clippy_lints/src/swap.rs

   1 use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
   2 use clippy_utils::source::snippet_with_applicability;
   3 use clippy_utils::sugg::Sugg;
   4 use clippy_utils::ty::is_type_diagnostic_item;
   5 use clippy_utils::{can_mut_borrow_both, differing_macro_contexts, eq_expr_value};
   6 use if_chain::if_chain;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::{BinOpKind, Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind};
   9 use rustc_lint::{LateContext, LateLintPass};
  10 use rustc_middle::ty;
  11 use rustc_session::{declare_lint_pass, declare_tool_lint};
  12 use rustc_span::source_map::Spanned;
  13 use rustc_span::{sym, Span};
  14
  15 declare_clippy_lint! {
  16     /// ### What it does
  17     /// Checks for manual swapping.
  18     ///
  19     /// ### Why is this bad?
  20     /// The `std::mem::swap` function exposes the intent better
  21     /// without deinitializing or copying either variable.
  22     ///
  23     /// ### Example
  24     /// ```rust
  25     /// let mut a = 42;
  26     /// let mut b = 1337;
  27     ///
  28     /// let t = b;
  29     /// b = a;
  30     /// a = t;
  31     /// ```
  32     /// Use std::mem::swap():
  33     /// ```rust
  34     /// let mut a = 1;
  35     /// let mut b = 2;
  36     /// std::mem::swap(&mut a, &mut b);
  37     /// ```
  38     pub MANUAL_SWAP,
  39     complexity,
  40     "manual swap of two variables"
  41 }
  42
  43 declare_clippy_lint! {
  44     /// ### What it does
  45     /// Checks for `foo = bar; bar = foo` sequences.
  46     ///
  47     /// ### Why is this bad?
  48     /// This looks like a failed attempt to swap.
  49     ///
  50     /// ### Example
  51     /// ```rust
  52     /// # let mut a = 1;
  53     /// # let mut b = 2;
  54     /// a = b;
  55     /// b = a;
  56     /// ```
  57     /// If swapping is intended, use `swap()` instead:
  58     /// ```rust
  59     /// # let mut a = 1;
  60     /// # let mut b = 2;
  61     /// std::mem::swap(&mut a, &mut b);
  62     /// ```
  63     pub ALMOST_SWAPPED,
  64     correctness,
  65     "`foo = bar; bar = foo` sequence"
  66 }
  67
  68 declare_lint_pass!(Swap => [MANUAL_SWAP, ALMOST_SWAPPED]);
  69
  70 impl<'tcx> LateLintPass<'tcx> for Swap {
  71     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'_>) {
  72         check_manual_swap(cx, block);
  73         check_suspicious_swap(cx, block);
  74         check_xor_swap(cx, block);
  75     }
  76 }
  77
  78 fn generate_swap_warning(cx: &LateContext<'_>, e1: &Expr<'_>, e2: &Expr<'_>, span: Span, is_xor_based: bool) {
  79     let mut applicability = Applicability::MachineApplicable;
  80
  81     if !can_mut_borrow_both(cx, e1, e2) {
  82         if let ExprKind::Index(lhs1, idx1) = e1.kind {
  83             if let ExprKind::Index(lhs2, idx2) = e2.kind {
  84                 if eq_expr_value(cx, lhs1, lhs2) {
  85                     let ty = cx.typeck_results().expr_ty(lhs1).peel_refs();
  86
  87                     if matches!(ty.kind(), ty::Slice(_))
  88                         || matches!(ty.kind(), ty::Array(_, _))
  89                         || is_type_diagnostic_item(cx, ty, sym::Vec)
  90                         || is_type_diagnostic_item(cx, ty, sym::VecDeque)
  91                     {
  92                         let slice = Sugg::hir_with_applicability(cx, lhs1, "<slice>", &mut applicability);
  93                         span_lint_and_sugg(
  94                             cx,
  95                             MANUAL_SWAP,
  96                             span,
  97                             &format!("this looks like you are swapping elements of `{}` manually", slice),
  98                             "try",
  99                             format!(
 100                                 "{}.swap({}, {})",
 101                                 slice.maybe_par(),
 102                                 snippet_with_applicability(cx, idx1.span, "..", &mut applicability),
 103                                 snippet_with_applicability(cx, idx2.span, "..", &mut applicability),
 104                             ),
 105                             applicability,
 106                         );
 107                     }
 108                 }
 109             }
 110         }
 111         return;
 112     }
 113
 114     let first = Sugg::hir_with_applicability(cx, e1, "..", &mut applicability);
 115     let second = Sugg::hir_with_applicability(cx, e2, "..", &mut applicability);
 116     span_lint_and_then(
 117         cx,
 118         MANUAL_SWAP,
 119         span,
 120         &format!("this looks like you are swapping `{}` and `{}` manually", first, second),
 121         |diag| {
 122             diag.span_suggestion(
 123                 span,
 124                 "try",
 125                 format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()),
 126                 applicability,
 127             );
 128             if !is_xor_based {
 129                 diag.note("or maybe you should use `std::mem::replace`?");
 130             }
 131         },
 132     );
 133 }
 134
 135 /// Implementation of the `MANUAL_SWAP` lint.
