clippy_lints/src/swap.rs

   1 use crate::utils::sugg::Sugg;
   2 use crate::utils::{
   3     differing_macro_contexts, is_type_diagnostic_item, match_type, paths, snippet_with_applicability,
   4     span_lint_and_then, walk_ptrs_ty, SpanlessEq,
   5 };
   6 use if_chain::if_chain;
   7 use matches::matches;
   8 use rustc::ty;
   9 use rustc_errors::Applicability;
  10 use rustc_hir::*;
  11 use rustc_lint::{LateContext, LateLintPass};
  12 use rustc_session::{declare_lint_pass, declare_tool_lint};
  13 use rustc_span::Symbol;
  14
  15 declare_clippy_lint! {
  16     /// **What it does:** Checks for manual swapping.
  17     ///
  18     /// **Why is this bad?** The `std::mem::swap` function exposes the intent better
  19     /// without deinitializing or copying either variable.
  20     ///
  21     /// **Known problems:** None.
  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:** Checks for `foo = bar; bar = foo` sequences.
  45     ///
  46     /// **Why is this bad?** This looks like a failed attempt to swap.
  47     ///
  48     /// **Known problems:** None.
  49     ///
  50     /// **Example:**
  51     /// ```rust
  52     /// # let mut a = 1;
  53     /// # let mut b = 2;
  54     /// a = b;
  55     /// b = a;
  56     /// ```
  57     /// Could be written as:
  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<'a, 'tcx> LateLintPass<'a, 'tcx> for Swap {
  71     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx Block<'_>) {
  72         check_manual_swap(cx, block);
  73         check_suspicious_swap(cx, block);
  74     }
  75 }
  76
  77 /// Implementation of the `MANUAL_SWAP` lint.
  78 fn check_manual_swap(cx: &LateContext<'_, '_>, block: &Block<'_>) {
  79     for w in block.stmts.windows(3) {
  80         if_chain! {
  81             // let t = foo();
  82             if let StmtKind::Local(ref tmp) = w[0].kind;
  83             if let Some(ref tmp_init) = tmp.init;
  84             if let PatKind::Binding(.., ident, None) = tmp.pat.kind;
  85
  86             // foo() = bar();
  87             if let StmtKind::Semi(ref first) = w[1].kind;
  88             if let ExprKind::Assign(ref lhs1, ref rhs1, _) = first.kind;
  89
  90             // bar() = t;
  91             if let StmtKind::Semi(ref second) = w[2].kind;
  92             if let ExprKind::Assign(ref lhs2, ref rhs2, _) = second.kind;
  93             if let ExprKind::Path(QPath::Resolved(None, ref rhs2)) = rhs2.kind;
  94             if rhs2.segments.len() == 1;
  95
  96             if ident.as_str() == rhs2.segments[0].ident.as_str();
  97             if SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1);
  98             if SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2);
  99             then {
 100                 if let ExprKind::Field(ref lhs1, _) = lhs1.kind {
 101                     if let ExprKind::Field(ref lhs2, _) = lhs2.kind {
 102                         if lhs1.hir_id.owner_def_id() == lhs2.hir_id.owner_def_id() {
 103                             return;
 104                         }
 105                     }
 106                 }
 107
 108                 let mut applicability = Applicability::MachineApplicable;
 109
 110                 let slice = check_for_slice(cx, lhs1, lhs2);
 111                 let (replace, what, sugg) = if let Slice::NotSwappable = slice {
 112                     return;
 113                 } else if let Slice::Swappable(slice, idx1, idx2) = slice {
 114                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 115                         (
 116                             false,
 117                             format!(" elements of `{}`", slice),
 118                             format!(
 119                                 "{}.swap({}, {})",
 120                                 slice.maybe_par(),
 121                                 snippet_with_applicability(cx, idx1.span, "..", &mut applicability),
 122                                 snippet_with_applicability(cx, idx2.span, "..", &mut applicability),
 123                             ),
 124                         )
 125                     } else {
 126                         (false, String::new(), String::new())
 127                     }
 128                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 129                     (
 130                         true,
 131                         format!(" `{}` and `{}`", first, second),
 132                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()),
 133                     )
 134                 } else {
 135                     (true, String::new(), String::new())
 136                 };
 137
 138                 let span = w[0].span.to(second.span);
 139
 140                 span_lint_and_then(
 141                     cx,
 142                     MANUAL_SWAP,
 143                     span,
 144                     &format!("this looks like you are swapping{} manually", what),
 145                     |db| {
 146                         if !sugg.is_empty() {
 147                             db.span_suggestion(
 148                                 span,
 149                                 "try",
 150                                 sugg,
 151                                 applicability,
 152                             );
 153
 154                             if replace {
 155                                 db.note("or maybe you should use `std::mem::replace`?");
 156                             }
 157                         }
 158                     }
 159                 );
 160             }
 161         }
 162     }
 163 }
 164
 165 enum Slice<'a> {
 166     /// `slice.swap(idx1, idx2)` can be used
 167     ///
 168     /// ## Example
 169     ///
 170     /// ```rust
 171     /// # let mut a = vec![0, 1];
 172     /// let t = a[1];
 173     /// a[1] = a[0];
 174     /// a[0] = t;
 175     /// // can be written as
 176     /// a.swap(0, 1);
 177     /// ```
 178     Swappable(&'a Expr<'a>, &'a Expr<'a>, &'a Expr<'a>),
 179     /// The `swap` function cannot be used.
