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_tool_lint, lint_array};
  11 use rustc_errors::Applicability;
  12
  13 /// **What it does:** Checks for manual swapping.
  14 ///
  15 /// **Why is this bad?** The `std::mem::swap` function exposes the intent better
  16 /// without deinitializing or copying either variable.
  17 ///
  18 /// **Known problems:** None.
  19 ///
  20 /// **Example:**
  21 /// ```rust,ignore
  22 /// let t = b;
  23 /// b = a;
  24 /// a = t;
  25 /// ```
  26 /// Use std::mem::swap():
  27 /// ```rust
  28 /// std::mem::swap(&mut a, &mut b);
  29 /// ```
  30 declare_clippy_lint! {
  31     pub MANUAL_SWAP,
  32     complexity,
  33     "manual swap of two variables"
  34 }
  35
  36 /// **What it does:** Checks for `foo = bar; bar = foo` sequences.
  37 ///
  38 /// **Why is this bad?** This looks like a failed attempt to swap.
  39 ///
  40 /// **Known problems:** None.
  41 ///
  42 /// **Example:**
  43 /// ```rust,ignore
  44 /// a = b;
  45 /// b = a;
  46 /// ```
  47 declare_clippy_lint! {
  48     pub ALMOST_SWAPPED,
  49     correctness,
  50     "`foo = bar; bar = foo` sequence"
  51 }
  52
  53 #[derive(Copy, Clone)]
  54 pub struct Swap;
  55
  56 impl LintPass for Swap {
  57     fn get_lints(&self) -> LintArray {
  58         lint_array![MANUAL_SWAP, ALMOST_SWAPPED]
  59     }
  60 }
  61
  62 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Swap {
  63     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx Block) {
  64         check_manual_swap(cx, block);
  65         check_suspicious_swap(cx, block);
  66     }
  67 }
  68
  69 /// Implementation of the `MANUAL_SWAP` lint.
  70 fn check_manual_swap(cx: &LateContext<'_, '_>, block: &Block) {
  71     for w in block.stmts.windows(3) {
  72         if_chain! {
  73             // let t = foo();
  74             if let StmtKind::Local(ref tmp) = w[0].node;
  75             if let Some(ref tmp_init) = tmp.init;
  76             if let PatKind::Binding(_, _, ident, None) = tmp.pat.node;
  77
  78             // foo() = bar();
  79             if let StmtKind::Semi(ref first) = w[1].node;
  80             if let ExprKind::Assign(ref lhs1, ref rhs1) = first.node;
  81
  82             // bar() = t;
  83             if let StmtKind::Semi(ref second) = w[2].node;
  84             if let ExprKind::Assign(ref lhs2, ref rhs2) = second.node;
  85             if let ExprKind::Path(QPath::Resolved(None, ref rhs2)) = rhs2.node;
  86             if rhs2.segments.len() == 1;
  87
  88             if ident.as_str() == rhs2.segments[0].ident.as_str();
  89             if SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1);
  90             if SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2);
  91             then {
  92                 fn check_for_slice<'a>(
  93                     cx: &LateContext<'_, '_>,
  94                     lhs1: &'a Expr,
  95                     lhs2: &'a Expr,
  96                 ) -> Option<(&'a Expr, &'a Expr, &'a Expr)> {
  97                     if let ExprKind::Index(ref lhs1, ref idx1) = lhs1.node {
  98                         if let ExprKind::Index(ref lhs2, ref idx2) = lhs2.node {
  99                             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
 100                                 let ty = walk_ptrs_ty(cx.tables.expr_ty(lhs1));
 101
 102                                 if matches!(ty.sty, ty::Slice(_)) ||
 103                                     matches!(ty.sty, ty::Array(_, _)) ||
 104                                     match_type(cx, ty, &paths::VEC) ||
 105                                     match_type(cx, ty, &paths::VEC_DEQUE) {
 106                                         return Some((lhs1, idx1, idx2));
 107                                 }
 108                             }
 109                         }
 110                     }
 111
 112                     None
 113                 }
 114
 115                 let (replace, what, sugg) = if let Some((slice, idx1, idx2)) = check_for_slice(cx, lhs1, lhs2) {
 116                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 117                         (false,
 118                          format!(" elements of `{}`", slice),
 119                          format!("{}.swap({}, {})",
 120                                  slice.maybe_par(),
 121                                  snippet(cx, idx1.span, ".."),
 122                                  snippet(cx, idx2.span, "..")))
 123                     } else {
 124                         (false, String::new(), String::new())
 125                     }
 126                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 127                     (true, format!(" `{}` and `{}`", first, second),
 128                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()))
 129                 } else {
 130                     (true, String::new(), String::new())
 131                 };
 132
 133                 let span = w[0].span.to(second.span);
 134
 135                 span_lint_and_then(cx,
 136                                    MANUAL_SWAP,
 137                                    span,
 138                                    &format!("this looks like you are swapping{} manually", what),
 139                                    |db| {
 140                                        if !sugg.is_empty() {
 141                                            db.span_suggestion_with_applicability(
 142                                                span,
 143                                                "try",
 144                                                sugg,
 145                                                Applicability::Unspecified,
 146                                            );
 147
 148                                            if replace {
 149                                                db.note("or maybe you should use `std::mem::replace`?");
 150                                            }
 151                                        }
 152                                    });
 153             }
 154         }
 155     }
 156 }
 157
 158 /// Implementation of the `ALMOST_SWAPPED` lint.
 159 fn check_suspicious_swap(cx: &LateContext<'_, '_>, block: &Block) {
 160     for w in block.stmts.windows(2) {
 161         if_chain! {
 162             if let StmtKind::Semi(ref first) = w[0].node;
 163             if let StmtKind::Semi(ref second) = w[1].node;
 164             if !differing_macro_contexts(first.span, second.span);
 165             if let ExprKind::Assign(ref lhs0, ref rhs0) = first.node;
 166             if let ExprKind::Assign(ref lhs1, ref rhs1) = second.node;
 167             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1);
 168             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0);
 169             then {
 170                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 171                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 172                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 173                     (
 174                         format!(" `{}` and `{}`", first, second),
 175                         first.mut_addr().to_string(),
 176                         second.mut_addr().to_string(),
 177                     )
 178                 } else {
 179                     (String::new(), String::new(), String::new())
 180                 };
 181
 182                 let span = first.span.to(second.span);
 183
 184                 span_lint_and_then(cx,
 185                                    ALMOST_SWAPPED,
 186                                    span,
 187                                    &format!("this looks like you are trying to swap{}", what),
 188                                    |db| {
 189                                        if !what.is_empty() {
 190                                            db.span_suggestion_with_applicability(
 191                                                span,
 192                                                "try",
 193                                                format!(
 194                                                    "std::mem::swap({}, {})",
 195                                                    lhs,
 196                                                    rhs,
 197                                                ),
 198                                                Applicability::MaybeIncorrect,
 199                                            );
 200                                            db.note("or maybe you should use `std::mem::replace`?");
 201                                        }
 202                                    });
 203             }
 204         }
 205     }
 206 }