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