clippy_lints/src/swap.rs

   1 use crate::utils::sugg::Sugg;
   2 use crate::utils::{
   3     differing_macro_contexts, match_type, paths, snippet, span_lint_and_then, walk_ptrs_ty, SpanlessEq,
   4 };
   5 use if_chain::if_chain;
   6 use matches::matches;
   7 use rustc::hir::*;
   8 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   9 use rustc::ty;
  10 use rustc::{declare_lint_pass, declare_tool_lint};
  11 use rustc_errors::Applicability;
  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     /// Could be written as:
  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<'a, 'tcx> LateLintPass<'a, 'tcx> for Swap {
  69     fn check_block(&mut self, cx: &LateContext<'a, '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].node;
  81             if let Some(ref tmp_init) = tmp.init;
  82             if let PatKind::Binding(.., ident, None) = tmp.pat.node;
  83
  84             // foo() = bar();
  85             if let StmtKind::Semi(ref first) = w[1].node;
  86             if let ExprKind::Assign(ref lhs1, ref rhs1) = first.node;
  87
  88             // bar() = t;
  89             if let StmtKind::Semi(ref second) = w[2].node;
  90             if let ExprKind::Assign(ref lhs2, ref rhs2) = second.node;
  91             if let ExprKind::Path(QPath::Resolved(None, ref rhs2)) = rhs2.node;
  92             if rhs2.segments.len() == 1;
  93
  94             if ident.as_str() == rhs2.segments[0].ident.as_str();
  95             if SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1);
  96             if SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2);
  97             then {
  98                 fn check_for_slice<'a>(
  99                     cx: &LateContext<'_, '_>,
 100                     lhs1: &'a Expr,
 101                     lhs2: &'a Expr,
 102                 ) -> Option<(&'a Expr, &'a Expr, &'a Expr)> {
 103                     if let ExprKind::Index(ref lhs1, ref idx1) = lhs1.node {
 104                         if let ExprKind::Index(ref lhs2, ref idx2) = lhs2.node {
 105                             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
 106                                 let ty = walk_ptrs_ty(cx.tables.expr_ty(lhs1));
 107
 108                                 if matches!(ty.sty, ty::Slice(_)) ||
 109                                     matches!(ty.sty, ty::Array(_, _)) ||
 110                                     match_type(cx, ty, &paths::VEC) ||
 111                                     match_type(cx, ty, &paths::VEC_DEQUE) {
 112                                         return Some((lhs1, idx1, idx2));
 113                                 }
 114                             }
 115                         }
 116                     }
 117
 118                     None
 119                 }
 120
 121                 if let ExprKind::Field(ref lhs1, _) = lhs1.node {
 122                     if let ExprKind::Field(ref lhs2, _) = lhs2.node {
 123                         if lhs1.hir_id.owner_def_id() == lhs2.hir_id.owner_def_id() {
 124                             return;
 125                         }
 126                     }
 127                 }
 128
 129                 let (replace, what, sugg) = if let Some((slice, idx1, idx2)) = check_for_slice(cx, lhs1, lhs2) {
 130                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 131                         (false,
 132                          format!(" elements of `{}`", slice),
 133                          format!("{}.swap({}, {})",
 134                                  slice.maybe_par(),
 135                                  snippet(cx, idx1.span, ".."),
 136                                  snippet(cx, idx2.span, "..")))
 137                     } else {
 138                         (false, String::new(), String::new())
 139                     }
 140                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 141                     (true, format!(" `{}` and `{}`", first, second),
 142                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()))
 143                 } else {
 144                     (true, String::new(), String::new())
 145                 };
 146
 147                 let span = w[0].span.to(second.span);
 148
 149                 span_lint_and_then(cx,
 150                                    MANUAL_SWAP,
 151                                    span,
 152                                    &format!("this looks like you are swapping{} manually", what),
 153                                    |db| {
 154                                        if !sugg.is_empty() {
 155                                            db.span_suggestion(
 156                                                span,
 157                                                "try",
 158                                                sugg,
 159                                                Applicability::Unspecified,
 160                                            );
 161
 162                                            if replace {
 163                                                db.note("or maybe you should use `std::mem::replace`?");
 164                                            }
 165                                        }
 166                                    });
 167             }
 168         }
 169     }
 170 }
 171
 172 /// Implementation of the `ALMOST_SWAPPED` lint.
 173 fn check_suspicious_swap(cx: &LateContext<'_, '_>, block: &Block) {
 174     for w in block.stmts.windows(2) {
 175         if_chain! {
 176             if let StmtKind::Semi(ref first) = w[0].node;
 177             if let StmtKind::Semi(ref second) = w[1].node;
 178             if !differing_macro_contexts(first.span, second.span);
 179             if let ExprKind::Assign(ref lhs0, ref rhs0) = first.node;
 180             if let ExprKind::Assign(ref lhs1, ref rhs1) = second.node;
 181             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1);
 182             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0);
 183             then {
 184                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 185                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 186                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 187                     (
 188                         format!(" `{}` and `{}`", first, second),
 189                         first.mut_addr().to_string(),
 190                         second.mut_addr().to_string(),
 191                     )
 192                 } else {
 193                     (String::new(), String::new(), String::new())
 194                 };
 195
 196                 let span = first.span.to(second.span);
 197
 198                 span_lint_and_then(cx,
 199                                    ALMOST_SWAPPED,
 200                                    span,
 201                                    &format!("this looks like you are trying to swap{}", what),
 202                                    |db| {
 203                                        if !what.is_empty() {
 204                                            db.span_suggestion(
 205                                                span,
 206                                                "try",
 207                                                format!(
 208                                                    "std::mem::swap({}, {})",
 209                                                    lhs,
 210                                                    rhs,
 211                                                ),
 212                                                Applicability::MaybeIncorrect,
 213                                            );
 214                                            db.note("or maybe you should use `std::mem::replace`?");
 215                                        }
 216                                    });
 217             }
 218         }
 219     }
 220 }