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
   7 /// **What it does:** Checks for manual swapping.
   8 ///
   9 /// **Why is this bad?** The `std::mem::swap` function exposes the intent better
  10 /// without deinitializing or copying either variable.
  11 ///
  12 /// **Known problems:** None.
  13 ///
  14 /// **Example:**
  15 /// ```rust,ignore
  16 /// let t = b;
  17 /// b = a;
  18 /// a = t;
  19 /// ```
  20 declare_lint! {
  21     pub MANUAL_SWAP,
  22     Warn,
  23     "manual swap of two variables"
  24 }
  25
  26 /// **What it does:** Checks for `foo = bar; bar = foo` sequences.
  27 ///
  28 /// **Why is this bad?** This looks like a failed attempt to swap.
  29 ///
  30 /// **Known problems:** None.
  31 ///
  32 /// **Example:**
  33 /// ```rust,ignore
  34 /// a = b;
  35 /// b = a;
  36 /// ```
  37 declare_lint! {
  38     pub ALMOST_SWAPPED,
  39     Warn,
  40     "`foo = bar; bar = foo` sequence"
  41 }
  42
  43 #[derive(Copy, Clone)]
  44 pub struct Swap;
  45
  46 impl LintPass for Swap {
  47     fn get_lints(&self) -> LintArray {
  48         lint_array![MANUAL_SWAP, ALMOST_SWAPPED]
  49     }
  50 }
  51
  52 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Swap {
  53     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx Block) {
  54         check_manual_swap(cx, block);
  55         check_suspicious_swap(cx, block);
  56     }
  57 }
  58
  59 /// Implementation of the `MANUAL_SWAP` lint.
  60 fn check_manual_swap(cx: &LateContext, block: &Block) {
  61     for w in block.stmts.windows(3) {
  62         if_chain! {
  63             // let t = foo();
  64             if let StmtDecl(ref tmp, _) = w[0].node;
  65             if let DeclLocal(ref tmp) = tmp.node;
  66             if let Some(ref tmp_init) = tmp.init;
  67             if let PatKind::Binding(_, _, ref tmp_name, None) = tmp.pat.node;
  68
  69             // foo() = bar();
  70             if let StmtSemi(ref first, _) = w[1].node;
  71             if let ExprAssign(ref lhs1, ref rhs1) = first.node;
  72
  73             // bar() = t;
  74             if let StmtSemi(ref second, _) = w[2].node;
  75             if let ExprAssign(ref lhs2, ref rhs2) = second.node;
  76             if let ExprPath(QPath::Resolved(None, ref rhs2)) = rhs2.node;
  77             if rhs2.segments.len() == 1;
  78
  79             if tmp_name.node.as_str() == rhs2.segments[0].name.as_str();
  80             if SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1);
  81             if SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2);
  82             then {
  83                 fn check_for_slice<'a>(
  84                     cx: &LateContext,
  85                     lhs1: &'a Expr,
  86                     lhs2: &'a Expr,
  87                 ) -> Option<(&'a Expr, &'a Expr, &'a Expr)> {
  88                     if let ExprIndex(ref lhs1, ref idx1) = lhs1.node {
  89                         if let ExprIndex(ref lhs2, ref idx2) = lhs2.node {
  90                             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
  91                                 let ty = walk_ptrs_ty(cx.tables.expr_ty(lhs1));
  92
  93                                 if matches!(ty.sty, ty::TySlice(_)) ||
  94                                     matches!(ty.sty, ty::TyArray(_, _)) ||
  95                                     match_type(cx, ty, &paths::VEC) ||
  96                                     match_type(cx, ty, &paths::VEC_DEQUE) {
  97                                         return Some((lhs1, idx1, idx2));
  98                                 }
  99                             }
 100                         }
 101                     }
 102
 103                     None
 104                 }
 105
 106                 let (replace, what, sugg) = if let Some((slice, idx1, idx2)) = check_for_slice(cx, lhs1, lhs2) {
 107                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 108                         (false,
 109                          format!(" elements of `{}`", slice),
 110                          format!("{}.swap({}, {})",
 111                                  slice.maybe_par(),
 112                                  snippet(cx, idx1.span, ".."),
 113                                  snippet(cx, idx2.span, "..")))
 114                     } else {
 115                         (false, "".to_owned(), "".to_owned())
 116                     }
 117                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 118                     (true, format!(" `{}` and `{}`", first, second),
 119                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()))
 120                 } else {
 121                     (true, "".to_owned(), "".to_owned())
 122                 };
 123
 124                 let span = w[0].span.to(second.span);
 125
 126                 span_lint_and_then(cx,
 127                                    MANUAL_SWAP,
 128                                    span,
 129                                    &format!("this looks like you are swapping{} manually", what),
 130                                    |db| {
 131                                        if !sugg.is_empty() {
 132                                            db.span_suggestion(span, "try", sugg);
 133
 134                                            if replace {
 135                                                db.note("or maybe you should use `std::mem::replace`?");
 136                                            }
 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_chain! {
 148             if let StmtSemi(ref first, _) = w[0].node;
 149             if let StmtSemi(ref second, _) = w[1].node;
 150             if !differing_macro_contexts(first.span, second.span);
 151             if let ExprAssign(ref lhs0, ref rhs0) = first.node;
 152             if let ExprAssign(ref lhs1, ref rhs1) = second.node;
 153             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1);
 154             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0);
 155             then {
 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                     (
 160                         format!(" `{}` and `{}`", first, second),
 161                         first.mut_addr().to_string(),
 162                         second.mut_addr().to_string(),
 163                     )
 164                 } else {
 165                     ("".to_owned(), "".to_owned(), "".to_owned())
 166                 };
 167
 168                 let span = first.span.to(second.span);
 169
 170                 span_lint_and_then(cx,
 171                                    ALMOST_SWAPPED,
 172                                    span,
 173                                    &format!("this looks like you are trying to swap{}", what),
 174                                    |db| {
 175                                        if !what.is_empty() {
 176                                            db.span_suggestion(span, "try",
 177                                                               format!("std::mem::swap({}, {})", lhs, rhs));
 178                                            db.note("or maybe you should use `std::mem::replace`?");
 179                                        }
 180                                    });
 181             }
 182         }
 183     }
 184 }