src/tools/clippy/clippy_lints/src/suspicious_operation_groupings.rs

   1 use clippy_utils::ast_utils::{eq_id, is_useless_with_eq_exprs, IdentIter};
   2 use clippy_utils::diagnostics::span_lint_and_sugg;
   3 use clippy_utils::source::snippet_with_applicability;
   4 use core::ops::{Add, AddAssign};
   5 use if_chain::if_chain;
   6 use rustc_ast::ast::{BinOpKind, Expr, ExprKind, StmtKind};
   7 use rustc_data_structures::fx::FxHashSet;
   8 use rustc_errors::Applicability;
   9 use rustc_lint::{EarlyContext, EarlyLintPass};
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use rustc_span::source_map::Spanned;
  12 use rustc_span::symbol::Ident;
  13 use rustc_span::Span;
  14
  15 declare_clippy_lint! {
  16     /// ### What it does
  17     /// Checks for unlikely usages of binary operators that are almost
  18     /// certainly typos and/or copy/paste errors, given the other usages
  19     /// of binary operators nearby.
  20     ///
  21     /// ### Why is this bad?
  22     /// They are probably bugs and if they aren't then they look like bugs
  23     /// and you should add a comment explaining why you are doing such an
  24     /// odd set of operations.
  25     ///
  26     /// ### Known problems
  27     /// There may be some false positives if you are trying to do something
  28     /// unusual that happens to look like a typo.
  29     ///
  30     /// ### Example
  31     /// ```rust
  32     /// struct Vec3 {
  33     ///     x: f64,
  34     ///     y: f64,
  35     ///     z: f64,
  36     /// }
  37     ///
  38     /// impl Eq for Vec3 {}
  39     ///
  40     /// impl PartialEq for Vec3 {
  41     ///     fn eq(&self, other: &Self) -> bool {
  42     ///         // This should trigger the lint because `self.x` is compared to `other.y`
  43     ///         self.x == other.y && self.y == other.y && self.z == other.z
  44     ///     }
  45     /// }
  46     /// ```
  47     /// Use instead:
  48     /// ```rust
  49     /// # struct Vec3 {
  50     /// #     x: f64,
  51     /// #     y: f64,
  52     /// #     z: f64,
  53     /// # }
  54     /// // same as above except:
  55     /// impl PartialEq for Vec3 {
  56     ///     fn eq(&self, other: &Self) -> bool {
  57     ///         // Note we now compare other.x to self.x
  58     ///         self.x == other.x && self.y == other.y && self.z == other.z
  59     ///     }
  60     /// }
  61     /// ```
  62     pub SUSPICIOUS_OPERATION_GROUPINGS,
  63     nursery,
  64     "groupings of binary operations that look suspiciously like typos"
  65 }
  66
  67 declare_lint_pass!(SuspiciousOperationGroupings => [SUSPICIOUS_OPERATION_GROUPINGS]);
  68
  69 impl EarlyLintPass for SuspiciousOperationGroupings {
  70     fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &Expr) {
  71         if expr.span.from_expansion() {
  72             return;
  73         }
  74
  75         if let Some(binops) = extract_related_binops(&expr.kind) {
  76             check_binops(cx, &binops.iter().collect::<Vec<_>>());
  77
  78             let mut op_types = Vec::with_capacity(binops.len());
  79             // We could use a hashmap, etc. to avoid being O(n*m) here, but
  80             // we want the lints to be emitted in a consistent order. Besides,
  81             // m, (the number of distinct `BinOpKind`s in `binops`)
  82             // will often be small, and does have an upper limit.
  83             binops.iter().map(|b| b.op).for_each(|op| {
  84                 if !op_types.contains(&op) {
  85                     op_types.push(op);
  86                 }
  87             });
  88
  89             for op_type in op_types {
  90                 let ops: Vec<_> = binops.iter().filter(|b| b.op == op_type).collect();
  91
  92                 check_binops(cx, &ops);
  93             }
  94         }
  95     }
  96 }
  97
  98 fn check_binops(cx: &EarlyContext<'_>, binops: &[&BinaryOp<'_>]) {
  99     let binop_count = binops.len();
 100     if binop_count < 2 {
 101         // Single binary operation expressions would likely be false
 102         // positives.
