clippy_lints/src/loops/needless_range_loop.rs

   1 use super::NEEDLESS_RANGE_LOOP;
   2 use clippy_utils::diagnostics::{multispan_sugg, span_lint_and_then};
   3 use clippy_utils::source::snippet;
   4 use clippy_utils::ty::has_iter_method;
   5 use clippy_utils::visitors::is_local_used;
   6 use clippy_utils::{contains_name, higher, is_integer_const, match_trait_method, paths, sugg, SpanlessEq};
   7 use if_chain::if_chain;
   8 use rustc_ast::ast;
   9 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  10 use rustc_hir::def::{DefKind, Res};
  11 use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
  12 use rustc_hir::{BinOpKind, BorrowKind, Expr, ExprKind, HirId, Mutability, Pat, PatKind, QPath};
  13 use rustc_lint::LateContext;
  14 use rustc_middle::hir::map::Map;
  15 use rustc_middle::middle::region;
  16 use rustc_middle::ty::{self, Ty};
  17 use rustc_span::symbol::{sym, Symbol};
  18 use std::iter::{self, Iterator};
  19 use std::mem;
  20
  21 /// Checks for looping over a range and then indexing a sequence with it.
  22 /// The iteratee must be a range literal.
  23 #[allow(clippy::too_many_lines)]
  24 pub(super) fn check<'tcx>(
  25     cx: &LateContext<'tcx>,
  26     pat: &'tcx Pat<'_>,
  27     arg: &'tcx Expr<'_>,
  28     body: &'tcx Expr<'_>,
  29     expr: &'tcx Expr<'_>,
  30 ) {
  31     if let Some(higher::Range {
  32         start: Some(start),
  33         ref end,
  34         limits,
  35     }) = higher::Range::hir(arg)
  36     {
  37         // the var must be a single name
  38         if let PatKind::Binding(_, canonical_id, ident, _) = pat.kind {
  39             let mut visitor = VarVisitor {
  40                 cx,
  41                 var: canonical_id,
  42                 indexed_mut: FxHashSet::default(),
  43                 indexed_indirectly: FxHashMap::default(),
  44                 indexed_directly: FxHashMap::default(),
  45                 referenced: FxHashSet::default(),
  46                 nonindex: false,
  47                 prefer_mutable: false,
  48             };
  49             walk_expr(&mut visitor, body);
  50
  51             // linting condition: we only indexed one variable, and indexed it directly
  52             if visitor.indexed_indirectly.is_empty() && visitor.indexed_directly.len() == 1 {
  53                 let (indexed, (indexed_extent, indexed_ty)) = visitor
  54                     .indexed_directly
  55                     .into_iter()
  56                     .next()
  57                     .expect("already checked that we have exactly 1 element");
  58
  59                 // ensure that the indexed variable was declared before the loop, see #601
  60                 if let Some(indexed_extent) = indexed_extent {
  61                     let parent_def_id = cx.tcx.hir().get_parent_item(expr.hir_id);
  62                     let region_scope_tree = cx.tcx.region_scope_tree(parent_def_id);
  63                     let pat_extent = region_scope_tree.var_scope(pat.hir_id.local_id);
  64                     if region_scope_tree.is_subscope_of(indexed_extent, pat_extent) {
  65                         return;
  66                     }
  67                 }
  68
  69                 // don't lint if the container that is indexed does not have .iter() method
  70                 let has_iter = has_iter_method(cx, indexed_ty);
  71                 if has_iter.is_none() {
  72                     return;
  73                 }
  74
  75                 // don't lint if the container that is indexed into is also used without
  76                 // indexing
  77                 if visitor.referenced.contains(&indexed) {
  78                     return;
  79                 }
  80
  81                 let starts_at_zero = is_integer_const(cx, start, 0);
  82
  83                 let skip = if starts_at_zero {
  84                     String::new()
  85                 } else if visitor.indexed_mut.contains(&indexed) && contains_name(indexed, start) {
  86                     return;
  87                 } else {
  88                     format!(".skip({})", snippet(cx, start.span, ".."))
