clippy_lints/src/needless_pass_by_value.rs

   1 use clippy_utils::diagnostics::{multispan_sugg, span_lint_and_then};
   2 use clippy_utils::ptr::get_spans;
   3 use clippy_utils::source::{snippet, snippet_opt};
   4 use clippy_utils::ty::{implements_trait, is_copy, is_type_diagnostic_item};
   5 use clippy_utils::{get_trait_def_id, is_self, paths};
   6 use if_chain::if_chain;
   7 use rustc_ast::ast::Attribute;
   8 use rustc_data_structures::fx::FxHashSet;
   9 use rustc_errors::{Applicability, Diagnostic};
  10 use rustc_hir::intravisit::FnKind;
  11 use rustc_hir::{BindingAnnotation, Body, FnDecl, GenericArg, HirId, Impl, ItemKind, Node, PatKind, QPath, TyKind};
  12 use rustc_hir::{HirIdMap, HirIdSet};
  13 use rustc_infer::infer::TyCtxtInferExt;
  14 use rustc_lint::{LateContext, LateLintPass};
  15 use rustc_middle::mir::FakeReadCause;
  16 use rustc_middle::ty::{self, TypeFoldable};
  17 use rustc_session::{declare_lint_pass, declare_tool_lint};
  18 use rustc_span::symbol::kw;
  19 use rustc_span::{sym, Span};
  20 use rustc_target::spec::abi::Abi;
  21 use rustc_trait_selection::traits;
  22 use rustc_trait_selection::traits::misc::can_type_implement_copy;
  23 use rustc_typeck::expr_use_visitor as euv;
  24 use std::borrow::Cow;
  25
  26 declare_clippy_lint! {
  27     /// ### What it does
  28     /// Checks for functions taking arguments by value, but not
  29     /// consuming them in its
  30     /// body.
  31     ///
  32     /// ### Why is this bad?
  33     /// Taking arguments by reference is more flexible and can
  34     /// sometimes avoid
  35     /// unnecessary allocations.
  36     ///
  37     /// ### Known problems
  38     /// * This lint suggests taking an argument by reference,
  39     /// however sometimes it is better to let users decide the argument type
  40     /// (by using `Borrow` trait, for example), depending on how the function is used.
  41     ///
  42     /// ### Example
  43     /// ```rust
  44     /// fn foo(v: Vec<i32>) {
  45     ///     assert_eq!(v.len(), 42);
  46     /// }
  47     /// ```
  48     /// should be
  49     /// ```rust
  50     /// fn foo(v: &[i32]) {
  51     ///     assert_eq!(v.len(), 42);
  52     /// }
  53     /// ```
  54     #[clippy::version = "pre 1.29.0"]
  55     pub NEEDLESS_PASS_BY_VALUE,
  56     pedantic,
  57     "functions taking arguments by value, but not consuming them in its body"
  58 }
  59
  60 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
  61
  62 macro_rules! need {
  63     ($e: expr) => {
  64         if let Some(x) = $e {
  65             x
  66         } else {
  67             return;
  68         }
  69     };
  70 }
  71
  72 impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
  73     #[allow(clippy::too_many_lines)]
  74     fn check_fn(
  75         &mut self,
  76         cx: &LateContext<'tcx>,
  77         kind: FnKind<'tcx>,
  78         decl: &'tcx FnDecl<'_>,
  79         body: &'tcx Body<'_>,
  80         span: Span,
  81         hir_id: HirId,
  82     ) {
  83         if span.from_expansion() {
  84             return;
  85         }
  86
  87         match kind {
  88             FnKind::ItemFn(.., header, _) => {
  89                 let attrs = cx.tcx.hir().attrs(hir_id);
  90                 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
  91                     return;
  92                 }
  93             },
  94             FnKind::Method(..) => (),
  95             FnKind::Closure => return,
  96         }
  97
  98         // Exclude non-inherent impls
  99         if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
 100             if matches!(
 101                 item.kind,
 102                 ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
 103             ) {
 104                 return;
 105             }
 106         }
 107
 108         // Allow `Borrow` or functions to be taken by value
 109         let allowed_traits = [
 110             need!(cx.tcx.lang_items().fn_trait()),
 111             need!(cx.tcx.lang_items().fn_once_trait()),
 112             need!(cx.tcx.lang_items().fn_mut_trait()),
 113             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
 114         ];
 115
 116         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 117
 118         let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
 119
 120         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
 121             .filter(|p| !p.is_global())
 122             .filter_map(|obligation| {
 123                 // Note that we do not want to deal with qualified predicates here.
 124                 match obligation.predicate.kind().no_bound_vars() {
 125                     Some(ty::PredicateKind::Trait(pred)) if pred.def_id() != sized_trait => Some(pred),
 126                     _ => None,
 127                 }
 128             })
 129             .collect::<Vec<_>>();
 130
 131         // Collect moved variables and spans which will need dereferencings from the
 132         // function body.
 133         let MovedVariablesCtxt {
 134             moved_vars,
 135             spans_need_deref,
 136             ..
 137         } = {
 138             let mut ctx = MovedVariablesCtxt::default();
 139             cx.tcx.infer_ctxt().enter(|infcx| {
 140                 euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results())
 141                     .consume_body(body);
 142             });
 143             ctx
 144         };
 145
 146         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 147         let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
 148
 149         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
 150             // All spans generated from a proc-macro invocation are the same...
 151             if span == input.span {
 152                 return;
 153             }
 154
 155             // Ignore `self`s.
 156             if idx == 0 {
 157                 if let PatKind::Binding(.., ident, _) = arg.pat.kind {
 158                     if ident.name == kw::SelfLower {
 159                         continue;
 160                     }
 161                 }
 162             }
 163
 164             //
 165             // * Exclude a type that is specifically bounded by `Borrow`.