 103         return;
 104     }
 105
 106     let mut one_ident_difference_count = 0;
 107     let mut no_difference_info = None;
 108     let mut double_difference_info = None;
 109     let mut expected_ident_loc = None;
 110
 111     let mut paired_identifiers = FxHashSet::default();
 112
 113     for (i, BinaryOp { left, right, op, .. }) in binops.iter().enumerate() {
 114         match ident_difference_expr(left, right) {
 115             IdentDifference::NoDifference => {
 116                 if is_useless_with_eq_exprs(*op) {
 117                     // The `eq_op` lint should catch this in this case.
 118                     return;
 119                 }
 120
 121                 no_difference_info = Some(i);
 122             },
 123             IdentDifference::Single(ident_loc) => {
 124                 one_ident_difference_count += 1;
 125                 if let Some(previous_expected) = expected_ident_loc {
 126                     if previous_expected != ident_loc {
 127                         // This expression doesn't match the form we're
 128                         // looking for.
 129                         return;
 130                     }
 131                 } else {
 132                     expected_ident_loc = Some(ident_loc);
 133                 }
 134
 135                 // If there was only a single difference, all other idents
 136                 // must have been the same, and thus were paired.
 137                 for id in skip_index(IdentIter::from(*left), ident_loc.index) {
 138                     paired_identifiers.insert(id);
 139                 }
 140             },
 141             IdentDifference::Double(ident_loc1, ident_loc2) => {
 142                 double_difference_info = Some((i, ident_loc1, ident_loc2));
 143             },
 144             IdentDifference::Multiple | IdentDifference::NonIdent => {
 145                 // It's too hard to know whether this is a bug or not.
 146                 return;
 147             },
 148         }
 149     }
 150
 151     let mut applicability = Applicability::MachineApplicable;
 152
 153     if let Some(expected_loc) = expected_ident_loc {
 154         match (no_difference_info, double_difference_info) {
 155             (Some(i), None) => attempt_to_emit_no_difference_lint(cx, binops, i, expected_loc),
 156             (None, Some((double_difference_index, ident_loc1, ident_loc2))) => {
 157                 if_chain! {
 158                     if one_ident_difference_count == binop_count - 1;
 159                     if let Some(binop) = binops.get(double_difference_index);
 160                     then {
 161                         let changed_loc = if ident_loc1 == expected_loc {
 162                             ident_loc2
 163                         } else if ident_loc2 == expected_loc {
 164                             ident_loc1
 165                         } else {
 166                             // This expression doesn't match the form we're
 167                             // looking for.
 168                             return;
 169                         };
 170
 171                         if let Some(sugg) = ident_swap_sugg(
 172                             cx,
 173                             &paired_identifiers,
 174                             binop,
 175                             changed_loc,
 176                             &mut applicability,
 177                         ) {
 178                             emit_suggestion(
 179                                 cx,
 180                                 binop.span,
 181                                 sugg,
 182                                 applicability,
 183                             );
 184                         }
 185                     }
 186                 }
 187             },
 188             _ => {},
 189         }
 190     }
 191 }
 192
 193 fn attempt_to_emit_no_difference_lint(
 194     cx: &EarlyContext<'_>,
 195     binops: &[&BinaryOp<'_>],
 196     i: usize,
 197     expected_loc: IdentLocation,
 198 ) {
 199     if let Some(binop) = binops.get(i).copied() {
 200         // We need to try and figure out which identifier we should
 201         // suggest using instead. Since there could be multiple
 202         // replacement candidates in a given expression, and we're
 203         // just taking the first one, we may get some bad lint
 204         // messages.