 136 fn check_manual_swap(cx: &LateContext<'_>, block: &Block<'_>) {
 137     for w in block.stmts.windows(3) {
 138         if_chain! {
 139             // let t = foo();
 140             if let StmtKind::Local(tmp) = w[0].kind;
 141             if let Some(tmp_init) = tmp.init;
 142             if let PatKind::Binding(.., ident, None) = tmp.pat.kind;
 143
 144             // foo() = bar();
 145             if let StmtKind::Semi(first) = w[1].kind;
 146             if let ExprKind::Assign(lhs1, rhs1, _) = first.kind;
 147
 148             // bar() = t;
 149             if let StmtKind::Semi(second) = w[2].kind;
 150             if let ExprKind::Assign(lhs2, rhs2, _) = second.kind;
 151             if let ExprKind::Path(QPath::Resolved(None, rhs2)) = rhs2.kind;
 152             if rhs2.segments.len() == 1;
 153
 154             if ident.name == rhs2.segments[0].ident.name;
 155             if eq_expr_value(cx, tmp_init, lhs1);
 156             if eq_expr_value(cx, rhs1, lhs2);
 157             then {
 158                 let span = w[0].span.to(second.span);
 159                 generate_swap_warning(cx, lhs1, lhs2, span, false);
 160             }
 161         }
 162     }
 163 }
 164
 165 /// Implementation of the `ALMOST_SWAPPED` lint.
 166 fn check_suspicious_swap(cx: &LateContext<'_>, block: &Block<'_>) {
 167     for w in block.stmts.windows(2) {
 168         if_chain! {
 169             if let StmtKind::Semi(first) = w[0].kind;
 170             if let StmtKind::Semi(second) = w[1].kind;
 171             if !differing_macro_contexts(first.span, second.span);
 172             if let ExprKind::Assign(lhs0, rhs0, _) = first.kind;
 173             if let ExprKind::Assign(lhs1, rhs1, _) = second.kind;
 174             if eq_expr_value(cx, lhs0, rhs1);
 175             if eq_expr_value(cx, lhs1, rhs0);
 176             then {
 177                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 178                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 179                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 180                     (
 181                         format!(" `{}` and `{}`", first, second),
 182                         first.mut_addr().to_string(),
 183                         second.mut_addr().to_string(),
 184                     )
 185                 } else {
 186                     (String::new(), String::new(), String::new())
 187                 };
 188
 189                 let span = first.span.to(second.span);
 190
 191                 span_lint_and_then(cx,
 192                                    ALMOST_SWAPPED,
 193                                    span,
 194                                    &format!("this looks like you are trying to swap{}", what),
 195                                    |diag| {
 196                                        if !what.is_empty() {
 197                                            diag.span_suggestion(
 198                                                span,
 199                                                "try",
 200                                                format!(
 201                                                    "std::mem::swap({}, {})",
 202                                                    lhs,
 203                                                    rhs,
 204                                                ),
 205                                                Applicability::MaybeIncorrect,
 206                                            );
 207                                            diag.note("or maybe you should use `std::mem::replace`?");
 208                                        }
 209                                    });
 210             }
 211         }
 212     }
 213 }
 214
 215 /// Implementation of the xor case for `MANUAL_SWAP` lint.
 216 fn check_xor_swap(cx: &LateContext<'_>, block: &Block<'_>) {
 217     for window in block.stmts.windows(3) {
 218         if_chain! {
 219             if let Some((lhs0, rhs0)) = extract_sides_of_xor_assign(&window[0]);
 220             if let Some((lhs1, rhs1)) = extract_sides_of_xor_assign(&window[1]);
 221             if let Some((lhs2, rhs2)) = extract_sides_of_xor_assign(&window[2]);
 222             if eq_expr_value(cx, lhs0, rhs1);
 223             if eq_expr_value(cx, lhs2, rhs1);
 224             if eq_expr_value(cx, lhs1, rhs0);
 225             if eq_expr_value(cx, lhs1, rhs2);
 226             then {
 227                 let span = window[0].span.to(window[2].span);
 228                 generate_swap_warning(cx, lhs0, rhs0, span, true);
 229             }
 230         };
 231     }
 232 }
 233
 234 /// Returns the lhs and rhs of an xor assignment statement.
 235 fn extract_sides_of_xor_assign<'a, 'hir>(stmt: &'a Stmt<'hir>) -> Option<(&'a Expr<'hir>, &'a Expr<'hir>)> {
 236     if let StmtKind::Semi(expr) = stmt.kind {
 237         if let ExprKind::AssignOp(
 238             Spanned {
 239                 node: BinOpKind::BitXor,
 240                 ..
 241             },
 242             lhs,
 243             rhs,
 244         ) = expr.kind
 245         {
 246             return Some((lhs, rhs));
 247         }
 248     }
 249     None
 250 }