 166             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 167             //   `serde::Serialize`)
 168             let (implements_borrow_trait, all_borrowable_trait) = {
 169                 let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::<Vec<_>>();
 170
 171                 (
 172                     preds.iter().any(|t| cx.tcx.is_diagnostic_item(sym::Borrow, t.def_id())),
 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.trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
 177                             implements_trait(cx, ty_empty_region, t.def_id(), &ty_params)
 178                         })
 179                     },
 180                 )
 181             };
 182
 183             if_chain! {
 184                 if !is_self(arg);
 185                 if !ty.is_mutable_ptr();
 186                 if !is_copy(cx, ty);
 187                 if !allowed_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 188                 if !implements_borrow_trait;
 189                 if !all_borrowable_trait;
 190
 191                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.kind;
 192                 if !moved_vars.contains(&canonical_id);
 193                 then {
 194                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 195                         continue;
 196                     }
 197
 198                     // Dereference suggestion
 199                     let sugg = |diag: &mut Diagnostic| {
 200                         if let ty::Adt(def, ..) = ty.kind() {
 201                             if let Some(span) = cx.tcx.hir().span_if_local(def.did()) {
 202                                 if can_type_implement_copy(cx.tcx, cx.param_env, ty, traits::ObligationCause::dummy_with_span(span)).is_ok() {
 203                                     diag.span_help(span, "consider marking this type as `Copy`");
 204                                 }
 205                             }
 206                         }
 207
 208                         let deref_span = spans_need_deref.get(&canonical_id);
 209                         if_chain! {
 210                             if is_type_diagnostic_item(cx, ty, sym::Vec);
 211                             if let Some(clone_spans) =
 212                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 213                             if let TyKind::Path(QPath::Resolved(_, path)) = input.kind;
 214                             if let Some(elem_ty) = path.segments.iter()
 215                                 .find(|seg| seg.ident.name == sym::Vec)
 216                                 .and_then(|ps| ps.args.as_ref())
 217                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 218                                     GenericArg::Type(ty) => Some(ty),
 219                                     _ => None,
 220                                 }).unwrap());
 221                             then {
 222                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 223                                 diag.span_suggestion(
 224                                     input.span,
 225                                     "consider changing the type to",
 226                                     slice_ty,
 227                                     Applicability::Unspecified,
 228                                 );
 229
 230                                 for (span, suggestion) in clone_spans {
 231                                     diag.span_suggestion(
 232                                         span,
 233                                         &snippet_opt(cx, span)
 234                                             .map_or(
 235                                                 "change the call to".into(),
 236                                                 |x| Cow::from(format!("change `{}` to", x)),
 237                                             ),
 238                                         suggestion.into(),
 239                                         Applicability::Unspecified,
 240                                     );
 241                                 }
 242
 243                                 // cannot be destructured, no need for `*` suggestion
 244                                 assert!(deref_span.is_none());
 245                                 return;
 246                             }
 247                         }
 248
 249                         if is_type_diagnostic_item(cx, ty, sym::String) {
 250                             if let Some(clone_spans) =
 251                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 252                                 diag.span_suggestion(
 253                                     input.span,
 254                                     "consider changing the type to",
 255                                     "&str".to_string(),
 256                                     Applicability::Unspecified,
 257                                 );
 258
 259                                 for (span, suggestion) in clone_spans {
 260                                     diag.span_suggestion(
 261                                         span,
 262                                         &snippet_opt(cx, span)
 263                                             .map_or(
 264                                                 "change the call to".into(),
 265                                                 |x| Cow::from(format!("change `{}` to", x))
 266                                             ),
 267                                         suggestion.into(),
 268                                         Applicability::Unspecified,
 269                                     );
 270                                 }
 271
 272                                 assert!(deref_span.is_none());
 273                                 return;
 274                             }
 275                         }
 276
 277                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 278
 279                         // Suggests adding `*` to dereference the added reference.
 280                         if let Some(deref_span) = deref_span {
 281                             spans.extend(
 282                                 deref_span
 283                                     .iter()
 284                                     .copied()
 285                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 286                             );
 287                             spans.sort_by_key(|&(span, _)| span);
 288                         }
 289                         multispan_sugg(diag, "consider taking a reference instead", spans);
 290                     };
 291
 292                     span_lint_and_then(
 293                         cx,
 294                         NEEDLESS_PASS_BY_VALUE,
 295                         input.span,
 296                         "this argument is passed by value, but not consumed in the function body",
 297                         sugg,
 298                     );
 299                 }
 300             }
 301         }
 302     }
 303 }
 304
 305 /// Functions marked with these attributes must have the exact signature.
 306 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
 307     attrs.iter().any(|attr| {
 308         [sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive]
 309             .iter()
 310             .any(|&allow| attr.has_name(allow))
 311     })
 312 }
 313
 314 #[derive(Default)]
 315 struct MovedVariablesCtxt {
 316     moved_vars: HirIdSet,
 317     /// Spans which need to be prefixed with `*` for dereferencing the
 318     /// suggested additional reference.
 319     spans_need_deref: HirIdMap<FxHashSet<Span>>,
 320 }
 321
 322 impl MovedVariablesCtxt {
 323     fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
 324         if let euv::PlaceBase::Local(vid) = cmt.place.base {
 325             self.moved_vars.insert(vid);
 326         }
 327     }
 328 }
 329
 330 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
 331     fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId) {
 332         self.move_common(cmt);
 333     }
 334
 335     fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {}
 336
 337     fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {}
 338
 339     fn fake_read(&mut self, _: rustc_typeck::expr_use_visitor::Place<'tcx>, _: FakeReadCause, _: HirId) {}
 340 }