 205         let mut applicability = Applicability::MaybeIncorrect;
 206
 207         // We assume that the correct ident is one used elsewhere in
 208         // the other binops, in a place that there was a single
 209         // difference between idents before.
 210         let old_left_ident = get_ident(binop.left, expected_loc);
 211         let old_right_ident = get_ident(binop.right, expected_loc);
 212
 213         for b in skip_index(binops.iter(), i) {
 214             if_chain! {
 215                 if let (Some(old_ident), Some(new_ident)) =
 216                 (old_left_ident, get_ident(b.left, expected_loc));
 217                 if old_ident != new_ident;
 218                 if let Some(sugg) = suggestion_with_swapped_ident(
 219                     cx,
 220                     binop.left,
 221                     expected_loc,
 222                     new_ident,
 223                     &mut applicability,
 224                 );
 225                 then {
 226                     emit_suggestion(
 227                         cx,
 228                         binop.span,
 229                         replace_left_sugg(cx, binop, &sugg, &mut applicability),
 230                         applicability,
 231                     );
 232                     return;
 233                 }
 234             }
 235
 236             if_chain! {
 237                 if let (Some(old_ident), Some(new_ident)) =
 238                     (old_right_ident, get_ident(b.right, expected_loc));
 239                 if old_ident != new_ident;
 240                 if let Some(sugg) = suggestion_with_swapped_ident(
 241                     cx,
 242                     binop.right,
 243                     expected_loc,
 244                     new_ident,
 245                     &mut applicability,
 246                 );
 247                 then {
 248                     emit_suggestion(
 249                         cx,
 250                         binop.span,
 251                         replace_right_sugg(cx, binop, &sugg, &mut applicability),
 252                         applicability,
 253                     );
 254                     return;
 255                 }
 256             }
 257         }
 258     }
 259 }
 260
 261 fn emit_suggestion(cx: &EarlyContext<'_>, span: Span, sugg: String, applicability: Applicability) {
 262     span_lint_and_sugg(
 263         cx,
 264         SUSPICIOUS_OPERATION_GROUPINGS,
 265         span,
 266         "this sequence of operators looks suspiciously like a bug",
 267         "did you mean",
 268         sugg,
 269         applicability,
 270     );
 271 }
 272
 273 fn ident_swap_sugg(
 274     cx: &EarlyContext<'_>,
 275     paired_identifiers: &FxHashSet<Ident>,
 276     binop: &BinaryOp<'_>,
 277     location: IdentLocation,
 278     applicability: &mut Applicability,
 279 ) -> Option<String> {
 280     let left_ident = get_ident(binop.left, location)?;
 281     let right_ident = get_ident(binop.right, location)?;
 282
 283     let sugg = match (
 284         paired_identifiers.contains(&left_ident),
 285         paired_identifiers.contains(&right_ident),
 286     ) {
 287         (true, true) | (false, false) => {
 288             // We don't have a good guess of what ident should be
 289             // used instead, in these cases.
 290             *applicability = Applicability::MaybeIncorrect;
 291
 292             // We arbitraily choose one side to suggest changing,
 293             // since we don't have a better guess. If the user
 294             // ends up duplicating a clause, the `logic_bug` lint
 295             // should catch it.
 296
 297             let right_suggestion = suggestion_with_swapped_ident(cx, binop.right, location, left_ident, applicability)?;
 298
 299             replace_right_sugg(cx, binop, &right_suggestion, applicability)
 300         },
 301         (false, true) => {
 302             // We haven't seen a pair involving the left one, so
 303             // it's probably what is wanted.
 304
 305             let right_suggestion = suggestion_with_swapped_ident(cx, binop.right, location, left_ident, applicability)?;
 306
 307             replace_right_sugg(cx, binop, &right_suggestion, applicability)
 308         },
 309         (true, false) => {
 310             // We haven't seen a pair involving the right one, so
 311             // it's probably what is wanted.
