clippy_lints/src/needless_pass_by_value.rs

   1 use rustc::hir::*;
   2 use rustc::hir::intravisit::FnKind;
   3 use rustc::lint::*;
   4 use rustc::ty::{self, RegionKind, TypeFoldable};
   5 use rustc::traits;
   6 use rustc::middle::expr_use_visitor as euv;
   7 use rustc::middle::mem_categorization as mc;
   8 use syntax::ast::NodeId;
   9 use syntax_pos::Span;
  10 use syntax::errors::DiagnosticBuilder;
  11 use utils::{get_trait_def_id, implements_trait, in_macro, is_copy, is_self, match_type, multispan_sugg, paths,
  12             snippet, snippet_opt, span_lint_and_then};
  13 use utils::ptr::get_spans;
  14 use std::collections::{HashMap, HashSet};
  15 use std::borrow::Cow;
  16
  17 /// **What it does:** Checks for functions taking arguments by value, but not
  18 /// consuming them in its
  19 /// body.
  20 ///
  21 /// **Why is this bad?** Taking arguments by reference is more flexible and can
  22 /// sometimes avoid
  23 /// unnecessary allocations.
  24 ///
  25 /// **Known problems:** Hopefully none.
  26 ///
  27 /// **Example:**
  28 /// ```rust
  29 /// fn foo(v: Vec<i32>) {
  30 ///     assert_eq!(v.len(), 42);
  31 /// }
  32 /// ```
  33 declare_lint! {
  34     pub NEEDLESS_PASS_BY_VALUE,
  35     Warn,
  36     "functions taking arguments by value, but not consuming them in its body"
  37 }
  38
  39 pub struct NeedlessPassByValue;
  40
  41 impl LintPass for NeedlessPassByValue {
  42     fn get_lints(&self) -> LintArray {
  43         lint_array![NEEDLESS_PASS_BY_VALUE]
  44     }
  45 }
  46
  47 macro_rules! need {
  48     ($e: expr) => { if let Some(x) = $e { x } else { return; } };
  49 }
  50
  51 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  52     fn check_fn(
  53         &mut self,
  54         cx: &LateContext<'a, 'tcx>,
  55         kind: FnKind<'tcx>,
  56         decl: &'tcx FnDecl,
  57         body: &'tcx Body,
  58         span: Span,
  59         node_id: NodeId,
  60     ) {
  61         if in_macro(span) {
  62             return;
  63         }
  64
  65         match kind {
  66             FnKind::ItemFn(.., attrs) => for a in attrs {
  67                 if_let_chain!{[
  68                     a.meta_item_list().is_some(),
  69                     let Some(name) = a.name(),
  70                     name == "proc_macro_derive",
  71                 ], {
  72                     return;
  73                 }}
  74             },
  75             _ => return,
  76         }
  77
  78         // Allow `Borrow` or functions to be taken by value
  79         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
  80         let fn_traits = [
  81             need!(cx.tcx.lang_items().fn_trait()),
  82             need!(cx.tcx.lang_items().fn_once_trait()),
  83             need!(cx.tcx.lang_items().fn_mut_trait()),
  84         ];
  85
  86         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
  87
  88         let fn_def_id = cx.tcx.hir.local_def_id(node_id);
  89
  90         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
  91             .filter(|p| !p.is_global())
  92             .filter_map(|pred| if let ty::Predicate::Trait(poly_trait_ref) = pred {
  93                 if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_regions() {
  94                     return None;
  95                 }
  96                 Some(poly_trait_ref)
  97             } else {
  98                 None
  99             })
 100             .collect::<Vec<_>>();
 101
 102         // Collect moved variables and spans which will need dereferencings from the
 103         // function body.
 104         let MovedVariablesCtxt {
 105             moved_vars,
 106             spans_need_deref,
 107             ..
