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 `{}`", ident.name, indexed),
 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!("{}.{}().enumerate(){}{}", indexed, method, 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 `{}`", ident.name, indexed),
 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, len_args, _) = expr.kind;
 192         if len_args.len() == 1;
 193         if method.ident.name == sym::len;
 194         if let ExprKind::Path(QPath::Resolved(_, path)) = len_args[0].kind;
 195         if path.segments.len() == 1;
 196         if path.segments[0].ident.name == var;
 197         then {
 198             return true;
 199         }
 200     }
 201
 202     false
 203 }
 204
 205 fn is_end_eq_array_len<'tcx>(
 206     cx: &LateContext<'tcx>,
 207     end: &Expr<'_>,
 208     limits: ast::RangeLimits,
 209     indexed_ty: Ty<'tcx>,
 210 ) -> bool {
 211     if_chain! {
 212         if let ExprKind::Lit(ref lit) = end.kind;
 213         if let ast::LitKind::Int(end_int, _) = lit.node;
 214         if let ty::Array(_, arr_len_const) = indexed_ty.kind();
 215         if let Some(arr_len) = arr_len_const.try_eval_usize(cx.tcx, cx.param_env);
 216         then {
 217             return match limits {
 218                 ast::RangeLimits::Closed => end_int + 1 >= arr_len.into(),
 219                 ast::RangeLimits::HalfOpen => end_int >= arr_len.into(),
 220             };
 221         }
 222     }
 223
 224     false
 225 }
 226
 227 struct VarVisitor<'a, 'tcx> {
 228     /// context reference
 229     cx: &'a LateContext<'tcx>,
 230     /// var name to look for as index
 231     var: HirId,
 232     /// indexed variables that are used mutably
 233     indexed_mut: FxHashSet<Symbol>,
 234     /// indirectly indexed variables (`v[(i + 4) % N]`), the extend is `None` for global
 235     indexed_indirectly: FxHashMap<Symbol, Option<region::Scope>>,
 236     /// subset of `indexed` of vars that are indexed directly: `v[i]`
 237     /// this will not contain cases like `v[calc_index(i)]` or `v[(i + 4) % N]`
 238     indexed_directly: FxHashMap<Symbol, (Option<region::Scope>, Ty<'tcx>)>,
 239     /// Any names that are used outside an index operation.
 240     /// Used to detect things like `&mut vec` used together with `vec[i]`
 241     referenced: FxHashSet<Symbol>,
 242     /// has the loop variable been used in expressions other than the index of
 243     /// an index op?
 244     nonindex: bool,
 245     /// Whether we are inside the `$` in `&mut $` or `$ = foo` or `$.bar`, where bar
 246     /// takes `&mut self`
 247     prefer_mutable: bool,
 248 }
 249
 250 impl<'a, 'tcx> VarVisitor<'a, 'tcx> {
 251     fn check(&mut self, idx: &'tcx Expr<'_>, seqexpr: &'tcx Expr<'_>, expr: &'tcx Expr<'_>) -> bool {
 252         if_chain! {
 253             // the indexed container is referenced by a name
 254             if let ExprKind::Path(ref seqpath) = seqexpr.kind;
 255             if let QPath::Resolved(None, seqvar) = *seqpath;
 256             if seqvar.segments.len() == 1;
 257             if is_local_used(self.cx, idx, self.var);
 258             then {
 259                 if self.prefer_mutable {
 260                     self.indexed_mut.insert(seqvar.segments[0].ident.name);
 261                 }
 262                 let index_used_directly = matches!(idx.kind, ExprKind::Path(_));
 263                 let res = self.cx.qpath_res(seqpath, seqexpr.hir_id);
 264                 match res {
 265                     Res::Local(hir_id) => {
 266                         let parent_def_id = self.cx.tcx.hir().get_parent_item(expr.hir_id);
 267                         let extent = self.cx
 268                             .tcx
 269                             .region_scope_tree(parent_def_id)
 270                             .var_scope(hir_id.local_id)
 271                             .unwrap();
 272                         if index_used_directly {
 273                             self.indexed_directly.insert(
 274                                 seqvar.segments[0].ident.name,
 275                                 (Some(extent), self.cx.typeck_results().node_type(seqexpr.hir_id)),
 276                             );
 277                         } else {
 278                             self.indexed_indirectly.insert(seqvar.segments[0].ident.name, Some(extent));
 279                         }
 280                         return false;  // no need to walk further *on the variable*