 312             let left_suggestion = suggestion_with_swapped_ident(cx, binop.left, location, right_ident, applicability)?;
 313
 314             replace_left_sugg(cx, binop, &left_suggestion, applicability)
 315         },
 316     };
 317
 318     Some(sugg)
 319 }
 320
 321 fn replace_left_sugg(
 322     cx: &EarlyContext<'_>,
 323     binop: &BinaryOp<'_>,
 324     left_suggestion: &str,
 325     applicability: &mut Applicability,
 326 ) -> String {
 327     format!(
 328         "{} {} {}",
 329         left_suggestion,
 330         binop.op.to_string(),
 331         snippet_with_applicability(cx, binop.right.span, "..", applicability),
 332     )
 333 }
 334
 335 fn replace_right_sugg(
 336     cx: &EarlyContext<'_>,
 337     binop: &BinaryOp<'_>,
 338     right_suggestion: &str,
 339     applicability: &mut Applicability,
 340 ) -> String {
 341     format!(
 342         "{} {} {}",
 343         snippet_with_applicability(cx, binop.left.span, "..", applicability),
 344         binop.op.to_string(),
 345         right_suggestion,
 346     )
 347 }
 348
 349 #[derive(Clone, Debug)]
 350 struct BinaryOp<'exprs> {
 351     op: BinOpKind,
 352     span: Span,
 353     left: &'exprs Expr,
 354     right: &'exprs Expr,
 355 }
 356
 357 impl BinaryOp<'exprs> {
 358     fn new(op: BinOpKind, span: Span, (left, right): (&'exprs Expr, &'exprs Expr)) -> Self {
 359         Self { op, span, left, right }
 360     }
 361 }
 362
 363 fn strip_non_ident_wrappers(expr: &Expr) -> &Expr {
 364     let mut output = expr;
 365     loop {
 366         output = match &output.kind {
 367             ExprKind::Paren(ref inner) | ExprKind::Unary(_, ref inner) => inner,
 368             _ => {
 369                 return output;
 370             },
 371         };
 372     }
 373 }
 374
 375 fn extract_related_binops(kind: &ExprKind) -> Option<Vec<BinaryOp<'_>>> {
 376     append_opt_vecs(chained_binops(kind), if_statment_binops(kind))
 377 }
 378
 379 fn if_statment_binops(kind: &ExprKind) -> Option<Vec<BinaryOp<'_>>> {
 380     match kind {
 381         ExprKind::If(ref condition, _, _) => chained_binops(&condition.kind),
 382         ExprKind::Paren(ref e) => if_statment_binops(&e.kind),
 383         ExprKind::Block(ref block, _) => {
 384             let mut output = None;
 385             for stmt in &block.stmts {
 386                 match stmt.kind {
 387                     StmtKind::Expr(ref e) | StmtKind::Semi(ref e) => {
 388                         output = append_opt_vecs(output, if_statment_binops(&e.kind));
 389                     },
 390                     _ => {},
 391                 }
 392             }
 393             output
 394         },
 395         _ => None,
 396     }
 397 }
 398
 399 fn append_opt_vecs<A>(target_opt: Option<Vec<A>>, source_opt: Option<Vec<A>>) -> Option<Vec<A>> {
 400     match (target_opt, source_opt) {
 401         (Some(mut target), Some(mut source)) => {
 402             target.reserve(source.len());
 403             for op in source.drain(..) {
 404                 target.push(op);
 405             }
 406             Some(target)
 407         },
 408         (Some(v), None) | (None, Some(v)) => Some(v),
 409         (None, None) => None,
 410     }
 411 }
 412
 413 fn chained_binops(kind: &ExprKind) -> Option<Vec<BinaryOp<'_>>> {
 414     match kind {
 415         ExprKind::Binary(_, left_outer, right_outer) => chained_binops_helper(left_outer, right_outer),
 416         ExprKind::Paren(ref e) | ExprKind::Unary(_, ref e) => chained_binops(&e.kind),
 417         _ => None,
 418     }
 419 }
 420
 421 fn chained_binops_helper(left_outer: &'expr Expr, right_outer: &'expr Expr) -> Option<Vec<BinaryOp<'expr>>> {
