clippy_lints/src/needless_pass_by_value.rs

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