 281                     }
 282                     Res::Def(DefKind::Static (_)| DefKind::Const, ..) => {
 283                         if index_used_directly {
 284                             self.indexed_directly.insert(
 285                                 seqvar.segments[0].ident.name,
 286                                 (None, self.cx.typeck_results().node_type(seqexpr.hir_id)),
 287                             );
 288                         } else {
 289                             self.indexed_indirectly.insert(seqvar.segments[0].ident.name, None);
 290                         }
 291                         return false;  // no need to walk further *on the variable*
 292                     }
 293                     _ => (),
 294                 }
 295             }
 296         }
 297         true
 298     }
 299 }
 300
 301 impl<'a, 'tcx> Visitor<'tcx> for VarVisitor<'a, 'tcx> {
 302     fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
 303         if_chain! {
 304             // a range index op
 305             if let ExprKind::MethodCall(meth, [args_0, args_1, ..], _) = &expr.kind;
 306             if (meth.ident.name == sym::index && match_trait_method(self.cx, expr, &paths::INDEX))
 307                 || (meth.ident.name == sym::index_mut && match_trait_method(self.cx, expr, &paths::INDEX_MUT));
 308             if !self.check(args_1, args_0, expr);
 309             then { return }
 310         }
 311
 312         if_chain! {
 313             // an index op
 314             if let ExprKind::Index(seqexpr, idx) = expr.kind;
 315             if !self.check(idx, seqexpr, expr);
 316             then { return }
 317         }
 318
 319         if_chain! {
 320             // directly using a variable
 321             if let ExprKind::Path(QPath::Resolved(None, path)) = expr.kind;
 322             if let Res::Local(local_id) = path.res;
 323             then {
 324                 if local_id == self.var {
 325                     self.nonindex = true;
 326                 } else {
 327                     // not the correct variable, but still a variable
 328                     self.referenced.insert(path.segments[0].ident.name);
 329                 }
 330             }
 331         }
 332
 333         let old = self.prefer_mutable;
 334         match expr.kind {
 335             ExprKind::AssignOp(_, lhs, rhs) | ExprKind::Assign(lhs, rhs, _) => {
 336                 self.prefer_mutable = true;
 337                 self.visit_expr(lhs);
 338                 self.prefer_mutable = false;
 339                 self.visit_expr(rhs);
 340             },
 341             ExprKind::AddrOf(BorrowKind::Ref, mutbl, expr) => {
 342                 if mutbl == Mutability::Mut {
 343                     self.prefer_mutable = true;
 344                 }
 345                 self.visit_expr(expr);
 346             },
 347             ExprKind::Call(f, args) => {
 348                 self.visit_expr(f);
 349                 for expr in args {
 350                     let ty = self.cx.typeck_results().expr_ty_adjusted(expr);
 351                     self.prefer_mutable = false;
 352                     if let ty::Ref(_, _, mutbl) = *ty.kind() {
 353                         if mutbl == Mutability::Mut {
 354                             self.prefer_mutable = true;
 355                         }
 356                     }
 357                     self.visit_expr(expr);
 358                 }
 359             },
 360             ExprKind::MethodCall(_, args, _) => {
 361                 let def_id = self.cx.typeck_results().type_dependent_def_id(expr.hir_id).unwrap();
 362                 for (ty, expr) in iter::zip(self.cx.tcx.fn_sig(def_id).inputs().skip_binder(), args) {
 363                     self.prefer_mutable = false;
 364                     if let ty::Ref(_, _, mutbl) = *ty.kind() {
 365                         if mutbl == Mutability::Mut {
 366                             self.prefer_mutable = true;
 367                         }
 368                     }
 369                     self.visit_expr(expr);
 370                 }
 371             },
 372             ExprKind::Closure(&Closure { body, .. }) => {
 373                 let body = self.cx.tcx.hir().body(body);
 374                 self.visit_expr(&body.value);
 375             },
 376             _ => walk_expr(self, expr),
 377         }
 378         self.prefer_mutable = old;
 379     }
 380 }