src/tools/clippy/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, Visitor};
  12 use rustc_hir::{BinOpKind, BorrowKind, Closure, Expr, ExprKind, HirId, Mutability, Pat, PatKind, QPath};
  13 use rustc_lint::LateContext;
  14 use rustc_middle::middle::region;
  15 use rustc_middle::ty::{self, Ty};
  16 use rustc_span::symbol::{sym, Symbol};
  17 use std::iter::{self, Iterator};
  18 use std::mem;
  19
  20 /// Checks for looping over a range and then indexing a sequence with it.
  21 /// The iteratee must be a range literal.
  22 #[expect(clippy::too_many_lines)]
  23 pub(super) fn check<'tcx>(
  24     cx: &LateContext<'tcx>,
  25     pat: &'tcx Pat<'_>,
  26     arg: &'tcx Expr<'_>,
  27     body: &'tcx Expr<'_>,
  28     expr: &'tcx Expr<'_>,
  29 ) {
  30     if let Some(higher::Range {
  31         start: Some(start),
  32         ref end,
  33         limits,
  34     }) = higher::Range::hir(arg)
  35     {
  36         // the var must be a single name
  37         if let PatKind::Binding(_, canonical_id, ident, _) = pat.kind {
  38             let mut visitor = VarVisitor {
  39                 cx,
  40                 var: canonical_id,
  41                 indexed_mut: FxHashSet::default(),
  42                 indexed_indirectly: FxHashMap::default(),
  43                 indexed_directly: FxHashMap::default(),
  44                 referenced: FxHashSet::default(),
  45                 nonindex: false,
  46                 prefer_mutable: false,
  47             };
  48             walk_expr(&mut visitor, body);
  49
  50             // linting condition: we only indexed one variable, and indexed it directly
  51             if visitor.indexed_indirectly.is_empty() && visitor.indexed_directly.len() == 1 {
  52                 let (indexed, (indexed_extent, indexed_ty)) = visitor
  53                     .indexed_directly
  54                     .into_iter()
  55                     .next()
  56                     .expect("already checked that we have exactly 1 element");
  57
  58                 // ensure that the indexed variable was declared before the loop, see #601
  59                 if let Some(indexed_extent) = indexed_extent {
  60                     let parent_def_id = cx.tcx.hir().get_parent_item(expr.hir_id);
  61                     let region_scope_tree = cx.tcx.region_scope_tree(parent_def_id);
  62                     let pat_extent = region_scope_tree.var_scope(pat.hir_id.local_id).unwrap();
  63                     if region_scope_tree.is_subscope_of(indexed_extent, pat_extent) {
  64                         return;
  65                     }
  66                 }
  67
  68                 // don't lint if the container that is indexed does not have .iter() method
  69                 let has_iter = has_iter_method(cx, indexed_ty);
  70                 if has_iter.is_none() {
  71                     return;
  72                 }
  73
  74                 // don't lint if the container that is indexed into is also used without
  75                 // indexing
  76                 if visitor.referenced.contains(&indexed) {
  77                     return;
  78                 }
  79
  80                 let starts_at_zero = is_integer_const(cx, start, 0);
  81
  82                 let skip = if starts_at_zero {
  83                     String::new()
  84                 } else if visitor.indexed_mut.contains(&indexed) && contains_name(indexed, start) {
  85                     return;
  86                 } else {
  87                     format!(".skip({})", snippet(cx, start.span, ".."))