 422     match (&left_outer.kind, &right_outer.kind) {
 423         (
 424             ExprKind::Paren(ref left_e) | ExprKind::Unary(_, ref left_e),
 425             ExprKind::Paren(ref right_e) | ExprKind::Unary(_, ref right_e),
 426         ) => chained_binops_helper(left_e, right_e),
 427         (ExprKind::Paren(ref left_e) | ExprKind::Unary(_, ref left_e), _) => chained_binops_helper(left_e, right_outer),
 428         (_, ExprKind::Paren(ref right_e) | ExprKind::Unary(_, ref right_e)) => {
 429             chained_binops_helper(left_outer, right_e)
 430         },
 431         (
 432             ExprKind::Binary(Spanned { node: left_op, .. }, ref left_left, ref left_right),
 433             ExprKind::Binary(Spanned { node: right_op, .. }, ref right_left, ref right_right),
 434         ) => match (
 435             chained_binops_helper(left_left, left_right),
 436             chained_binops_helper(right_left, right_right),
 437         ) {
 438             (Some(mut left_ops), Some(mut right_ops)) => {
 439                 left_ops.reserve(right_ops.len());
 440                 for op in right_ops.drain(..) {
 441                     left_ops.push(op);
 442                 }
 443                 Some(left_ops)
 444             },
 445             (Some(mut left_ops), _) => {
 446                 left_ops.push(BinaryOp::new(*right_op, right_outer.span, (right_left, right_right)));
 447                 Some(left_ops)
 448             },
 449             (_, Some(mut right_ops)) => {
 450                 right_ops.insert(0, BinaryOp::new(*left_op, left_outer.span, (left_left, left_right)));
 451                 Some(right_ops)
 452             },
 453             (None, None) => Some(vec![
 454                 BinaryOp::new(*left_op, left_outer.span, (left_left, left_right)),
 455                 BinaryOp::new(*right_op, right_outer.span, (right_left, right_right)),
 456             ]),
 457         },
 458         _ => None,
 459     }
 460 }
 461
 462 #[derive(Clone, Copy, PartialEq, Eq, Default, Debug)]
 463 struct IdentLocation {
 464     index: usize,
 465 }
 466
 467 impl Add for IdentLocation {
 468     type Output = IdentLocation;
 469
 470     fn add(self, other: Self) -> Self::Output {
 471         Self {
 472             index: self.index + other.index,
 473         }
 474     }
 475 }
 476
 477 impl AddAssign for IdentLocation {
 478     fn add_assign(&mut self, other: Self) {
 479         *self = *self + other;
 480     }
 481 }
 482
 483 #[derive(Clone, Copy, Debug)]
 484 enum IdentDifference {
 485     NoDifference,
 486     Single(IdentLocation),
 487     Double(IdentLocation, IdentLocation),
 488     Multiple,
 489     NonIdent,
 490 }
 491
 492 impl Add for IdentDifference {
 493     type Output = IdentDifference;
 494
 495     fn add(self, other: Self) -> Self::Output {
 496         match (self, other) {
 497             (Self::NoDifference, output) | (output, Self::NoDifference) => output,
 498             (Self::Multiple, _)
 499             | (_, Self::Multiple)
 500             | (Self::Double(_, _), Self::Single(_))
 501             | (Self::Single(_) | Self::Double(_, _), Self::Double(_, _)) => Self::Multiple,
 502             (Self::NonIdent, _) | (_, Self::NonIdent) => Self::NonIdent,
 503             (Self::Single(il1), Self::Single(il2)) => Self::Double(il1, il2),
 504         }
 505     }
 506 }
 507
 508 impl AddAssign for IdentDifference {
 509     fn add_assign(&mut self, other: Self) {
 510         *self = *self + other;
 511     }
 512 }
 513
 514 impl IdentDifference {
 515     /// Returns true if learning about more differences will not change the value
 516     /// of this `IdentDifference`, and false otherwise.
