let TyKind::Path(QPath::Resolved(_, ref path)) = input.kind;
 209                             if let Some(elem_ty) = path.segments.iter()
 210                                 .find(|seg| seg.ident.name == sym::Vec)
 211                                 .and_then(|ps| ps.args.as_ref())
 212                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 213                                     GenericArg::Type(ty) => Some(ty),
 214                                     _ => None,
 215                                 }).unwrap());
 216                             then {
 217                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 218                                 diag.span_suggestion(
 219                                     input.span,
 220                                     "consider changing the type to",
 221                                     slice_ty,
 222                                     Applicability::Unspecified,
 223                                 );
 224
 225                                 for (span, suggestion) in clone_spans {
 226                                     diag.span_suggestion(
 227                                         span,
 228                                         &snippet_opt(cx, span)
 229                                             .map_or(
 230                                                 "change the call to".into(),
 231                                                 |x| Cow::from(format!("change `{}` to", x)),
 232                                             ),
 233                                         suggestion.into(),
 234                                         Applicability::Unspecified,
 235                                     );
 236                                 }
 237
 238                                 // cannot be destructured, no need for `*` suggestion
 239                                 assert!(deref_span.is_none());
 240                                 return;
 241                             }
 242                         }
 243
 244                         if is_type_diagnostic_item(cx, ty, sym::string_type) {
 245                             if let Some(clone_spans) =
 246                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 247                                 diag.span_suggestion(
 248                                     input.span,
 249                                     "consider changing the type to",
 250                                     "&str".to_string(),
 251                                     Applicability::Unspecified,
 252                                 );
 253
 254                                 for (span, suggestion) in clone_spans {
 255                                     diag.span_suggestion(
 256                                         span,
 257                                         &snippet_opt(cx, span)
 258                                             .map_or(
 259                                                 "change the call to".into(),
 260                                                 |x| Cow::from(format!("change `{}` to", x))
 261                                             ),
 262                                         suggestion.into(),
 263                                         Applicability::Unspecified,
 264                                     );
 265                                 }
 266
 267                                 assert!(deref_span.is_none());
 268                                 return;
 269                             }
 270                         }
 271
 272                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 273
 274                         // Suggests adding `*` to dereference the added reference.
 275                         if let Some(deref_span) = deref_span {
 276                             spans.extend(
 277                                 deref_span
 278                                     .iter()
 279                                     .cloned()
 280                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 281                             );
 282                             spans.sort_by_key(|&(span, _)| span);
 283                         }
 284                         multispan_sugg(diag, "consider taking a reference instead", spans);
 285                     };
 286
 287                     span_lint_and_then(
 288                         cx,
 289                         NEEDLESS_PASS_BY_VALUE,
 290                         input.span,
 291                         "this argument is passed by value, but not consumed in the function body",
 292                         sugg,
 293                     );
 294                 }
 295             }
 296         }
 297     }
 298 }
 299
 300 /// Functions marked with these attributes must have the exact signature.
 301 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
 302     attrs.iter().any(|attr| {
 303         [sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive]
 304             .iter()
 305             .any(|&allow| attr.has_name(allow))
 306     })
 307 }
 308
 309 #[derive(Default)]
 310 struct MovedVariablesCtxt {
 311     moved_vars: FxHashSet<HirId>,
 312     /// Spans which need to be prefixed with `*` for dereferencing the
 313     /// suggested additional reference.
 314     spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
 315 }
 316
 317 impl MovedVariablesCtxt {
 318     fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
 319         if let euv::PlaceBase::Local(vid) = cmt.place.base {
 320             self.moved_vars.insert(vid);
 321         }
 322     }
 323 }
 324
 325 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
 326     fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId, mode: euv::ConsumeMode) {
 327         if let euv::ConsumeMode::Move = mode {
 328             self.move_common(cmt);
 329         }
 330     }
 331
 332     fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {}
 333
 334     fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {}
 335 }