  88                 };
  89
  90                 let mut end_is_start_plus_val = false;
  91
  92                 let take = if let Some(end) = *end {
  93                     let mut take_expr = end;
  94
  95                     if let ExprKind::Binary(ref op, left, right) = end.kind {
  96                         if op.node == BinOpKind::Add {
  97                             let start_equal_left = SpanlessEq::new(cx).eq_expr(start, left);
  98                             let start_equal_right = SpanlessEq::new(cx).eq_expr(start, right);
  99
 100                             if start_equal_left {
 101                                 take_expr = right;
 102                             } else if start_equal_right {
 103                                 take_expr = left;
 104                             }
 105
 106                             end_is_start_plus_val = start_equal_left | start_equal_right;
 107                         }
 108                     }
 109
 110                     if is_len_call(end, indexed) || is_end_eq_array_len(cx, end, limits, indexed_ty) {
 111                         String::new()
 112                     } else if visitor.indexed_mut.contains(&indexed) && contains_name(indexed, take_expr) {
 113                         return;
 114                     } else {
 115                         match limits {
 116                             ast::RangeLimits::Closed => {
 117                                 let take_expr = sugg::Sugg::hir(cx, take_expr, "<count>");
 118                                 format!(".take({})", take_expr + sugg::ONE)
 119                             },
 120                             ast::RangeLimits::HalfOpen => {
 121                                 format!(".take({})", snippet(cx, take_expr.span, ".."))
 122                             },
 123                         }
 124                     }
 125                 } else {
 126                     String::new()
 127                 };
 128
 129                 let (ref_mut, method) = if visitor.indexed_mut.contains(&indexed) {
 130                     ("mut ", "iter_mut")
 131                 } else {
 132                     ("", "iter")
 133                 };
 134
 135                 let take_is_empty = take.is_empty();
 136                 let mut method_1 = take;
 137                 let mut method_2 = skip;
 138
 139                 if end_is_start_plus_val {
 140                     mem::swap(&mut method_1, &mut method_2);
 141                 }
 142
 143                 if visitor.nonindex {
 144                     span_lint_and_then(
 145                         cx,
 146                         NEEDLESS_RANGE_LOOP,
 147                         arg.span,
 148                         &format!("the loop variable `{}` is used to index `{indexed}`", ident.name),
 149                         |diag| {
 150                             multispan_sugg(
 151                                 diag,
 152                                 "consider using an iterator",
 153                                 vec![
 154                                     (pat.span, format!("({}, <item>)", ident.name)),
 155                                     (
 156                                         arg.span,
 157                                         format!("{indexed}.{method}().enumerate(){method_1}{method_2}"),
 158                                     ),
 159                                 ],
 160                             );
 161                         },
 162                     );
 163                 } else {
 164                     let repl = if starts_at_zero && take_is_empty {
 165                         format!("&{ref_mut}{indexed}")
 166                     } else {
 167                         format!("{indexed}.{method}(){method_1}{method_2}")
 168                     };
 169
 170                     span_lint_and_then(
 171                         cx,
 172                         NEEDLESS_RANGE_LOOP,
 173                         arg.span,
 174                         &format!("the loop variable `{}` is only used to index `{indexed}`", ident.name),
 175                         |diag| {
 176                             multispan_sugg(
 177                                 diag,
 178                                 "consider using an iterator",
 179                                 vec![(pat.span, "<item>".to_string()), (arg.span, repl)],
 180                             );
 181                         },
 182                     );
 183                 }
 184             }
 185         }
 186     }
 187 }
 188
 189 fn is_len_call(expr: &Expr<'_>, var: Symbol) -> bool {
 190     if_chain! {
 191         if let ExprKind::MethodCall(method, recv, [], _) = expr.kind;
 192         if method.ident.name == sym::len;
 193         if let ExprKind::Path(QPath::Resolved(_, path)) = recv.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
 267                             .tcx
 268                             .region_scope_tree(parent_def_id)
 269                             .var_scope(hir_id.local_id)
 270                             .unwrap();
