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, 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::Span;
  17 use rustc_target::spec::abi::Abi;
  18 use rustc_trait_selection::traits;
  19 use rustc_trait_selection::traits::misc::can_type_implement_copy;
  20 use rustc_typeck::expr_use_visitor as euv;
  21 use std::borrow::Cow;
  22
  23 declare_clippy_lint! {
  24     /// **What it does:** Checks for functions taking arguments by value, but not
  25     /// consuming them in its
  26     /// body.
  27     ///
  28     /// **Why is this bad?** Taking arguments by reference is more flexible and can
  29     /// sometimes avoid
  30     /// unnecessary allocations.
  31     ///
  32     /// **Known problems:**
  33     /// * This lint suggests taking an argument by reference,
  34     /// however sometimes it is better to let users decide the argument type
  35     /// (by using `Borrow` trait, for example), depending on how the function is used.
  36     ///
  37     /// **Example:**
  38     /// ```rust
  39     /// fn foo(v: Vec<i32>) {
  40     ///     assert_eq!(v.len(), 42);
  41     /// }
  42     /// ```
  43     ///
  44     /// ```rust
  45     /// // should be
  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<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  68     #[allow(clippy::too_many_lines)]
  69     fn check_fn(
  70         &mut self,
  71         cx: &LateContext<'a, '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             _ => 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!(item.kind, ItemKind::Impl{ of_trait: Some(_), .. } |
  95                 ItemKind::Trait(..))
  96             {
  97                 return;
  98             }
  99         }
 100
 101         // Allow `Borrow` or functions to be taken by value
 102         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 103         let whitelisted_traits = [
 104             need!(cx.tcx.lang_items().fn_trait()),
 105             need!(cx.tcx.lang_items().fn_once_trait()),
 106             need!(cx.tcx.lang_items().fn_mut_trait()),
 107             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
 108         ];
 109
 110         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 111
 112         let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
 113
 114         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.iter().copied())
 115             .filter(|p| !p.is_global())
 116             .filter_map(|obligation| {
 117                 if let ty::Predicate::Trait(poly_trait_ref, _) = obligation.predicate {
 118                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars()
 119                     {
 120                         return None;
 121                     }
 122                     Some(poly_trait_ref)
 123                 } else {
 124                     None
 125                 }
 126             })
 127             .collect::<Vec<_>>();
 128
 129         // Collect moved variables and spans which will need dereferencings from the
 130         // function body.
 131         let MovedVariablesCtxt {
 132             moved_vars,
 133             spans_need_deref,
 134             ..
 135         } = {
 136             let mut ctx = MovedVariablesCtxt::default();
 137             cx.tcx.infer_ctxt().enter(|infcx| {
 138                 euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.tables).consume_body(body);
 139             });
 140             ctx
 141         };
 142
 143         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 144         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 145
 146         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
 147             // All spans generated from a proc-macro invocation are the same...
 148             if span == input.span {
 149                 return;
 150             }
 151
 152             // Ignore `self`s.
 153             if idx == 0 {
 154                 if let PatKind::Binding(.., ident, _) = arg.pat.kind {
 155                     if ident.as_str() == "self" {
 156                         continue;
 157                     }
 158                 }
 159             }
 160
 161             //
 162             // * Exclude a type that is specifically bounded by `Borrow`.
 163             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 164             //   `serde::Serialize`)
 165             let (implements_borrow_trait, all_borrowable_trait) = {
 166                 let preds = preds
 167                     .iter()
 168                     .filter(|t| t.skip_binder().self_ty() == ty)
 169                     .collect::<Vec<_>>();
 170
 171                 (
 172                     preds.iter().any(|t| t.def_id() == borrow_trait),
 173                     !preds.is_empty() && {
 174                         let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_root_empty, ty);
 175                         preds.iter().all(|t| {
 176                             let ty_params = &t
 177                                 .skip_binder()
 178                                 .trait_ref
 179                                 .substs
 180                                 .iter()
 181                                 .skip(1)
 182                                 .cloned()
 183                                 .collect::<Vec<_>>();
 184                             implements_trait(cx, ty_empty_region, t.def_id(), ty_params)
 185                         })
 186                     },
 187                 )
 188             };
 189
 190             if_chain! {
 191                 if !is_self(arg);
 192                 if !ty.is_mutable_ptr();
 193                 if !is_copy(cx, ty);
 194                 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 195                 if !implements_borrow_trait;
 196                 if !all_borrowable_trait;
 197
 198                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.kind;
 199                 if !moved_vars.contains(&canonical_id);
 200                 then {
 201                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 202                         continue;
 203                     }
 204
 205                     // Dereference suggestion
 206                     let sugg = |diag: &mut DiagnosticBuilder<'_>| {
 207                         if let ty::Adt(def, ..) = ty.kind {
 208                             if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
 209                                 if can_type_implement_copy(cx.tcx, cx.param_env, ty).is_ok() {
