clippy_lints/src/swap.rs

   1 use rustc::hir::*;
   2 use rustc::lint::*;
   3 use rustc::ty;
   4 use syntax::codemap::mk_sp;
   5 use utils::{differing_macro_contexts, match_type, paths, snippet, snippet_opt, span_lint_and_then, walk_ptrs_ty, SpanlessEq};
   6
   7 /// **What it does:** This lints manual swapping.
   8 ///
   9 /// **Why is this bad?** The `std::mem::swap` function exposes the intent better without
  10 /// 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"
  24 }
  25
  26 /// **What it does:** This lints `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 LateLintPass for Swap {
  53     fn check_block(&mut self, cx: &LateContext, block: &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_let_chain!{[
  63             // let t = foo();
  64             let StmtDecl(ref tmp, _) = w[0].node,
  65             let DeclLocal(ref tmp) = tmp.node,
  66             let Some(ref tmp_init) = tmp.init,
  67             let PatKind::Binding(_, ref tmp_name, None) = tmp.pat.node,
  68
  69             // foo() = bar();
  70             let StmtSemi(ref first, _) = w[1].node,
  71             let ExprAssign(ref lhs1, ref rhs1) = first.node,
  72
  73             // bar() = t;
  74             let StmtSemi(ref second, _) = w[2].node,
  75             let ExprAssign(ref lhs2, ref rhs2) = second.node,
  76             let ExprPath(None, ref rhs2) = rhs2.node,
  77             rhs2.segments.len() == 1,
  78
  79             tmp_name.node.as_str() == rhs2.segments[0].name.as_str(),
  80             SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1),
  81             SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2)
  82         ], {
  83             fn check_for_slice<'a>(cx: &LateContext, lhs1: &'a Expr, lhs2: &'a Expr) -> Option<(&'a Expr, &'a Expr, &'a Expr)> {
  84                 if let ExprIndex(ref lhs1, ref idx1) = lhs1.node {
  85                     if let ExprIndex(ref lhs2, ref idx2) = lhs2.node {
  86                         if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
  87                             let ty = walk_ptrs_ty(cx.tcx.expr_ty(lhs1));
  88
  89                             if matches!(ty.sty, ty::TySlice(_)) ||
  90                                 matches!(ty.sty, ty::TyArray(_, _)) ||
  91                                 match_type(cx, ty, &paths::VEC) ||
  92                                 match_type(cx, ty, &paths::VEC_DEQUE) {
  93                                     return Some((lhs1, idx1, idx2));
  94                             }
  95                         }
  96                     }
  97                 }
  98
  99                 None
 100             }
 101
 102             let (replace, what, sugg) = if let Some((slice, idx1, idx2)) = check_for_slice(cx, lhs1, lhs2) {
 103                 if let Some(slice) = snippet_opt(cx, slice.span) {
 104                     (false,
 105                      format!(" elements of `{}`", slice),
 106                      format!("{}.swap({}, {})",slice,  snippet(cx, idx1.span, ".."), snippet(cx, idx2.span, "..")))
 107                 } else {
 108                     (false, "".to_owned(), "".to_owned())
 109                 }
 110             } else {
 111                  if let (Some(first), Some(second)) = (snippet_opt(cx, lhs1.span), snippet_opt(cx, rhs1.span)) {
 112                     (true, format!(" `{}` and `{}`", first, second),
 113                      format!("std::mem::swap(&mut {}, &mut {})", first, second))
 114                 } else {
 115                     (true, "".to_owned(), "".to_owned())
 116                 }
 117             };
 118
 119             let span = mk_sp(w[0].span.lo, second.span.hi);
 120
 121             span_lint_and_then(cx,
 122                                MANUAL_SWAP,
 123                                span,
 124                                &format!("this looks like you are swapping{} manually", what),
 125                                |db| {
 126                                    if !sugg.is_empty() {
 127                                        db.span_suggestion(span, "try", sugg);
 128
 129                                        if replace {
 130                                            db.note("or maybe you should use `std::mem::replace`?");
 131                                        }
 132                                    }
 133                                });
 134         }}
 135     }
 136 }
 137
 138 /// Implementation of the `ALMOST_SWAPPED` lint.
 139 fn check_suspicious_swap(cx: &LateContext, block: &Block) {
 140     for w in block.stmts.windows(2) {
 141         if_let_chain!{[
 142             let StmtSemi(ref first, _) = w[0].node,
 143             let StmtSemi(ref second, _) = w[1].node,
 144             !differing_macro_contexts(first.span, second.span),
 145             let ExprAssign(ref lhs0, ref rhs0) = first.node,
 146             let ExprAssign(ref lhs1, ref rhs1) = second.node,
 147             SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1),
 148             SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0)
 149         ], {
 150             let (what, lhs, rhs) = if let (Some(first), Some(second)) = (snippet_opt(cx, lhs0.span), snippet_opt(cx, rhs0.span)) {
 151                 (format!(" `{}` and `{}`", first, second), first, second)
 152             } else {
 153                 ("".to_owned(), "".to_owned(), "".to_owned())
 154             };
 155
 156             let span = mk_sp(first.span.lo, second.span.hi);
 157
 158             span_lint_and_then(cx,
 159                                ALMOST_SWAPPED,
 160                                span,
 161                                &format!("this looks like you are trying to swap{}", what),
 162                                |db| {
 163                                    if !what.is_empty() {
 164                                        db.span_suggestion(span, "try",
 165                                                           format!("std::mem::swap(&mut {}, &mut {})", lhs, rhs));
 166                                        db.note("or maybe you should use `std::mem::replace`?");
 167                                    }
 168                                });
 169         }}
 170     }
 171 }