  89                 };
  90
  91                 let mut end_is_start_plus_val = false;
  92
  93                 let take = if let Some(end) = *end {
  94                     let mut take_expr = end;
  95
  96                     if let ExprKind::Binary(ref op, left, right) = end.kind {
  97                         if op.node == BinOpKind::Add {
  98                             let start_equal_left = SpanlessEq::new(cx).eq_expr(start, left);
  99                             let start_equal_right = SpanlessEq::new(cx).eq_expr(start, right);
 100
 101                             if start_equal_left {
 102                                 take_expr = right;
 103                             } else if start_equal_right {
 104                                 take_expr = left;
 105                             }
 106
 107                             end_is_start_plus_val = start_equal_left | start_equal_right;
 108                         }
 109                     }
 110
 111                     if is_len_call(end, indexed) || is_end_eq_array_len(cx, end, limits, indexed_ty) {
 112                         String::new()
 113                     } else if visitor.indexed_mut.contains(&indexed) && contains_name(indexed, take_expr) {
 114                         return;
 115                     } else {
 116                         match limits {
 117                             ast::RangeLimits::Closed => {
 118                                 let take_expr = sugg::Sugg::hir(cx, take_expr, "<count>");
 119                                 format!(".take({})", take_expr + sugg::ONE)
 120                             },
 121                             ast::RangeLimits::HalfOpen => format!(".take({})", snippet(cx, take_expr.span, "..")),
 122                         }
 123                     }
 124                 } else {
 125                     String::new()
 126                 };
 127
 128                 let (ref_mut, method) = if visitor.indexed_mut.contains(&indexed) {
 129                     ("mut ", "iter_mut")
 130                 } else {
 131                     ("", "iter")
 132                 };
 133
 134                 let take_is_empty = take.is_empty();
 135                 let mut method_1 = take;
 136                 let mut method_2 = skip;
 137
 138                 if end_is_start_plus_val {
 139                     mem::swap(&mut method_1, &mut method_2);
 140                 }
 141
 142                 if visitor.nonindex {
 143                     span_lint_and_then(
 144                         cx,
 145                         NEEDLESS_RANGE_LOOP,
 146                         arg.span,
 147                         &format!("the loop variable `{}` is used to index `{}`", ident.name, indexed),
 148                         |diag| {
 149                             multispan_sugg(
 150                                 diag,
 151                                 "consider using an iterator",
 152                                 vec![
 153                                     (pat.span, format!("({}, <item>)", ident.name)),
 154                                     (
 155                                         arg.span,
 156                                         format!("{}.{}().enumerate(){}{}", indexed, method, method_1, method_2),
 157                                     ),
 158                                 ],
 159                             );
 160                         },
 161                     );
 162                 } else {
 163                     let repl = if starts_at_zero && take_is_empty {
 164                         format!("&{}{}", ref_mut, indexed)
 165                     } else {
 166                         format!("{}.{}(){}{}", indexed, method, method_1, method_2)
 167                     };
 168
 169                     span_lint_and_then(
 170                         cx,
 171                         NEEDLESS_RANGE_LOOP,
 172                         arg.span,
 173                         &format!("the loop variable `{}` is only used to index `{}`", ident.name, indexed),
 174                         |diag| {
 175                             multispan_sugg(
 176                                 diag,
 177                                 "consider using an iterator",
 178                                 vec![(pat.span, "<item>".to_string()), (arg.span, repl)],
 179                             );
 180                         },
