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::declare_lint_pass;
   9 use rustc::hir::*;
  10 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  11 use rustc::ty;
  12 use rustc_errors::Applicability;
  13 use rustc_session::declare_tool_lint;
  14 use syntax_pos::Symbol;
  15
  16 declare_clippy_lint! {
  17     /// **What it does:** Checks for manual swapping.
  18     ///
  19     /// **Why is this bad?** The `std::mem::swap` function exposes the intent better
  20     /// without deinitializing or copying either variable.
  21     ///
  22     /// **Known problems:** None.
  23     ///
  24     /// **Example:**
  25     /// ```rust
  26     /// let mut a = 42;
  27     /// let mut b = 1337;
  28     ///
  29     /// let t = b;
  30     /// b = a;
  31     /// a = t;
  32     /// ```
  33     /// Use std::mem::swap():
  34     /// ```rust
  35     /// let mut a = 1;
  36     /// let mut b = 2;
  37     /// std::mem::swap(&mut a, &mut b);
  38     /// ```
  39     pub MANUAL_SWAP,
  40     complexity,
  41     "manual swap of two variables"
  42 }
  43
  44 declare_clippy_lint! {
  45     /// **What it does:** Checks for `foo = bar; bar = foo` sequences.
  46     ///
  47     /// **Why is this bad?** This looks like a failed attempt to swap.
  48     ///
  49     /// **Known problems:** None.
  50     ///
  51     /// **Example:**
  52     /// ```rust
  53     /// # let mut a = 1;
  54     /// # let mut b = 2;
  55     /// a = b;
  56     /// b = a;
  57     /// ```
  58     /// Could be written as:
  59     /// ```rust
  60     /// # let mut a = 1;
  61     /// # let mut b = 2;
  62     /// std::mem::swap(&mut a, &mut b);
  63     /// ```
  64     pub ALMOST_SWAPPED,
  65     correctness,
  66     "`foo = bar; bar = foo` sequence"
  67 }
  68
  69 declare_lint_pass!(Swap => [MANUAL_SWAP, ALMOST_SWAPPED]);
  70
  71 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Swap {
  72     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx Block) {
  73         check_manual_swap(cx, block);
  74         check_suspicious_swap(cx, block);
  75     }
  76 }
  77
  78 /// Implementation of the `MANUAL_SWAP` lint.
  79 fn check_manual_swap(cx: &LateContext<'_, '_>, block: &Block) {
  80     for w in block.stmts.windows(3) {
  81         if_chain! {
  82             // let t = foo();
  83             if let StmtKind::Local(ref tmp) = w[0].kind;
  84             if let Some(ref tmp_init) = tmp.init;
  85             if let PatKind::Binding(.., ident, None) = tmp.pat.kind;
  86
  87             // foo() = bar();
  88             if let StmtKind::Semi(ref first) = w[1].kind;
  89             if let ExprKind::Assign(ref lhs1, ref rhs1) = first.kind;
  90
  91             // bar() = t;
  92             if let StmtKind::Semi(ref second) = w[2].kind;
  93             if let ExprKind::Assign(ref lhs2, ref rhs2) = second.kind;
  94             if let ExprKind::Path(QPath::Resolved(None, ref rhs2)) = rhs2.kind;
  95             if rhs2.segments.len() == 1;
  96
  97             if ident.as_str() == rhs2.segments[0].ident.as_str();
  98             if SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1);
  99             if SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2);
 100             then {
 101                 if let ExprKind::Field(ref lhs1, _) = lhs1.kind {
 102                     if let ExprKind::Field(ref lhs2, _) = lhs2.kind {
 103                         if lhs1.hir_id.owner_def_id() == lhs2.hir_id.owner_def_id() {
 104                             return;
 105                         }
 106                     }
 107                 }
 108
 109                 let mut applicability = Applicability::MachineApplicable;
 110
 111                 let slice = check_for_slice(cx, lhs1, lhs2);
 112                 let (replace, what, sugg) = if let Slice::NotSwappable = slice {
 113                     return;
 114                 } else if let Slice::Swappable(slice, idx1, idx2) = slice {
 115                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 116                         (
 117                             false,
 118                             format!(" elements of `{}`", slice),
 119                             format!(
 120                                 "{}.swap({}, {})",
 121                                 slice.maybe_par(),
 122                                 snippet_with_applicability(cx, idx1.span, "..", &mut applicability),
 123                                 snippet_with_applicability(cx, idx2.span, "..", &mut applicability),
 124                             ),
 125                         )
 126                     } else {
 127                         (false, String::new(), String::new())
 128                     }
 129                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 130                     (
 131                         true,
 132                         format!(" `{}` and `{}`", first, second),
 133                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()),
 134                     )
 135                 } else {
 136                     (true, String::new(), String::new())
 137                 };
 138
 139                 let span = w[0].span.to(second.span);
 140
