clippy_lints/src/swap.rs

   1 use crate::utils::sugg::Sugg;
   2 use crate::utils::{
   3     differing_macro_contexts, eq_expr_value, is_type_diagnostic_item, snippet_with_applicability, span_lint_and_then,
   4 };
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{Block, Expr, ExprKind, PatKind, QPath, StmtKind};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::ty;
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use rustc_span::sym;
  12
  13 declare_clippy_lint! {
  14     /// **What it does:** Checks for manual swapping.
  15     ///
  16     /// **Why is this bad?** The `std::mem::swap` function exposes the intent better
  17     /// without deinitializing or copying either variable.
  18     ///
  19     /// **Known problems:** None.
  20     ///
  21     /// **Example:**
  22     /// ```rust
  23     /// let mut a = 42;
  24     /// let mut b = 1337;
  25     ///
  26     /// let t = b;
  27     /// b = a;
  28     /// a = t;
  29     /// ```
  30     /// Use std::mem::swap():
  31     /// ```rust
  32     /// let mut a = 1;
  33     /// let mut b = 2;
  34     /// std::mem::swap(&mut a, &mut b);
  35     /// ```
  36     pub MANUAL_SWAP,
  37     complexity,
  38     "manual swap of two variables"
  39 }
  40
  41 declare_clippy_lint! {
  42     /// **What it does:** Checks for `foo = bar; bar = foo` sequences.
  43     ///
  44     /// **Why is this bad?** This looks like a failed attempt to swap.
  45     ///
  46     /// **Known problems:** None.
  47     ///
  48     /// **Example:**
  49     /// ```rust
  50     /// # let mut a = 1;
  51     /// # let mut b = 2;
  52     /// a = b;
  53     /// b = a;
  54     /// ```
  55     /// If swapping is intended, use `swap()` instead:
  56     /// ```rust
  57     /// # let mut a = 1;
  58     /// # let mut b = 2;
  59     /// std::mem::swap(&mut a, &mut b);
  60     /// ```
  61     pub ALMOST_SWAPPED,
  62     correctness,
  63     "`foo = bar; bar = foo` sequence"
  64 }
  65
  66 declare_lint_pass!(Swap => [MANUAL_SWAP, ALMOST_SWAPPED]);
  67
  68 impl<'tcx> LateLintPass<'tcx> for Swap {
  69     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'_>) {
  70         check_manual_swap(cx, block);
  71         check_suspicious_swap(cx, block);
  72     }
  73 }
  74
  75 /// Implementation of the `MANUAL_SWAP` lint.
  76 fn check_manual_swap(cx: &LateContext<'_>, block: &Block<'_>) {
  77     for w in block.stmts.windows(3) {
  78         if_chain! {
  79             // let t = foo();
  80             if let StmtKind::Local(ref tmp) = w[0].kind;
  81             if let Some(ref tmp_init) = tmp.init;
  82             if let PatKind::Binding(.., ident, None) = tmp.pat.kind;
  83
  84             // foo() = bar();
  85             if let StmtKind::Semi(ref first) = w[1].kind;
  86             if let ExprKind::Assign(ref lhs1, ref rhs1, _) = first.kind;
  87
  88             // bar() = t;
  89             if let StmtKind::Semi(ref second) = w[2].kind;
  90             if let ExprKind::Assign(ref lhs2, ref rhs2, _) = second.kind;
  91             if let ExprKind::Path(QPath::Resolved(None, ref rhs2)) = rhs2.kind;
  92             if rhs2.segments.len() == 1;
  93
  94             if ident.name == rhs2.segments[0].ident.name;
  95             if eq_expr_value(cx, tmp_init, lhs1);
  96             if eq_expr_value(cx, rhs1, lhs2);
  97             then {
  98                 if let ExprKind::Field(ref lhs1, _) = lhs1.kind {
  99                     if let ExprKind::Field(ref lhs2, _) = lhs2.kind {
 100                         if lhs1.hir_id.owner == lhs2.hir_id.owner {
 101                             return;
 102                         }
 103                     }
 104                 }
 105
 106                 let mut applicability = Applicability::MachineApplicable;
 107
 108                 let slice = check_for_slice(cx, lhs1, lhs2);
 109                 let (replace, what, sugg) = if let Slice::NotSwappable = slice {
 110                     return;
 111                 } else if let Slice::Swappable(slice, idx1, idx2) = slice {
 112                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 113                         (
 114                             false,
 115                             format!(" elements of `{}`", slice),
 116                             format!(
 117                                 "{}.swap({}, {})",
 118                                 slice.maybe_par(),
 119                                 snippet_with_applicability(cx, idx1.span, "..", &mut applicability),
 120                                 snippet_with_applicability(cx, idx2.span, "..", &mut applicability),
 121                             ),
 122                         )
 123                     } else {
 124                         (false, String::new(), String::new())
 125                     }
 126                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 127                     (
 128                         true,
 129                         format!(" `{}` and `{}`", first, second),
 130                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()),
 131                     )
 132                 } else {
 133                     (true, String::new(), String::new())
 134                 };
 135
