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 t = b;
  24     /// b = a;
  25     /// a = t;
  26     /// ```
  27     /// Use std::mem::swap():
  28     /// ```rust
  29     /// std::mem::swap(&mut a, &mut b);
  30     /// ```
  31     pub MANUAL_SWAP,
  32     complexity,
  33     "manual swap of two variables"
  34 }
  35
  36 declare_clippy_lint! {
  37     /// **What it does:** Checks for `foo = bar; bar = foo` sequences.
  38     ///
  39     /// **Why is this bad?** This looks like a failed attempt to swap.
  40     ///
  41     /// **Known problems:** None.
  42     ///
  43     /// **Example:**
  44     /// ```rust
  45     /// # let mut a = 1;
  46     /// # let mut b = 2;
  47     /// a = b;
  48     /// b = a;
  49     /// ```
  50     pub ALMOST_SWAPPED,
  51     correctness,
  52     "`foo = bar; bar = foo` sequence"
  53 }
  54
  55 declare_lint_pass!(Swap => [MANUAL_SWAP, ALMOST_SWAPPED]);
  56
  57 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Swap {
  58     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx Block) {
  59         check_manual_swap(cx, block);
  60         check_suspicious_swap(cx, block);
  61     }
  62 }
  63
  64 /// Implementation of the `MANUAL_SWAP` lint.
  65 fn check_manual_swap(cx: &LateContext<'_, '_>, block: &Block) {
  66     for w in block.stmts.windows(3) {
  67         if_chain! {
  68             // let t = foo();
  69             if let StmtKind::Local(ref tmp) = w[0].node;
  70             if let Some(ref tmp_init) = tmp.init;
  71             if let PatKind::Binding(.., ident, None) = tmp.pat.node;
  72
  73             // foo() = bar();
  74             if let StmtKind::Semi(ref first) = w[1].node;
  75             if let ExprKind::Assign(ref lhs1, ref rhs1) = first.node;
  76
  77             // bar() = t;
  78             if let StmtKind::Semi(ref second) = w[2].node;
  79             if let ExprKind::Assign(ref lhs2, ref rhs2) = second.node;
  80             if let ExprKind::Path(QPath::Resolved(None, ref rhs2)) = rhs2.node;
  81             if rhs2.segments.len() == 1;
  82
  83             if ident.as_str() == rhs2.segments[0].ident.as_str();
  84             if SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1);
  85             if SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2);
  86             then {
  87                 fn check_for_slice<'a>(
  88                     cx: &LateContext<'_, '_>,
  89                     lhs1: &'a Expr,
  90                     lhs2: &'a Expr,
  91                 ) -> Option<(&'a Expr, &'a Expr, &'a Expr)> {
  92                     if let ExprKind::Index(ref lhs1, ref idx1) = lhs1.node {
  93                         if let ExprKind::Index(ref lhs2, ref idx2) = lhs2.node {
  94                             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
  95                                 let ty = walk_ptrs_ty(cx.tables.expr_ty(lhs1));
  96
  97                                 if matches!(ty.sty, ty::Slice(_)) ||
  98                                     matches!(ty.sty, ty::Array(_, _)) ||
  99                                     match_type(cx, ty, &paths::VEC) ||
 100                                     match_type(cx, ty, &paths::VEC_DEQUE) {
 101                                         return Some((lhs1, idx1, idx2));
 102                                 }
 103                             }
 104                         }
 105                     }
 106
 107                     None
 108                 }
 109
 110                 let (replace, what, sugg) = if let Some((slice, idx1, idx2)) = check_for_slice(cx, lhs1, lhs2) {
 111                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 112                         (false,
 113                          format!(" elements of `{}`", slice),
 114                          format!("{}.swap({}, {})",
 115                                  slice.maybe_par(),
 116                                  snippet(cx, idx1.span, ".."),
 117                                  snippet(cx, idx2.span, "..")))
 118                     } else {
 119                         (false, String::new(), String::new())
 120                     }
 121                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 122                     (true, format!(" `{}` and `{}`", first, second),
 123                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()))
 124                 } else {
 125                     (true, String::new(), String::new())
 126                 };
 127
 128                 let span = w[0].span.to(second.span);
 129
 130                 span_lint_and_then(cx,
 131                                    MANUAL_SWAP,
 132                                    span,
 133                                    &format!("this looks like you are swapping{} manually", what),
 134                                    |db| {
 135                                        if !sugg.is_empty() {
 136                                            db.span_suggestion(
 137                                                span,
 138                                                "try",
 139                                                sugg,
 140                                                Applicability::Unspecified,
 141                                            );
 142
 143                                            if replace {
 144                                                db.note("or maybe you should use `std::mem::replace`?");
 145                                            }
 146                                        }
 147                                    });
 148             }
 149         }
 150     }
 151 }
 152
 153 /// Implementation of the `ALMOST_SWAPPED` lint.
 154 fn check_suspicious_swap(cx: &LateContext<'_, '_>, block: &Block) {
 155     for w in block.stmts.windows(2) {
 156         if_chain! {
 157             if let StmtKind::Semi(ref first) = w[0].node;
 158             if let StmtKind::Semi(ref second) = w[1].node;
 159             if !differing_macro_contexts(first.span, second.span);
 160             if let ExprKind::Assign(ref lhs0, ref rhs0) = first.node;
 161             if let ExprKind::Assign(ref lhs1, ref rhs1) = second.node;
 162             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1);
 163             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0);
 164             then {
 165                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 166                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 167                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 168                     (
 169                         format!(" `{}` and `{}`", first, second),
 170                         first.mut_addr().to_string(),
 171                         second.mut_addr().to_string(),
 172                     )
 173                 } else {
 174                     (String::new(), String::new(), String::new())
 175                 };
 176
 177                 let span = first.span.to(second.span);
 178
 179                 span_lint_and_then(cx,
 180                                    ALMOST_SWAPPED,
 181                                    span,
 182                                    &format!("this looks like you are trying to swap{}", what),
 183                                    |db| {
 184                                        if !what.is_empty() {
 185                                            db.span_suggestion(
 186                                                span,
 187                                                "try",
 188                                                format!(
 189                                                    "std::mem::swap({}, {})",
 190                                                    lhs,
 191                                                    rhs,
 192                                                ),
 193                                                Applicability::MaybeIncorrect,
 194                                            );
 195                                            db.note("or maybe you should use `std::mem::replace`?");
 196                                        }
 197                                    });
 198             }
 199         }
 200     }
 201 }