 181                     );
 182                 }
 183             }
 184         }
 185     }
 186 }
 187
 188 fn is_len_call(expr: &Expr<'_>, var: Symbol) -> bool {
 189     if_chain! {
 190         if let ExprKind::MethodCall(method, _, len_args, _) = expr.kind;
 191         if len_args.len() == 1;
 192         if method.ident.name == sym::len;
 193         if let ExprKind::Path(QPath::Resolved(_, path)) = len_args[0].kind;
 194         if path.segments.len() == 1;
 195         if path.segments[0].ident.name == var;
 196         then {
 197             return true;
 198         }
 199     }
 200
 201     false
 202 }
 203
 204 fn is_end_eq_array_len<'tcx>(
 205     cx: &LateContext<'tcx>,
 206     end: &Expr<'_>,
 207     limits: ast::RangeLimits,
 208     indexed_ty: Ty<'tcx>,
 209 ) -> bool {
 210     if_chain! {
 211         if let ExprKind::Lit(ref lit) = end.kind;
 212         if let ast::LitKind::Int(end_int, _) = lit.node;
 213         if let ty::Array(_, arr_len_const) = indexed_ty.kind();
 214         if let Some(arr_len) = arr_len_const.try_eval_usize(cx.tcx, cx.param_env);
 215         then {
 216             return match limits {
 217                 ast::RangeLimits::Closed => end_int + 1 >= arr_len.into(),
 218                 ast::RangeLimits::HalfOpen => end_int >= arr_len.into(),
 219             };
 220         }
 221     }
 222
 223     false
 224 }
 225
 226 struct VarVisitor<'a, 'tcx> {
 227     /// context reference
 228     cx: &'a LateContext<'tcx>,
 229     /// var name to look for as index
 230     var: HirId,
 231     /// indexed variables that are used mutably
 232     indexed_mut: FxHashSet<Symbol>,
 233     /// indirectly indexed variables (`v[(i + 4) % N]`), the extend is `None` for global
 234     indexed_indirectly: FxHashMap<Symbol, Option<region::Scope>>,
 235     /// subset of `indexed` of vars that are indexed directly: `v[i]`
 236     /// this will not contain cases like `v[calc_index(i)]` or `v[(i + 4) % N]`
 237     indexed_directly: FxHashMap<Symbol, (Option<region::Scope>, Ty<'tcx>)>,
 238     /// Any names that are used outside an index operation.
 239     /// Used to detect things like `&mut vec` used together with `vec[i]`
 240     referenced: FxHashSet<Symbol>,
 241     /// has the loop variable been used in expressions other than the index of
 242     /// an index op?
 243     nonindex: bool,
 244     /// Whether we are inside the `$` in `&mut $` or `$ = foo` or `$.bar`, where bar
 245     /// takes `&mut self`
 246     prefer_mutable: bool,
 247 }
 248
 249 impl<'a, 'tcx> VarVisitor<'a, 'tcx> {
 250     fn check(&mut self, idx: &'tcx Expr<'_>, seqexpr: &'tcx Expr<'_>, expr: &'tcx Expr<'_>) -> bool {
 251         if_chain! {
 252             // the indexed container is referenced by a name
 253             if let ExprKind::Path(ref seqpath) = seqexpr.kind;
 254             if let QPath::Resolved(None, seqvar) = *seqpath;
 255             if seqvar.segments.len() == 1;
 256             if is_local_used(self.cx, idx, self.var);
 257             then {
 258                 if self.prefer_mutable {
 259                     self.indexed_mut.insert(seqvar.segments[0].ident.name);
 260                 }
 261                 let index_used_directly = matches!(idx.kind, ExprKind::Path(_));
 262                 let res = self.cx.qpath_res(seqpath, seqexpr.hir_id);
 263                 match res {
 264                     Res::Local(hir_id) => {
 265                         let parent_def_id = self.cx.tcx.hir().get_parent_item(expr.hir_id);
 266                         let extent = self.cx.tcx.region_scope_tree(parent_def_id).var_scope(hir_id.local_id);
 267                         if index_used_directly {
 268                             self.indexed_directly.insert(
 269                                 seqvar.segments[0].ident.name,
 270                                 (Some(extent), self.cx.typeck_results().node_type(seqexpr.hir_id)),
 271                             );
 272                         } else {
 273                             self.indexed_indirectly.insert(seqvar.segments[0].ident.name, Some(extent));
 274                         }
 275                         return false;  // no need to walk further *on the variable*
 276                     }
 277                     Res::Def(DefKind::Static | DefKind::Const, ..) => {
 278                         if index_used_directly {