 180     ///
 181     /// ## Example
 182     ///
 183     /// ```rust
 184     /// # let mut a = [vec![1, 2], vec![3, 4]];
 185     /// let t = a[0][1];
 186     /// a[0][1] = a[1][0];
 187     /// a[1][0] = t;
 188     /// ```
 189     NotSwappable,
 190     /// Not a slice
 191     None,
 192 }
 193
 194 /// Checks if both expressions are index operations into "slice-like" types.
 195 fn check_for_slice<'a>(cx: &LateContext<'_, '_>, lhs1: &'a Expr<'_>, lhs2: &'a Expr<'_>) -> Slice<'a> {
 196     if let ExprKind::Index(ref lhs1, ref idx1) = lhs1.kind {
 197         if let ExprKind::Index(ref lhs2, ref idx2) = lhs2.kind {
 198             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
 199                 let ty = walk_ptrs_ty(cx.tables.expr_ty(lhs1));
 200
 201                 if matches!(ty.kind, ty::Slice(_))
 202                     || matches!(ty.kind, ty::Array(_, _))
 203                     || is_type_diagnostic_item(cx, ty, Symbol::intern("vec_type"))
 204                     || match_type(cx, ty, &paths::VEC_DEQUE)
 205                 {
 206                     return Slice::Swappable(lhs1, idx1, idx2);
 207                 }
 208             } else {
 209                 return Slice::NotSwappable;
 210             }
 211         }
 212     }
 213
 214     Slice::None
 215 }
 216
 217 /// Implementation of the `ALMOST_SWAPPED` lint.
 218 fn check_suspicious_swap(cx: &LateContext<'_, '_>, block: &Block<'_>) {
 219     for w in block.stmts.windows(2) {
 220         if_chain! {
 221             if let StmtKind::Semi(ref first) = w[0].kind;
 222             if let StmtKind::Semi(ref second) = w[1].kind;
 223             if !differing_macro_contexts(first.span, second.span);
 224             if let ExprKind::Assign(ref lhs0, ref rhs0, _) = first.kind;
 225             if let ExprKind::Assign(ref lhs1, ref rhs1, _) = second.kind;
 226             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1);
 227             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0);
 228             then {
 229                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 230                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 231                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 232                     (
 233                         format!(" `{}` and `{}`", first, second),
 234                         first.mut_addr().to_string(),
 235                         second.mut_addr().to_string(),
 236                     )
 237                 } else {
 238                     (String::new(), String::new(), String::new())
 239                 };
 240
 241                 let span = first.span.to(second.span);
 242
 243                 span_lint_and_then(cx,
 244                                    ALMOST_SWAPPED,
 245                                    span,
 246                                    &format!("this looks like you are trying to swap{}", what),
 247                                    |db| {
 248                                        if !what.is_empty() {
 249                                            db.span_suggestion(
 250                                                span,
 251                                                "try",
 252                                                format!(
 253                                                    "std::mem::swap({}, {})",
 254                                                    lhs,
 255                                                    rhs,
 256                                                ),
 257                                                Applicability::MaybeIncorrect,
 258                                            );
 259                                            db.note("or maybe you should use `std::mem::replace`?");
 260                                        }
 261                                    });
 262             }
 263         }
 264     }
 265 }