 210                                     diag.span_help(span, "consider marking this type as `Copy`");
 211                                 }
 212                             }
 213                         }
 214
 215                         let deref_span = spans_need_deref.get(&canonical_id);
 216                         if_chain! {
 217                             if is_type_diagnostic_item(cx, ty, sym!(vec_type));
 218                             if let Some(clone_spans) =
 219                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 220                             if let TyKind::Path(QPath::Resolved(_, ref path)) = input.kind;
 221                             if let Some(elem_ty) = path.segments.iter()
 222                                 .find(|seg| seg.ident.name == sym!(Vec))
 223                                 .and_then(|ps| ps.args.as_ref())
 224                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 225                                     GenericArg::Type(ty) => Some(ty),
 226                                     _ => None,
 227                                 }).unwrap());
 228                             then {
 229                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 230                                 diag.span_suggestion(
 231                                     input.span,
 232                                     "consider changing the type to",
 233                                     slice_ty,
 234                                     Applicability::Unspecified,
 235                                 );
 236
 237                                 for (span, suggestion) in clone_spans {
 238                                     diag.span_suggestion(
 239                                         span,
 240                                         &snippet_opt(cx, span)
 241                                             .map_or(
 242                                                 "change the call to".into(),
 243                                                 |x| Cow::from(format!("change `{}` to", x)),
 244                                             ),
 245                                         suggestion.into(),
 246                                         Applicability::Unspecified,
 247                                     );
 248                                 }
 249
 250                                 // cannot be destructured, no need for `*` suggestion
 251                                 assert!(deref_span.is_none());
 252                                 return;
 253                             }
 254                         }
 255
 256                         if is_type_diagnostic_item(cx, ty, sym!(string_type)) {
 257                             if let Some(clone_spans) =
 258                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 259                                 diag.span_suggestion(
 260                                     input.span,
 261                                     "consider changing the type to",
 262                                     "&str".to_string(),
 263                                     Applicability::Unspecified,
 264                                 );
 265
 266                                 for (span, suggestion) in clone_spans {
 267                                     diag.span_suggestion(
 268                                         span,
 269                                         &snippet_opt(cx, span)
 270                                             .map_or(
 271                                                 "change the call to".into(),
 272                                                 |x| Cow::from(format!("change `{}` to", x))
 273                                             ),
 274                                         suggestion.into(),
 275                                         Applicability::Unspecified,
 276                                     );
 277                                 }
 278
 279                                 assert!(deref_span.is_none());
 280                                 return;
 281                             }
 282                         }
 283
 284                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 285
 286                         // Suggests adding `*` to dereference the added reference.
 287                         if let Some(deref_span) = deref_span {
 288                             spans.extend(
 289                                 deref_span
 290                                     .iter()
 291                                     .cloned()
 292                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 293                             );
 294                             spans.sort_by_key(|&(span, _)| span);
 295                         }
 296                         multispan_sugg(diag, "consider taking a reference instead", spans);
 297                     };
 298
 299                     span_lint_and_then(
 300                         cx,
 301                         NEEDLESS_PASS_BY_VALUE,
 302                         input.span,
 303                         "this argument is passed by value, but not consumed in the function body",
 304                         sugg,
 305                     );
 306                 }
 307             }
 308         }
 309     }
 310 }
 311
 312 /// Functions marked with these attributes must have the exact signature.
 313 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
 314     attrs.iter().any(|attr| {
 315         [sym!(proc_macro), sym!(proc_macro_attribute), sym!(proc_macro_derive)]
 316             .iter()
 317             .any(|&allow| attr.check_name(allow))
 318     })
 319 }
 320
 321 #[derive(Default)]
 322 struct MovedVariablesCtxt {
 323     moved_vars: FxHashSet<HirId>,
 324     /// Spans which need to be prefixed with `*` for dereferencing the
 325     /// suggested additional reference.
 326     spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
 327 }
 328
 329 impl MovedVariablesCtxt {
 330     fn move_common(&mut self, cmt: &euv::Place<'_>) {
 331         if let euv::PlaceBase::Local(vid) = cmt.base {
 332             self.moved_vars.insert(vid);
 333         }
 334     }
 335 }
 336
 337 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
 338     fn consume(&mut self, cmt: &euv::Place<'tcx>, mode: euv::ConsumeMode) {
 339         if let euv::ConsumeMode::Move = mode {
 340             self.move_common(cmt);
 341         }
 342     }
 343
 344     fn borrow(&mut self, _: &euv::Place<'tcx>, _: ty::BorrowKind) {}
 345
 346     fn mutate(&mut self, _: &euv::Place<'tcx>) {}
 347 }