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 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 #[derive(Copy, Clone)]
  56 pub struct Swap;
  57
  58 impl LintPass for Swap {
  59     fn get_lints(&self) -> LintArray {
  60         lint_array![MANUAL_SWAP, ALMOST_SWAPPED]
  61     }
  62
  63     fn name(&self) -> &'static str {
  64         "Swap"
  65     }
  66 }
  67
  68 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Swap {
  69     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx Block) {
  70         check_manual_swap(cx, block);
  71         check_suspicious_swap(cx, block);
  72     }
  73 }
  74
  75 /// Implementation of the `MANUAL_SWAP` lint.
  76 fn check_manual_swap(cx: &LateContext<'_, '_>, block: &Block) {
  77     for w in block.stmts.windows(3) {
  78         if_chain! {
  79             // let t = foo();
  80             if let StmtKind::Local(ref tmp) = w[0].node;
  81             if let Some(ref tmp_init) = tmp.init;
  82             if let PatKind::Binding(.., ident, None) = tmp.pat.node;
  83
  84             // foo() = bar();
  85             if let StmtKind::Semi(ref first) = w[1].node;
  86             if let ExprKind::Assign(ref lhs1, ref rhs1) = first.node;
  87
  88             // bar() = t;
  89             if let StmtKind::Semi(ref second) = w[2].node;
  90             if let ExprKind::Assign(ref lhs2, ref rhs2) = second.node;
  91             if let ExprKind::Path(QPath::Resolved(None, ref rhs2)) = rhs2.node;
  92             if rhs2.segments.len() == 1;
  93
  94             if ident.as_str() == rhs2.segments[0].ident.as_str();
  95             if SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1);
  96             if SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2);
  97             then {
  98                 fn check_for_slice<'a>(
  99                     cx: &LateContext<'_, '_>,
 100                     lhs1: &'a Expr,
 101                     lhs2: &'a Expr,
 102                 ) -> Option<(&'a Expr, &'a Expr, &'a Expr)> {
 103                     if let ExprKind::Index(ref lhs1, ref idx1) = lhs1.node {
 104                         if let ExprKind::Index(ref lhs2, ref idx2) = lhs2.node {
 105                             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
 106                                 let ty = walk_ptrs_ty(cx.tables.expr_ty(lhs1));
 107
 108                                 if matches!(ty.sty, ty::Slice(_)) ||
 109                                     matches!(ty.sty, ty::Array(_, _)) ||
 110                                     match_type(cx, ty, &paths::VEC) ||
 111                                     match_type(cx, ty, &paths::VEC_DEQUE) {
 112                                         return Some((lhs1, idx1, idx2));
 113                                 }
 114                             }
 115                         }
 116                     }
 117
 118                     None
 119                 }
 120
 121                 let (replace, what, sugg) = if let Some((slice, idx1, idx2)) = check_for_slice(cx, lhs1, lhs2) {
 122                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 123                         (false,
 124                          format!(" elements of `{}`", slice),
 125                          format!("{}.swap({}, {})",
 126                                  slice.maybe_par(),
 127                                  snippet(cx, idx1.span, ".."),
 128                                  snippet(cx, idx2.span, "..")))
 129                     } else {
 130                         (false, String::new(), String::new())
 131                     }
 132                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 133                     (true, format!(" `{}` and `{}`", first, second),
 134                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()))
 135                 } else {
 136                     (true, String::new(), String::new())
 137                 };
 138
 139                 let span = w[0].span.to(second.span);
 140
 141                 span_lint_and_then(cx,
 142                                    MANUAL_SWAP,
 143                                    span,
 144                                    &format!("this looks like you are swapping{} manually", what),
 145                                    |db| {
 146                                        if !sugg.is_empty() {
 147                                            db.span_suggestion(
 148                                                span,
 149                                                "try",
 150                                                sugg,
 151                                                Applicability::Unspecified,
 152                                            );
 153
 154                                            if replace {
 155                                                db.note("or maybe you should use `std::mem::replace`?");
 156                                            }
 157                                        }
 158                                    });
 159             }
 160         }
 161     }
 162 }
 163
 164 /// Implementation of the `ALMOST_SWAPPED` lint.
 165 fn check_suspicious_swap(cx: &LateContext<'_, '_>, block: &Block) {
 166     for w in block.stmts.windows(2) {
 167         if_chain! {
 168             if let StmtKind::Semi(ref first) = w[0].node;
 169             if let StmtKind::Semi(ref second) = w[1].node;
 170             if !differing_macro_contexts(first.span, second.span);
 171             if let ExprKind::Assign(ref lhs0, ref rhs0) = first.node;
 172             if let ExprKind::Assign(ref lhs1, ref rhs1) = second.node;
 173             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1);
 174             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0);
 175             then {
 176                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 177                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 178                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 179                     (
 180                         format!(" `{}` and `{}`", first, second),
 181                         first.mut_addr().to_string(),
 182                         second.mut_addr().to_string(),
 183                     )
 184                 } else {
 185                     (String::new(), String::new(), String::new())
 186                 };
 187
 188                 let span = first.span.to(second.span);
 189
 190                 span_lint_and_then(cx,
 191                                    ALMOST_SWAPPED,
 192                                    span,
 193                                    &format!("this looks like you are trying to swap{}", what),
 194                                    |db| {
 195                                        if !what.is_empty() {
 196                                            db.span_suggestion(
 197                                                span,
 198                                                "try",
 199                                                format!(
 200                                                    "std::mem::swap({}, {})",
 201                                                    lhs,
 202                                                    rhs,
 203                                                ),
 204                                                Applicability::MaybeIncorrect,
 205                                            );
 206                                            db.note("or maybe you should use `std::mem::replace`?");
 207                                        }
 208                                    });
 209             }
 210         }
 211     }
 212 }