clippy_lints/src/swap.rs

   1 use rustc::hir::*;
   2 use rustc::lint::*;
   3 use rustc::ty;
   4 use utils::{differing_macro_contexts, match_type, paths, snippet, span_lint_and_then, walk_ptrs_ty, SpanlessEq};
   5 use utils::sugg::Sugg;
   6 use syntax_pos::{Span, NO_EXPANSION};
   7
   8 /// **What it does:** Checks for manual swapping.
   9 ///
  10 /// **Why is this bad?** The `std::mem::swap` function exposes the intent better
  11 /// without deinitializing or copying either variable.
  12 ///
  13 /// **Known problems:** None.
  14 ///
  15 /// **Example:**
  16 /// ```rust,ignore
  17 /// let t = b;
  18 /// b = a;
  19 /// a = t;
  20 /// ```
  21 declare_lint! {
  22     pub MANUAL_SWAP,
  23     Warn,
  24     "manual swap of two variables"
  25 }
  26
  27 /// **What it does:** Checks for `foo = bar; bar = foo` sequences.
  28 ///
  29 /// **Why is this bad?** This looks like a failed attempt to swap.
  30 ///
  31 /// **Known problems:** None.
  32 ///
  33 /// **Example:**
  34 /// ```rust,ignore
  35 /// a = b;
  36 /// b = a;
  37 /// ```
  38 declare_lint! {
  39     pub ALMOST_SWAPPED,
  40     Warn,
  41     "`foo = bar; bar = foo` sequence"
  42 }
  43
  44 #[derive(Copy,Clone)]
  45 pub struct Swap;
  46
  47 impl LintPass for Swap {
  48     fn get_lints(&self) -> LintArray {
  49         lint_array![MANUAL_SWAP, ALMOST_SWAPPED]
  50     }
  51 }
  52
  53 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Swap {
  54     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx Block) {
  55         check_manual_swap(cx, block);
  56         check_suspicious_swap(cx, block);
  57     }
  58 }
  59
  60 /// Implementation of the `MANUAL_SWAP` lint.
  61 fn check_manual_swap(cx: &LateContext, block: &Block) {
  62     for w in block.stmts.windows(3) {
  63         if_let_chain!{[
  64             // let t = foo();
  65             let StmtDecl(ref tmp, _) = w[0].node,
  66             let DeclLocal(ref tmp) = tmp.node,
  67             let Some(ref tmp_init) = tmp.init,
  68             let PatKind::Binding(_, _, ref tmp_name, None) = tmp.pat.node,
  69
  70             // foo() = bar();
  71             let StmtSemi(ref first, _) = w[1].node,
  72             let ExprAssign(ref lhs1, ref rhs1) = first.node,
  73
  74             // bar() = t;
  75             let StmtSemi(ref second, _) = w[2].node,
  76             let ExprAssign(ref lhs2, ref rhs2) = second.node,
  77             let ExprPath(QPath::Resolved(None, ref rhs2)) = rhs2.node,
  78             rhs2.segments.len() == 1,
  79
  80             tmp_name.node.as_str() == rhs2.segments[0].name.as_str(),
  81             SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1),
  82             SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2)
  83         ], {
  84             fn check_for_slice<'a>(
  85                 cx: &LateContext,
  86                 lhs1: &'a Expr,
  87                 lhs2: &'a Expr,
  88             ) -> Option<(&'a Expr, &'a Expr, &'a Expr)> {
  89                 if let ExprIndex(ref lhs1, ref idx1) = lhs1.node {
  90                     if let ExprIndex(ref lhs2, ref idx2) = lhs2.node {
  91                         if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
  92                             let ty = walk_ptrs_ty(cx.tables.expr_ty(lhs1));
  93
  94                             if matches!(ty.sty, ty::TySlice(_)) ||
  95                                 matches!(ty.sty, ty::TyArray(_, _)) ||
  96                                 match_type(cx, ty, &paths::VEC) ||
  97                                 match_type(cx, ty, &paths::VEC_DEQUE) {
  98                                     return Some((lhs1, idx1, idx2));
  99                             }
 100                         }
 101                     }
 102                 }
 103
 104                 None
 105             }
 106
 107             let (replace, what, sugg) = if let Some((slice, idx1, idx2)) = check_for_slice(cx, lhs1, lhs2) {
 108                 if let Some(slice) = Sugg::hir_opt(cx, slice) {
 109                     (false,
 110                      format!(" elements of `{}`", slice),
 111                      format!("{}.swap({}, {})",
 112                              slice.maybe_par(),
 113                              snippet(cx, idx1.span, ".."),
 114                              snippet(cx, idx2.span, "..")))
 115                 } else {
 116                     (false, "".to_owned(), "".to_owned())
 117                 }
 118             } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 119                 (true, format!(" `{}` and `{}`", first, second),
 120                     format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()))
 121             } else {
 122                 (true, "".to_owned(), "".to_owned())
 123             };
 124
 125             let span = Span { lo: w[0].span.lo, hi: second.span.hi, ctxt: NO_EXPANSION};
 126
 127             span_lint_and_then(cx,
 128                                MANUAL_SWAP,
 129                                span,
 130                                &format!("this looks like you are swapping{} manually", what),
 131                                |db| {
 132                                    if !sugg.is_empty() {
 133                                        db.span_suggestion(span, "try", sugg);
 134
 135                                        if replace {
 136                                            db.note("or maybe you should use `std::mem::replace`?");
 137                                        }
 138                                    }
 139                                });
 140         }}
 141     }
 142 }
 143
 144 /// Implementation of the `ALMOST_SWAPPED` lint.
 145 fn check_suspicious_swap(cx: &LateContext, block: &Block) {
 146     for w in block.stmts.windows(2) {
 147         if_let_chain!{[
 148             let StmtSemi(ref first, _) = w[0].node,
 149             let StmtSemi(ref second, _) = w[1].node,
 150             !differing_macro_contexts(first.span, second.span),
 151             let ExprAssign(ref lhs0, ref rhs0) = first.node,
 152             let ExprAssign(ref lhs1, ref rhs1) = second.node,
 153             SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1),
 154             SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0)
 155         ], {
 156             let lhs0 = Sugg::hir_opt(cx, lhs0);
 157             let rhs0 = Sugg::hir_opt(cx, rhs0);
 158             let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 159                 (format!(" `{}` and `{}`", first, second), first.mut_addr().to_string(), second.mut_addr().to_string())
 160             } else {
 161                 ("".to_owned(), "".to_owned(), "".to_owned())
 162             };
 163
 164             let span = Span{ lo: first.span.lo, hi: second.span.hi, ctxt: NO_EXPANSION};
 165
 166             span_lint_and_then(cx,
 167                                ALMOST_SWAPPED,
 168                                span,
 169                                &format!("this looks like you are trying to swap{}", what),
 170                                |db| {
 171                                    if !what.is_empty() {
 172                                        db.span_suggestion(span, "try",
 173                                                           format!("std::mem::swap({}, {})", lhs, rhs));
 174                                        db.note("or maybe you should use `std::mem::replace`?");
 175                                    }
 176                                });
 177         }}
 178     }
 179 }