]> git.lizzy.rs Git - rust.git/blob - clippy_lints/src/needless_pass_by_value.rs
Merge commit '0cce3f643bfcbb92d5a1bb71858c9cbaff749d6b' into clippyup
[rust.git] / 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, 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_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     pub NEEDLESS_PASS_BY_VALUE,
55     pedantic,
56     "functions taking arguments by value, but not consuming them in its body"
57 }
58
59 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
60
61 macro_rules! need {
62     ($e: expr) => {
63         if let Some(x) = $e {
64             x
65         } else {
66             return;
67         }
68     };
69 }
70
71 impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
72     #[allow(clippy::too_many_lines)]
73     fn check_fn(
74         &mut self,
75         cx: &LateContext<'tcx>,
76         kind: FnKind<'tcx>,
77         decl: &'tcx FnDecl<'_>,
78         body: &'tcx Body<'_>,
79         span: Span,
80         hir_id: HirId,
81     ) {
82         if span.from_expansion() {
83             return;
84         }
85
86         match kind {
87             FnKind::ItemFn(.., header, _) => {
88                 let attrs = cx.tcx.hir().attrs(hir_id);
89                 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
90                     return;
91                 }
92             },
93             FnKind::Method(..) => (),
94             FnKind::Closure => return,
95         }
96
97         // Exclude non-inherent impls
98         if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
99             if matches!(
100                 item.kind,
101                 ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
102             ) {
103                 return;
104             }
105         }
106
107         // Allow `Borrow` or functions to be taken by value
108         let allowed_traits = [
109             need!(cx.tcx.lang_items().fn_trait()),
110             need!(cx.tcx.lang_items().fn_once_trait()),
111             need!(cx.tcx.lang_items().fn_mut_trait()),
112             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
113         ];
114
115         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
116
117         let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
118
119         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
120             .filter(|p| !p.is_global())
121             .filter_map(|obligation| {
122                 // Note that we do not want to deal with qualified predicates here.
123                 match obligation.predicate.kind().no_bound_vars() {
124                     Some(ty::PredicateKind::Trait(pred, _)) if pred.def_id() != sized_trait => Some(pred),
125                     _ => None,
126                 }
127             })
128             .collect::<Vec<_>>();
129
130         // Collect moved variables and spans which will need dereferencings from the
131         // function body.
132         let MovedVariablesCtxt {
133             moved_vars,
134             spans_need_deref,
135             ..
136         } = {
137             let mut ctx = MovedVariablesCtxt::default();
138             cx.tcx.infer_ctxt().enter(|infcx| {
139                 euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results())
140                     .consume_body(body);
141             });
142             ctx
143         };
144
145         let fn_sig = cx.tcx.fn_sig(fn_def_id);
146         let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
147
148         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
149             // All spans generated from a proc-macro invocation are the same...
150             if span == input.span {
151                 return;
152             }
153
154             // Ignore `self`s.
155             if idx == 0 {
156                 if let PatKind::Binding(.., ident, _) = arg.pat.kind {
157                     if ident.name == kw::SelfLower {
158                         continue;
159                     }
160                 }
161             }
162
163             //
164             // * Exclude a type that is specifically bounded by `Borrow`.