 136                 let span = w[0].span.to(second.span);
 137
 138                 span_lint_and_then(
 139                     cx,
 140                     MANUAL_SWAP,
 141                     span,
 142                     &format!("this looks like you are swapping{} manually", what),
 143                     |diag| {
 144                         if !sugg.is_empty() {
 145                             diag.span_suggestion(
 146                                 span,
 147                                 "try",
 148                                 sugg,
 149                                 applicability,
 150                             );
 151
 152                             if replace {
 153                                 diag.note("or maybe you should use `std::mem::replace`?");
 154                             }
 155                         }
 156                     }
 157                 );
 158             }
 159         }
 160     }
 161 }
 162
 163 enum Slice<'a> {
 164     /// `slice.swap(idx1, idx2)` can be used
 165     ///
 166     /// ## Example
 167     ///
 168     /// ```rust
 169     /// # let mut a = vec![0, 1];
 170     /// let t = a[1];
 171     /// a[1] = a[0];
 172     /// a[0] = t;
 173     /// // can be written as
 174     /// a.swap(0, 1);
 175     /// ```
 176     Swappable(&'a Expr<'a>, &'a Expr<'a>, &'a Expr<'a>),
 177     /// The `swap` function cannot be used.
 178     ///
 179     /// ## Example
 180     ///
 181     /// ```rust
 182     /// # let mut a = [vec![1, 2], vec![3, 4]];
 183     /// let t = a[0][1];
 184     /// a[0][1] = a[1][0];
 185     /// a[1][0] = t;
 186     /// ```
 187     NotSwappable,
 188     /// Not a slice
 189     None,
 190 }
 191
 192 /// Checks if both expressions are index operations into "slice-like" types.
 193 fn check_for_slice<'a>(cx: &LateContext<'_>, lhs1: &'a Expr<'_>, lhs2: &'a Expr<'_>) -> Slice<'a> {
 194     if let ExprKind::Index(ref lhs1, ref idx1) = lhs1.kind {
 195         if let ExprKind::Index(ref lhs2, ref idx2) = lhs2.kind {
 196             if eq_expr_value(cx, lhs1, lhs2) {
 197                 let ty = cx.typeck_results().expr_ty(lhs1).peel_refs();
 198
 199                 if matches!(ty.kind(), ty::Slice(_))
 200                     || matches!(ty.kind(), ty::Array(_, _))
 201                     || is_type_diagnostic_item(cx, ty, sym::vec_type)
 202                     || is_type_diagnostic_item(cx, ty, sym!(vecdeque_type))
 203                 {
 204                     return Slice::Swappable(lhs1, idx1, idx2);
 205                 }
 206             } else {
 207                 return Slice::NotSwappable;
 208             }
 209         }
 210     }
 211
 212     Slice::None
 213 }
 214
 215 /// Implementation of the `ALMOST_SWAPPED` lint.
 216 fn check_suspicious_swap(cx: &LateContext<'_>, block: &Block<'_>) {
 217     for w in block.stmts.windows(2) {
 218         if_chain! {
 219             if let StmtKind::Semi(ref first) = w[0].kind;
 220             if let StmtKind::Semi(ref second) = w[1].kind;
 221             if !differing_macro_contexts(first.span, second.span);
 222             if let ExprKind::Assign(ref lhs0, ref rhs0, _) = first.kind;
 223             if let ExprKind::Assign(ref lhs1, ref rhs1, _) = second.kind;
 224             if eq_expr_value(cx, lhs0, rhs1);
 225             if eq_expr_value(cx, lhs1, rhs0);
 226             then {
 227                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 228                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 229                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 230                     (
 231                         format!(" `{}` and `{}`", first, second),
 232                         first.mut_addr().to_string(),
 233                         second.mut_addr().to_string(),
 234                     )
 235                 } else {
 236                     (String::new(), String::new(), String::new())
 237                 };
 238
 239                 let span = first.span.to(second.span);
 240
 241                 span_lint_and_then(cx,
 242                                    ALMOST_SWAPPED,
 243                                    span,
 244                                    &format!("this looks like you are trying to swap{}", what),
 245                                    |diag| {
 246                                        if !what.is_empty() {
 247                                            diag.span_suggestion(
 248                                                span,
 249                                                "try",
 250                                                format!(
 251                                                    "std::mem::swap({}, {})",
 252                                                    lhs,
 253                                                    rhs,
 254                                                ),
 255                                                Applicability::MaybeIncorrect,
 256                                            );
 257                                            diag.note("or maybe you should use `std::mem::replace`?");
 258                                        }
 259                                    });
 260             }
 261         }
 262     }
 263 }