clippy_lints/src/swap.rs

   1 use clippy_utils::diagnostics::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::{differing_macro_contexts, eq_expr_value};
   6 use if_chain::if_chain;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::{Block, Expr, ExprKind, PatKind, QPath, 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::sym;
  13
  14 declare_clippy_lint! {
  15     /// ### What it does
  16     /// Checks for manual swapping.
  17     ///
  18     /// ### Why is this bad?
  19     /// The `std::mem::swap` function exposes the intent better
  20     /// without deinitializing or copying either variable.
  21     ///
  22     /// ### Example
  23     /// ```rust
  24     /// let mut a = 42;
  25     /// let mut b = 1337;
  26     ///
  27     /// let t = b;
  28     /// b = a;
  29     /// a = t;
  30     /// ```
  31     /// Use std::mem::swap():
  32     /// ```rust
  33     /// let mut a = 1;
  34     /// let mut b = 2;
  35     /// std::mem::swap(&mut a, &mut b);
  36     /// ```
  37     pub MANUAL_SWAP,
  38     complexity,
  39     "manual swap of two variables"
  40 }
  41
  42 declare_clippy_lint! {
  43     /// ### What it does
  44     /// Checks for `foo = bar; bar = foo` sequences.
  45     ///
  46     /// ### Why is this bad?
  47     /// This looks like a failed attempt to swap.
  48     ///
  49     /// ### Example
  50     /// ```rust
  51     /// # let mut a = 1;
  52     /// # let mut b = 2;
  53     /// a = b;
  54     /// b = a;
  55     /// ```
  56     /// If swapping is intended, use `swap()` instead:
  57     /// ```rust
  58     /// # let mut a = 1;
  59     /// # let mut b = 2;
  60     /// std::mem::swap(&mut a, &mut b);
  61     /// ```
  62     pub ALMOST_SWAPPED,
  63     correctness,
  64     "`foo = bar; bar = foo` sequence"
  65 }
  66
  67 declare_lint_pass!(Swap => [MANUAL_SWAP, ALMOST_SWAPPED]);
  68
  69 impl<'tcx> LateLintPass<'tcx> for Swap {
  70     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'_>) {
  71         check_manual_swap(cx, block);
  72         check_suspicious_swap(cx, block);
  73     }
  74 }
  75
  76 /// Implementation of the `MANUAL_SWAP` lint.
  77 fn check_manual_swap(cx: &LateContext<'_>, block: &Block<'_>) {
  78     for w in block.stmts.windows(3) {
  79         if_chain! {
  80             // let t = foo();
  81             if let StmtKind::Local(tmp) = w[0].kind;
  82             if let Some(tmp_init) = tmp.init;
  83             if let PatKind::Binding(.., ident, None) = tmp.pat.kind;
  84
  85             // foo() = bar();
  86             if let StmtKind::Semi(first) = w[1].kind;
  87             if let ExprKind::Assign(lhs1, rhs1, _) = first.kind;
  88
  89             // bar() = t;
  90             if let StmtKind::Semi(second) = w[2].kind;
  91             if let ExprKind::Assign(lhs2, rhs2, _) = second.kind;
  92             if let ExprKind::Path(QPath::Resolved(None, rhs2)) = rhs2.kind;
  93             if rhs2.segments.len() == 1;
  94
  95             if ident.name == rhs2.segments[0].ident.name;
  96             if eq_expr_value(cx, tmp_init, lhs1);
  97             if eq_expr_value(cx, rhs1, lhs2);
  98             then {
  99                 if let ExprKind::Field(lhs1, _) = lhs1.kind {
 100                     if let ExprKind::Field(lhs2, _) = lhs2.kind {
 101                         if lhs1.hir_id.owner == lhs2.hir_id.owner {
 102                             return;
 103                         }
 104                     }
 105                 }
 106
 107                 let mut applicability = Applicability::MachineApplicable;
 108
 109                 let slice = check_for_slice(cx, lhs1, lhs2);
 110                 let (replace, what, sugg) = if let Slice::NotSwappable = slice {
 111                     return;
 112                 } else if let Slice::Swappable(slice, idx1, idx2) = slice {
 113                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 114                         (
 115                             false,
 116                             format!(" elements of `{}`", slice),
 117                             format!(
 118                                 "{}.swap({}, {})",
 119                                 slice.maybe_par(),
 120                                 snippet_with_applicability(cx, idx1.span, "..", &mut applicability),
 121                                 snippet_with_applicability(cx, idx2.span, "..", &mut applicability),
 122                             ),
 123                         )
 124                     } else {
 125                         (false, String::new(), String::new())
 126                     }
 127                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 128                     (
 129                         true,
 130                         format!(" `{}` and `{}`", first, second),
 131                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()),
 132                     )
 133                 } else {
 134                     (true, String::new(), String::new())
 135                 };
 136
 137                 let span = w[0].span.to(second.span);
 138
 139                 span_lint_and_then(
 140                     cx,
 141                     MANUAL_SWAP,
 142                     span,
 143                     &format!("this looks like you are swapping{} manually", what),
 144                     |diag| {
 145                         if !sugg.is_empty() {
 146                             diag.span_suggestion(
 147                                 span,
 148                                 "try",
 149                                 sugg,
 150                                 applicability,
 151                             );
 152
 153                             if replace {
 154                                 diag.note("or maybe you should use `std::mem::replace`?");
 155                             }
 156                         }
 157                     }
 158                 );
 159             }
 160         }
 161     }
 162 }
 163
 164 enum Slice<'a> {
 165     /// `slice.swap(idx1, idx2)` can be used
 166     ///
 167     /// ## Example
 168     ///
 169     /// ```rust
 170     /// # let mut a = vec![0, 1];
 171     /// let t = a[1];
 172     /// a[1] = a[0];
 173     /// a[0] = t;
 174     /// // can be written as
 175     /// a.swap(0, 1);
 176     /// ```
 177     Swappable(&'a Expr<'a>, &'a Expr<'a>, &'a Expr<'a>),
 178     /// The `swap` function cannot be used.