 108         } = {
 109             let mut ctx = MovedVariablesCtxt::new(cx);
 110             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 111             euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables).consume_body(body);
 112             ctx
 113         };
 114
 115         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 116         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 117
 118         for (idx, ((input, &ty), arg)) in decl.inputs
 119             .iter()
 120             .zip(fn_sig.inputs())
 121             .zip(&body.arguments)
 122             .enumerate()
 123         {
 124             // * Exclude a type that is specifically bounded by `Borrow`.
 125             // * Exclude a type whose reference also fulfills its bound.
 126             //   (e.g. `std::convert::AsRef`, `serde::Serialize`)
 127             let (implements_borrow_trait, all_borrowable_trait) = {
 128                 let preds = preds
 129                     .iter()
 130                     .filter(|t| t.skip_binder().self_ty() == ty)
 131                     .collect::<Vec<_>>();
 132
 133                 (
 134                     preds.iter().any(|t| t.def_id() == borrow_trait),
 135                     !preds.is_empty() && preds.iter().all(|t| {
 136                         implements_trait(
 137                             cx,
 138                             cx.tcx.mk_imm_ref(&RegionKind::ReErased, ty),
 139                             t.def_id(),
 140                             &t.skip_binder().input_types().skip(1).collect::<Vec<_>>(),
 141                         )
 142                     }),
 143                 )
 144             };
 145
 146             if_let_chain! {[
 147                 !is_self(arg),
 148                 !ty.is_mutable_pointer(),
 149                 !is_copy(cx, ty),
 150                 !fn_traits.iter().any(|&t| implements_trait(cx, ty, t, &[])),
 151                 !implements_borrow_trait,
 152                 !all_borrowable_trait,
 153
 154                 let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node,
 155                 !moved_vars.contains(&canonical_id),
 156             ], {
 157                 if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 158                     continue;
 159                 }
 160
 161                 // Dereference suggestion
 162                 let sugg = |db: &mut DiagnosticBuilder| {
 163                     let deref_span = spans_need_deref.get(&canonical_id);
 164                     if_let_chain! {[
 165                         match_type(cx, ty, &paths::VEC),
 166                         let Some(clone_spans) =
 167                             get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]),
 168                         let TyPath(QPath::Resolved(_, ref path)) = input.node,
 169                         let Some(elem_ty) = path.segments.iter()
 170                             .find(|seg| seg.name == "Vec")
 171                             .and_then(|ps| ps.parameters.as_ref())
 172                             .map(|params| &params.types[0]),
 173                     ], {
 174                         let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 175                         db.span_suggestion(input.span,
 176                                         "consider changing the type to",
 177                                         slice_ty);
 178
 179                         for (span, suggestion) in clone_spans {
 180                             db.span_suggestion(
 181                                 span,
 182                                 &snippet_opt(cx, span)
 183                                     .map_or(
 184                                         "change the call to".into(),
 185                                         |x| Cow::from(format!("change `{}` to", x)),
 186                                     ),
 187                                 suggestion.into()
 188                             );
 189                         }
 190
 191                         // cannot be destructured, no need for `*` suggestion
 192                         assert!(deref_span.is_none());
 193                         return;
 194                     }}
 195
 196                     if match_type(cx, ty, &paths::STRING) {
 197                         if let Some(clone_spans) =
 198                             get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 199                             db.span_suggestion(input.span, "consider changing the type to", "&str".to_string());
 200
 201                             for (span, suggestion) in clone_spans {
 202                                 db.span_suggestion(
 203                                     span,
 204                                     &snippet_opt(cx, span)
 205                                         .map_or(
 206                                             "change the call to".into(),
 207                                             |x| Cow::from(format!("change `{}` to", x))
 208                                         ),
 209                                     suggestion.into(),
 210                                 );
 211                             }
 212
 213                             assert!(deref_span.is_none());
 214                             return;
 215                         }
 216                     }
 217
 218                     let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 219
 220                     // Suggests adding `*` to dereference the added reference.