 141                 span_lint_and_then(
 142                     cx,
 143                     MANUAL_SWAP,
 144                     span,
 145                     &format!("this looks like you are swapping{} manually", what),
 146                     |db| {
 147                         if !sugg.is_empty() {
 148                             db.span_suggestion(
 149                                 span,
 150                                 "try",
 151                                 sugg,
 152                                 applicability,
 153                             );
 154
 155                             if replace {
 156                                 db.note("or maybe you should use `std::mem::replace`?");
 157                             }
 158                         }
 159                     }
 160                 );
 161             }
 162         }
 163     }
 164 }
 165
 166 enum Slice<'a> {
 167     /// `slice.swap(idx1, idx2)` can be used
 168     ///
 169     /// ## Example
 170     ///
 171     /// ```rust
 172     /// # let mut a = vec![0, 1];
 173     /// let t = a[1];
 174     /// a[1] = a[0];
 175     /// a[0] = t;
 176     /// // can be written as
 177     /// a.swap(0, 1);
 178     /// ```
 179     Swappable(&'a Expr, &'a Expr, &'a Expr),
 180     /// The `swap` function cannot be used.
 181     ///
 182     /// ## Example
 183     ///
 184     /// ```rust
 185     /// # let mut a = [vec![1, 2], vec![3, 4]];
 186     /// let t = a[0][1];
 187     /// a[0][1] = a[1][0];
 188     /// a[1][0] = t;
 189     /// ```
 190     NotSwappable,
 191     /// Not a slice
 192     None,
 193 }
 194
 195 /// Checks if both expressions are index operations into "slice-like" types.
 196 fn check_for_slice<'a>(cx: &LateContext<'_, '_>, lhs1: &'a Expr, lhs2: &'a Expr) -> Slice<'a> {
 197     if let ExprKind::Index(ref lhs1, ref idx1) = lhs1.kind {
 198         if let ExprKind::Index(ref lhs2, ref idx2) = lhs2.kind {
 199             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
 200                 let ty = walk_ptrs_ty(cx.tables.expr_ty(lhs1));
 201
 202                 if matches!(ty.kind, ty::Slice(_))
 203                     || matches!(ty.kind, ty::Array(_, _))
 204                     || is_type_diagnostic_item(cx, ty, Symbol::intern("vec_type"))
 205                     || match_type(cx, ty, &paths::VEC_DEQUE)
 206                 {
 207                     return Slice::Swappable(lhs1, idx1, idx2);
 208                 }
 209             } else {
 210                 return Slice::NotSwappable;
 211             }
 212         }
 213     }
 214
 215     Slice::None
 216 }
 217
 218 /// Implementation of the `ALMOST_SWAPPED` lint.
 219 fn check_suspicious_swap(cx: &LateContext<'_, '_>, block: &Block) {
 220     for w in block.stmts.windows(2) {
 221         if_chain! {
 222             if let StmtKind::Semi(ref first) = w[0].kind;
 223             if let StmtKind::Semi(ref second) = w[1].kind;
 224             if !differing_macro_contexts(first.span, second.span);
 225             if let ExprKind::Assign(ref lhs0, ref rhs0) = first.kind;
 226             if let ExprKind::Assign(ref lhs1, ref rhs1) = second.kind;
 227             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1);
 228             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0);
 229             then {
 230                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 231                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 232                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 233                     (
 234                         format!(" `{}` and `{}`", first, second),
 235                         first.mut_addr().to_string(),
 236                         second.mut_addr().to_string(),
 237                     )
 238                 } else {
 239                     (String::new(), String::new(), String::new())
 240                 };
 241
 242                 let span = first.span.to(second.span);
 243
 244                 span_lint_and_then(cx,
 245                                    ALMOST_SWAPPED,
 246                                    span,
 247                                    &format!("this looks like you are trying to swap{}", what),
 248                                    |db| {
 249                                        if !what.is_empty() {
 250                                            db.span_suggestion(
 251                                                span,
 252                                                "try",
 253                                                format!(
 254                                                    "std::mem::swap({}, {})",
 255                                                    lhs,
 256                                                    rhs,
 257                                                ),
 258                                                Applicability::MaybeIncorrect,
 259                                            );
 260                                            db.note("or maybe you should use `std::mem::replace`?");
 261                                        }
 262                                    });
 263             }
 264         }
 265     }
 266 }