165             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
166             //   `serde::Serialize`)
167             let (implements_borrow_trait, all_borrowable_trait) = {
168                 let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::<Vec<_>>();
169
170                 (
171                     preds.iter().any(|t| cx.tcx.is_diagnostic_item(sym::Borrow, t.def_id())),
172                     !preds.is_empty() && {
173                         let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_root_empty, ty);
174                         preds.iter().all(|t| {
175                             let ty_params = t.trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
176                             implements_trait(cx, ty_empty_region, t.def_id(), &ty_params)
177                         })
178                     },
179                 )
180             };
181
182             if_chain! {
183                 if !is_self(arg);
184                 if !ty.is_mutable_ptr();
185                 if !is_copy(cx, ty);
186                 if !allowed_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
187                 if !implements_borrow_trait;
188                 if !all_borrowable_trait;
189
190                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.kind;
191                 if !moved_vars.contains(&canonical_id);
192                 then {
193                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
194                         continue;
195                     }
196
197                     // Dereference suggestion
198                     let sugg = |diag: &mut DiagnosticBuilder<'_>| {
199                         if let ty::Adt(def, ..) = ty.kind() {
200                             if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
201                                 if can_type_implement_copy(cx.tcx, cx.param_env, ty).is_ok() {
202                                     diag.span_help(span, "consider marking this type as `Copy`");
203                                 }
204                             }
205                         }
206
207                         let deref_span = spans_need_deref.get(&canonical_id);
208                         if_chain! {
209                             if is_type_diagnostic_item(cx, ty, sym::vec_type);
210                             if let Some(clone_spans) =
211                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
212                             if let TyKind::Path(QPath::Resolved(_, path)) = input.kind;
213                             if let Some(elem_ty) = path.segments.iter()
214                                 .find(|seg| seg.ident.name == sym::Vec)
215                                 .and_then(|ps| ps.args.as_ref())
216                                 .map(|params| params.args.iter().find_map(|arg| match arg {
217                                     GenericArg::Type(ty) => Some(ty),
218                                     _ => None,
219                                 }).unwrap());
220                             then {
221                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
222                                 diag.span_suggestion(
223                                     input.span,
224                                     "consider changing the type to",
225                                     slice_ty,
226                                     Applicability::Unspecified,
227                                 );
228
229                                 for (span, suggestion) in clone_spans {
230                                     diag.span_suggestion(
231                                         span,
232                                         &snippet_opt(cx, span)
233                                             .map_or(
234                                                 "change the call to".into(),
235                                                 |x| Cow::from(format!("change `{}` to", x)),
236                                             ),
237                                         suggestion.into(),
238                                         Applicability::Unspecified,
239                                     );
240                                 }
241
242                                 // cannot be destructured, no need for `*` suggestion
243                                 assert!(deref_span.is_none());
244                                 return;
245                             }
246                         }
247
248                         if is_type_diagnostic_item(cx, ty, sym::string_type) {
249                             if let Some(clone_spans) =
250                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
251                                 diag.span_suggestion(
252                                     input.span,
253                                     "consider changing the type to",
254                                     "&str".to_string(),
255                                     Applicability::Unspecified,
256                                 );
257
258                                 for (span, suggestion) in clone_spans {
259                                     diag.span_suggestion(
260                                         span,
261                                         &snippet_opt(cx, span)
262                                             .map_or(
263                                                 "change the call to".into(),
264                                                 |x| Cow::from(format!("change `{}` to", x))
265                                             ),
266                                         suggestion.into(),
267                                         Applicability::Unspecified,
268                                     );
269                                 }
270
271                                 assert!(deref_span.is_none());
272                                 return;
273                             }
274                         }
275
276                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
277
278                         // Suggests adding `*` to dereference the added reference.
279                         if let Some(deref_span) = deref_span {
280                             spans.extend(
281                                 deref_span
282                                     .iter()
283                                     .copied()
284                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
285                             );
286                             spans.sort_by_key(|&(span, _)| span);
287                         }
288                         multispan_sugg(diag, "consider taking a reference instead", spans);
289                     };
290
291                     span_lint_and_then(
292                         cx,
293                         NEEDLESS_PASS_BY_VALUE,
294                         input.span,
295                         "this argument is passed by value, but not consumed in the function body",
296                         sugg,
297                     );
298                 }
299             }
300         }
301     }
302 }
303
304 /// Functions marked with these attributes must have the exact signature.
305 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
306     attrs.iter().any(|attr| {
307         [sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive]
308             .iter()
309             .any(|&allow| attr.has_name(allow))
310     })
311 }
312
313 #[derive(Default)]
314 struct MovedVariablesCtxt {
315     moved_vars: HirIdSet,
316     /// Spans which need to be prefixed with `*` for dereferencing the
317     /// suggested additional reference.
318     spans_need_deref: HirIdMap<FxHashSet<Span>>,
319 }
320
321 impl MovedVariablesCtxt {
322     fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
323         if let euv::PlaceBase::Local(vid) = cmt.place.base {
324             self.moved_vars.insert(vid);
325         }
326     }
327 }
328
329 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
330     fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId) {
331         self.move_common(cmt);
332     }
333
334     fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {}
335
336     fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {}
337
338     fn fake_read(&mut self, _: rustc_typeck::expr_use_visitor::Place<'tcx>, _: FakeReadCause, _: HirId) {}
339 }