 179     ///
 180     /// ## Example
 181     ///
 182     /// ```rust
 183     /// # let mut a = [vec![1, 2], vec![3, 4]];
 184     /// let t = a[0][1];
 185     /// a[0][1] = a[1][0];
 186     /// a[1][0] = t;
 187     /// ```
 188     NotSwappable,
 189     /// Not a slice
 190     None,
 191 }
 192
 193 /// Checks if both expressions are index operations into "slice-like" types.
 194 fn check_for_slice<'a>(cx: &LateContext<'_>, lhs1: &'a Expr<'_>, lhs2: &'a Expr<'_>) -> Slice<'a> {
 195     if let ExprKind::Index(lhs1, idx1) = lhs1.kind {
 196         if let ExprKind::Index(lhs2, idx2) = lhs2.kind {
 197             if eq_expr_value(cx, lhs1, lhs2) {
 198                 let ty = cx.typeck_results().expr_ty(lhs1).peel_refs();
 199
 200                 if matches!(ty.kind(), ty::Slice(_))
 201                     || matches!(ty.kind(), ty::Array(_, _))
 202                     || is_type_diagnostic_item(cx, ty, sym::vec_type)
 203                     || is_type_diagnostic_item(cx, ty, sym::vecdeque_type)
 204                 {
 205                     return Slice::Swappable(lhs1, idx1, idx2);
 206                 }
 207             } else {
 208                 return Slice::NotSwappable;
 209             }
 210         }
 211     }
 212
 213     Slice::None
 214 }
 215
 216 /// Implementation of the `ALMOST_SWAPPED` lint.
 217 fn check_suspicious_swap(cx: &LateContext<'_>, block: &Block<'_>) {
 218     for w in block.stmts.windows(2) {
 219         if_chain! {
 220             if let StmtKind::Semi(first) = w[0].kind;
 221             if let StmtKind::Semi(second) = w[1].kind;
 222             if !differing_macro_contexts(first.span, second.span);
 223             if let ExprKind::Assign(lhs0, rhs0, _) = first.kind;
 224             if let ExprKind::Assign(lhs1, rhs1, _) = second.kind;
 225             if eq_expr_value(cx, lhs0, rhs1);
 226             if eq_expr_value(cx, lhs1, rhs0);
 227             then {
 228                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 229                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 230                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 231                     (
 232                         format!(" `{}` and `{}`", first, second),
 233                         first.mut_addr().to_string(),
 234                         second.mut_addr().to_string(),
 235                     )
 236                 } else {
 237                     (String::new(), String::new(), String::new())
 238                 };
 239
 240                 let span = first.span.to(second.span);
 241
 242                 span_lint_and_then(cx,
 243                                    ALMOST_SWAPPED,
 244                                    span,
 245                                    &format!("this looks like you are trying to swap{}", what),
 246                                    |diag| {
 247                                        if !what.is_empty() {
 248                                            diag.span_suggestion(
 249                                                span,
 250                                                "try",
 251                                                format!(
 252                                                    "std::mem::swap({}, {})",
 253                                                    lhs,
 254                                                    rhs,
 255                                                ),
 256                                                Applicability::MaybeIncorrect,
 257                                            );
 258                                            diag.note("or maybe you should use `std::mem::replace`?");
 259                                        }
 260                                    });
 261             }
 262         }
 263     }
 264 }