 517     fn is_complete(&self) -> bool {
 518         match self {
 519             Self::NoDifference | Self::Single(_) | Self::Double(_, _) => false,
 520             Self::Multiple | Self::NonIdent => true,
 521         }
 522     }
 523 }
 524
 525 fn ident_difference_expr(left: &Expr, right: &Expr) -> IdentDifference {
 526     ident_difference_expr_with_base_location(left, right, IdentLocation::default()).0
 527 }
 528
 529 fn ident_difference_expr_with_base_location(
 530     left: &Expr,
 531     right: &Expr,
 532     mut base: IdentLocation,
 533 ) -> (IdentDifference, IdentLocation) {
 534     // Ideally, this function should not use IdentIter because it should return
 535     // early if the expressions have any non-ident differences. We want that early
 536     // return because if without that restriction the lint would lead to false
 537     // positives.
 538     //
 539     // But, we cannot (easily?) use a `rustc_ast::visit::Visitor`, since we need
 540     // the two expressions to be walked in lockstep. And without a `Visitor`, we'd
 541     // have to do all the AST traversal ourselves, which is a lot of work, since to
 542     // do it properly we'd need to be able to handle more or less every possible
 543     // AST node since `Item`s can be written inside `Expr`s.
 544     //
 545     // In practice, it seems likely that expressions, above a certain size, that
 546     // happen to use the exact same idents in the exact same order, and which are
 547     // not structured the same, would be rare. Therefore it seems likely that if
 548     // we do only the first layer of matching ourselves and eventually fallback on
 549     // IdentIter, then the output of this function will be almost always be correct
 550     // in practice.
 551     //
 552     // If it turns out that problematic cases are more prelavent than we assume,
 553     // then we should be able to change this function to do the correct traversal,
 554     // without needing to change the rest of the code.
 555
 556     #![allow(clippy::enum_glob_use)]
 557     use ExprKind::*;
 558
 559     match (
 560         &strip_non_ident_wrappers(left).kind,
 561         &strip_non_ident_wrappers(right).kind,
 562     ) {
 563         (Yield(_), Yield(_))
 564         | (Try(_), Try(_))
 565         | (Paren(_), Paren(_))
 566         | (Repeat(_, _), Repeat(_, _))
 567         | (Struct(_), Struct(_))
 568         | (MacCall(_), MacCall(_))
 569         | (LlvmInlineAsm(_), LlvmInlineAsm(_))
 570         | (InlineAsm(_), InlineAsm(_))
 571         | (Ret(_), Ret(_))
 572         | (Continue(_), Continue(_))
 573         | (Break(_, _), Break(_, _))
 574         | (AddrOf(_, _, _), AddrOf(_, _, _))
 575         | (Path(_, _), Path(_, _))
 576         | (Range(_, _, _), Range(_, _, _))
 577         | (Index(_, _), Index(_, _))
 578         | (Field(_, _), Field(_, _))
 579         | (AssignOp(_, _, _), AssignOp(_, _, _))
 580         | (Assign(_, _, _), Assign(_, _, _))
 581         | (TryBlock(_), TryBlock(_))
 582         | (Await(_), Await(_))
 583         | (Async(_, _, _), Async(_, _, _))
 584         | (Block(_, _), Block(_, _))
 585         | (Closure(_, _, _, _, _, _), Closure(_, _, _, _, _, _))
 586         | (Match(_, _), Match(_, _))
 587         | (Loop(_, _), Loop(_, _))
 588         | (ForLoop(_, _, _, _), ForLoop(_, _, _, _))
 589         | (While(_, _, _), While(_, _, _))
 590         | (If(_, _, _), If(_, _, _))
 591         | (Let(_, _, _), Let(_, _, _))
 592         | (Type(_, _), Type(_, _))
 593         | (Cast(_, _), Cast(_, _))
 594         | (Lit(_), Lit(_))