 221                     if let Some(deref_span) = deref_span {
 222                         spans.extend(
 223                             deref_span
 224                                 .iter()
 225                                 .cloned()
 226                                 .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 227                         );
 228                         spans.sort_by_key(|&(span, _)| span);
 229                     }
 230                     multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 231                 };
 232
 233                 span_lint_and_then(
 234                     cx,
 235                     NEEDLESS_PASS_BY_VALUE,
 236                     input.span,
 237                     "this argument is passed by value, but not consumed in the function body",
 238                     sugg,
 239                 );
 240             }}
 241         }
 242     }
 243 }
 244
 245 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 246     cx: &'a LateContext<'a, 'tcx>,
 247     moved_vars: HashSet<NodeId>,
 248     /// Spans which need to be prefixed with `*` for dereferencing the
 249     /// suggested additional reference.
 250     spans_need_deref: HashMap<NodeId, HashSet<Span>>,
 251 }
 252
 253 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 254     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 255         Self {
 256             cx: cx,
 257             moved_vars: HashSet::new(),
 258             spans_need_deref: HashMap::new(),
 259         }
 260     }
 261
 262     fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: mc::cmt<'tcx>) {
 263         let cmt = unwrap_downcast_or_interior(cmt);
 264
 265         if let mc::Categorization::Local(vid) = cmt.cat {
 266             self.moved_vars.insert(vid);
 267         }
 268     }
 269
 270     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>) {
 271         let cmt = unwrap_downcast_or_interior(cmt);
 272
 273         if let mc::Categorization::Local(vid) = cmt.cat {
 274             let mut id = matched_pat.id;
 275             loop {
 276                 let parent = self.cx.tcx.hir.get_parent_node(id);
 277                 if id == parent {
 278                     // no parent
 279                     return;
 280                 }
 281                 id = parent;
 282
 283                 if let Some(node) = self.cx.tcx.hir.find(id) {
 284                     match node {
 285                         map::Node::NodeExpr(e) => {
 286                             // `match` and `if let`
 287                             if let ExprMatch(ref c, ..) = e.node {
 288                                 self.spans_need_deref
 289                                     .entry(vid)
 290                                     .or_insert_with(HashSet::new)
 291                                     .insert(c.span);
 292                             }
 293                         },
 294
 295                         map::Node::NodeStmt(s) => {
 296                             // `let <pat> = x;`
 297                             if_let_chain! {[
 298                                 let StmtDecl(ref decl, _) = s.node,
 299                                 let DeclLocal(ref local) = decl.node,
 300                             ], {
 301                                 self.spans_need_deref
 302                                     .entry(vid)
 303                                     .or_insert_with(HashSet::new)
 304                                     .insert(local.init
 305                                         .as_ref()
 306                                         .map(|e| e.span)
 307                                         .expect("`let` stmt without init aren't caught by match_pat"));
 308                             }}
 309                         },
 310
 311                         _ => {},
 312                     }
 313                 }
 314             }
 315         }
 316     }
 317 }
 318
 319 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 320     fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 321         if let euv::ConsumeMode::Move(_) = mode {
 322             self.move_common(consume_id, consume_span, cmt);
 323         }
 324     }
 325
 326     fn matched_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::MatchMode) {
 327         if let euv::MatchMode::MovingMatch = mode {
 328             self.move_common(matched_pat.id, matched_pat.span, cmt);
 329         } else {
 330             self.non_moving_pat(matched_pat, cmt);
 331         }
 332     }
 333
 334     fn consume_pat(&mut self, consume_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 335         if let euv::ConsumeMode::Move(_) = mode {
 336             self.move_common(consume_pat.id, consume_pat.span, cmt);
 337         }
 338     }
 339
 340     fn borrow(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: ty::Region, _: ty::BorrowKind, _: euv::LoanCause) {}
 341
 342     fn mutate(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: euv::MutateMode) {}
 343
 344     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 345 }
 346
 347
 348 fn unwrap_downcast_or_interior(mut cmt: mc::cmt) -> mc::cmt {
 349     loop {
 350         match cmt.cat.clone() {
 351             mc::Categorization::Downcast(c, _) | mc::Categorization::Interior(c, _) => {
 352                 cmt = c;
 353             },
 354             _ => return cmt,
 355         }
 356     }
 357 }