 279                             self.indexed_directly.insert(
 280                                 seqvar.segments[0].ident.name,
 281                                 (None, self.cx.typeck_results().node_type(seqexpr.hir_id)),
 282                             );
 283                         } else {
 284                             self.indexed_indirectly.insert(seqvar.segments[0].ident.name, None);
 285                         }
 286                         return false;  // no need to walk further *on the variable*
 287                     }
 288                     _ => (),
 289                 }
 290             }
 291         }
 292         true
 293     }
 294 }
 295
 296 impl<'a, 'tcx> Visitor<'tcx> for VarVisitor<'a, 'tcx> {
 297     type Map = Map<'tcx>;
 298
 299     fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
 300         if_chain! {
 301             // a range index op
 302             if let ExprKind::MethodCall(meth, _, [args_0, args_1, ..], _) = &expr.kind;
 303             if (meth.ident.name == sym::index && match_trait_method(self.cx, expr, &paths::INDEX))
 304                 || (meth.ident.name == sym::index_mut && match_trait_method(self.cx, expr, &paths::INDEX_MUT));
 305             if !self.check(args_1, args_0, expr);
 306             then { return }
 307         }
 308
 309         if_chain! {
 310             // an index op
 311             if let ExprKind::Index(seqexpr, idx) = expr.kind;
 312             if !self.check(idx, seqexpr, expr);
 313             then { return }
 314         }
 315
 316         if_chain! {
 317             // directly using a variable
 318             if let ExprKind::Path(QPath::Resolved(None, path)) = expr.kind;
 319             if let Res::Local(local_id) = path.res;
 320             then {
 321                 if local_id == self.var {
 322                     self.nonindex = true;
 323                 } else {
 324                     // not the correct variable, but still a variable
 325                     self.referenced.insert(path.segments[0].ident.name);
 326                 }
 327             }
 328         }
 329
 330         let old = self.prefer_mutable;
 331         match expr.kind {
 332             ExprKind::AssignOp(_, lhs, rhs) | ExprKind::Assign(lhs, rhs, _) => {
 333                 self.prefer_mutable = true;
 334                 self.visit_expr(lhs);
 335                 self.prefer_mutable = false;
 336                 self.visit_expr(rhs);
 337             },
 338             ExprKind::AddrOf(BorrowKind::Ref, mutbl, expr) => {
 339                 if mutbl == Mutability::Mut {
 340                     self.prefer_mutable = true;
 341                 }
 342                 self.visit_expr(expr);
 343             },
 344             ExprKind::Call(f, args) => {
 345                 self.visit_expr(f);
 346                 for expr in args {
 347                     let ty = self.cx.typeck_results().expr_ty_adjusted(expr);
 348                     self.prefer_mutable = false;
 349                     if let ty::Ref(_, _, mutbl) = *ty.kind() {
 350                         if mutbl == Mutability::Mut {
 351                             self.prefer_mutable = true;
 352                         }
 353                     }
 354                     self.visit_expr(expr);
 355                 }
 356             },
 357             ExprKind::MethodCall(_, _, args, _) => {
 358                 let def_id = self.cx.typeck_results().type_dependent_def_id(expr.hir_id).unwrap();
 359                 for (ty, expr) in iter::zip(self.cx.tcx.fn_sig(def_id).inputs().skip_binder(), args) {
 360                     self.prefer_mutable = false;
 361                     if let ty::Ref(_, _, mutbl) = *ty.kind() {
 362                         if mutbl == Mutability::Mut {
 363                             self.prefer_mutable = true;
 364                         }
 365                     }
 366                     self.visit_expr(expr);
 367                 }
 368             },
 369             ExprKind::Closure(_, _, body_id, ..) => {
 370                 let body = self.cx.tcx.hir().body(body_id);
 371                 self.visit_expr(&body.value);
 372             },
 373             _ => walk_expr(self, expr),
 374         }
 375         self.prefer_mutable = old;
 376     }
 377     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 378         NestedVisitorMap::None
 379     }
 380 }