 595         | (Unary(_, _), Unary(_, _))
 596         | (Binary(_, _, _), Binary(_, _, _))
 597         | (Tup(_), Tup(_))
 598         | (MethodCall(_, _, _), MethodCall(_, _, _))
 599         | (Call(_, _), Call(_, _))
 600         | (ConstBlock(_), ConstBlock(_))
 601         | (Array(_), Array(_))
 602         | (Box(_), Box(_)) => {
 603             // keep going
 604         },
 605         _ => {
 606             return (IdentDifference::NonIdent, base);
 607         },
 608     }
 609
 610     let mut difference = IdentDifference::NoDifference;
 611
 612     for (left_attr, right_attr) in left.attrs.iter().zip(right.attrs.iter()) {
 613         let (new_difference, new_base) =
 614             ident_difference_via_ident_iter_with_base_location(left_attr, right_attr, base);
 615         base = new_base;
 616         difference += new_difference;
 617         if difference.is_complete() {
 618             return (difference, base);
 619         }
 620     }
 621
 622     let (new_difference, new_base) = ident_difference_via_ident_iter_with_base_location(left, right, base);
 623     base = new_base;
 624     difference += new_difference;
 625
 626     (difference, base)
 627 }
 628
 629 fn ident_difference_via_ident_iter_with_base_location<Iterable: Into<IdentIter>>(
 630     left: Iterable,
 631     right: Iterable,
 632     mut base: IdentLocation,
 633 ) -> (IdentDifference, IdentLocation) {
 634     // See the note in `ident_difference_expr_with_base_location` about `IdentIter`
 635     let mut difference = IdentDifference::NoDifference;
 636
 637     let mut left_iterator = left.into();
 638     let mut right_iterator = right.into();
 639
 640     loop {
 641         match (left_iterator.next(), right_iterator.next()) {
 642             (Some(left_ident), Some(right_ident)) => {
 643                 if !eq_id(left_ident, right_ident) {
 644                     difference += IdentDifference::Single(base);
 645                     if difference.is_complete() {
 646                         return (difference, base);
 647                     }
 648                 }
 649             },
 650             (Some(_), None) | (None, Some(_)) => {
 651                 return (IdentDifference::NonIdent, base);
 652             },
 653             (None, None) => {
 654                 return (difference, base);
 655             },
 656         }
 657         base += IdentLocation { index: 1 };
 658     }
 659 }
 660
 661 fn get_ident(expr: &Expr, location: IdentLocation) -> Option<Ident> {
 662     IdentIter::from(expr).nth(location.index)
 663 }
 664
 665 fn suggestion_with_swapped_ident(
 666     cx: &EarlyContext<'_>,
 667     expr: &Expr,
 668     location: IdentLocation,
 669     new_ident: Ident,
 670     applicability: &mut Applicability,
 671 ) -> Option<String> {
 672     get_ident(expr, location).and_then(|current_ident| {
 673         if eq_id(current_ident, new_ident) {
 674             // We never want to suggest a non-change
 675             return None;
 676         }
 677
 678         Some(format!(
 679             "{}{}{}",
 680             snippet_with_applicability(cx, expr.span.with_hi(current_ident.span.lo()), "..", applicability),
 681             new_ident,
 682             snippet_with_applicability(cx, expr.span.with_lo(current_ident.span.hi()), "..", applicability),
 683         ))
 684     })
 685 }
 686
 687 fn skip_index<A, Iter>(iter: Iter, index: usize) -> impl Iterator<Item = A>
 688 where
 689     Iter: Iterator<Item = A>,
 690 {
 691     iter.enumerate()
 692         .filter_map(move |(i, a)| if i == index { None } else { Some(a) })
 693 }