 271                         if index_used_directly {
 272                             self.indexed_directly.insert(
 273                                 seqvar.segments[0].ident.name,
 274                                 (Some(extent), self.cx.typeck_results().node_type(seqexpr.hir_id)),
 275                             );
 276                         } else {
 277                             self.indexed_indirectly.insert(seqvar.segments[0].ident.name, Some(extent));
 278                         }
 279                         return false;  // no need to walk further *on the variable*
 280                     }
 281                     Res::Def(DefKind::Static (_)| DefKind::Const, ..) => {
 282                         if index_used_directly {
 283                             self.indexed_directly.insert(
 284                                 seqvar.segments[0].ident.name,
 285                                 (None, self.cx.typeck_results().node_type(seqexpr.hir_id)),
 286                             );
 287                         } else {
 288                             self.indexed_indirectly.insert(seqvar.segments[0].ident.name, None);
 289                         }
 290                         return false;  // no need to walk further *on the variable*
 291                     }
 292                     _ => (),
 293                 }
 294             }
 295         }
 296         true
 297     }
 298 }
 299
 300 impl<'a, 'tcx> Visitor<'tcx> for VarVisitor<'a, 'tcx> {
 301     fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
 302         if_chain! {
 303             // a range index op
 304             if let ExprKind::MethodCall(meth, args_0, [args_1, ..], _) = &expr.kind;
 305             if (meth.ident.name == sym::index && match_trait_method(self.cx, expr, &paths::INDEX))
 306                 || (meth.ident.name == sym::index_mut && match_trait_method(self.cx, expr, &paths::INDEX_MUT));
 307             if !self.check(args_1, args_0, expr);
 308             then { return }
 309         }
 310
 311         if_chain! {
 312             // an index op
 313             if let ExprKind::Index(seqexpr, idx) = expr.kind;
 314             if !self.check(idx, seqexpr, expr);
 315             then { return }
 316         }
 317
 318         if_chain! {
 319             // directly using a variable
 320             if let ExprKind::Path(QPath::Resolved(None, path)) = expr.kind;
 321             if let Res::Local(local_id) = path.res;
 322             then {
 323                 if local_id == self.var {
 324                     self.nonindex = true;
 325                 } else {
 326                     // not the correct variable, but still a variable
 327                     self.referenced.insert(path.segments[0].ident.name);
 328                 }
 329             }
 330         }
 331
 332         let old = self.prefer_mutable;
 333         match expr.kind {
 334             ExprKind::AssignOp(_, lhs, rhs) | ExprKind::Assign(lhs, rhs, _) => {
 335                 self.prefer_mutable = true;
 336                 self.visit_expr(lhs);
 337                 self.prefer_mutable = false;
 338                 self.visit_expr(rhs);
 339             },
 340             ExprKind::AddrOf(BorrowKind::Ref, mutbl, expr) => {
 341                 if mutbl == Mutability::Mut {
 342                     self.prefer_mutable = true;
 343                 }
 344                 self.visit_expr(expr);
 345             },
 346             ExprKind::Call(f, args) => {
 347                 self.visit_expr(f);
 348                 for expr in args {
 349                     let ty = self.cx.typeck_results().expr_ty_adjusted(expr);
 350                     self.prefer_mutable = false;
 351                     if let ty::Ref(_, _, mutbl) = *ty.kind() {
 352                         if mutbl == Mutability::Mut {
 353                             self.prefer_mutable = true;
 354                         }
 355                     }
 356                     self.visit_expr(expr);
 357                 }
 358             },
 359             ExprKind::MethodCall(_, receiver, args, _) => {
 360                 let def_id = self.cx.typeck_results().type_dependent_def_id(expr.hir_id).unwrap();
 361                 for (ty, expr) in iter::zip(
 362                     self.cx.tcx.fn_sig(def_id).inputs().skip_binder(),
 363                     std::iter::once(receiver).chain(args.iter()),
 364                 ) {
 365                     self.prefer_mutable = false;
 366                     if let ty::Ref(_, _, mutbl) = *ty.kind() {
 367                         if mutbl == Mutability::Mut {
 368                             self.prefer_mutable = true;
 369                         }
 370                     }
 371                     self.visit_expr(expr);
 372                 }
 373             },
 374             ExprKind::Closure(&Closure { body, .. }) => {
 375                 let body = self.cx.tcx.hir().body(body);
 376                 self.visit_expr(body.value);
 377             },
 378             _ => walk_expr(self, expr),
 379         }
 380         self.prefer_mutable = old;
 381     }
 382 }