clippy_lints/src/swap.rs

   1 use crate::utils::sugg::Sugg;
   2 use crate::utils::{
   3     differing_macro_contexts, match_type, is_type_diagnostic_item, paths, snippet,
   4     span_lint_and_then, walk_ptrs_ty, SpanlessEq,
   5 };
   6 use if_chain::if_chain;
   7 use matches::matches;
   8 use rustc::hir::*;
   9 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  10 use rustc::ty;
  11 use rustc::{declare_lint_pass, declare_tool_lint};
  12 use rustc_errors::Applicability;
  13 use syntax_pos::Symbol;
  14
  15 declare_clippy_lint! {
  16     /// **What it does:** Checks for manual swapping.
  17     ///
  18     /// **Why is this bad?** The `std::mem::swap` function exposes the intent better
  19     /// without deinitializing or copying either variable.
  20     ///
  21     /// **Known problems:** None.
  22     ///
  23     /// **Example:**
  24     /// ```rust
  25     /// let mut a = 42;
  26     /// let mut b = 1337;
  27     ///
  28     /// let t = b;
  29     /// b = a;
  30     /// a = t;
  31     /// ```
  32     /// Use std::mem::swap():
  33     /// ```rust
  34     /// let mut a = 1;
  35     /// let mut b = 2;
  36     /// std::mem::swap(&mut a, &mut b);
  37     /// ```
  38     pub MANUAL_SWAP,
  39     complexity,
  40     "manual swap of two variables"
  41 }
  42
  43 declare_clippy_lint! {
  44     /// **What it does:** Checks for `foo = bar; bar = foo` sequences.
  45     ///
  46     /// **Why is this bad?** This looks like a failed attempt to swap.
  47     ///
  48     /// **Known problems:** None.
  49     ///
  50     /// **Example:**
  51     /// ```rust
  52     /// # let mut a = 1;
  53     /// # let mut b = 2;
  54     /// a = b;
  55     /// b = a;
  56     /// ```
  57     /// Could be written as:
  58     /// ```rust
  59     /// # let mut a = 1;
  60     /// # let mut b = 2;
  61     /// std::mem::swap(&mut a, &mut b);
  62     /// ```
  63     pub ALMOST_SWAPPED,
  64     correctness,
  65     "`foo = bar; bar = foo` sequence"
  66 }
  67
  68 declare_lint_pass!(Swap => [MANUAL_SWAP, ALMOST_SWAPPED]);
  69
  70 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Swap {
  71     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx Block) {
  72         check_manual_swap(cx, block);
  73         check_suspicious_swap(cx, block);
  74     }
  75 }
  76
  77 /// Implementation of the `MANUAL_SWAP` lint.
  78 fn check_manual_swap(cx: &LateContext<'_, '_>, block: &Block) {
  79     for w in block.stmts.windows(3) {
  80         if_chain! {
  81             // let t = foo();
  82             if let StmtKind::Local(ref tmp) = w[0].node;
  83             if let Some(ref tmp_init) = tmp.init;
  84             if let PatKind::Binding(.., ident, None) = tmp.pat.node;
  85
  86             // foo() = bar();
  87             if let StmtKind::Semi(ref first) = w[1].node;
  88             if let ExprKind::Assign(ref lhs1, ref rhs1) = first.node;
  89
  90             // bar() = t;
  91             if let StmtKind::Semi(ref second) = w[2].node;
  92             if let ExprKind::Assign(ref lhs2, ref rhs2) = second.node;
  93             if let ExprKind::Path(QPath::Resolved(None, ref rhs2)) = rhs2.node;
  94             if rhs2.segments.len() == 1;
  95
  96             if ident.as_str() == rhs2.segments[0].ident.as_str();
  97             if SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1);
  98             if SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2);
  99             then {
 100                 fn check_for_slice<'a>(
 101                     cx: &LateContext<'_, '_>,
 102                     lhs1: &'a Expr,
 103                     lhs2: &'a Expr,
 104                 ) -> Option<(&'a Expr, &'a Expr, &'a Expr)> {
 105                     if let ExprKind::Index(ref lhs1, ref idx1) = lhs1.node {
 106                         if let ExprKind::Index(ref lhs2, ref idx2) = lhs2.node {
 107                             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
 108                                 let ty = walk_ptrs_ty(cx.tables.expr_ty(lhs1));
 109
 110                                 if matches!(ty.sty, ty::Slice(_)) ||
 111                                     matches!(ty.sty, ty::Array(_, _)) ||
 112                                     is_type_diagnostic_item(cx, ty, Symbol::intern("vec_type")) ||
 113                                     match_type(cx, ty, &paths::VEC_DEQUE) {
 114                                         return Some((lhs1, idx1, idx2));
 115                                 }
 116                             }
 117                         }
 118                     }
 119
 120                     None
 121                 }
 122
 123                 let (replace, what, sugg) = if let Some((slice, idx1, idx2)) = check_for_slice(cx, lhs1, lhs2) {
 124                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 125                         (false,
 126                          format!(" elements of `{}`", slice),
 127                          format!("{}.swap({}, {})",
 128                                  slice.maybe_par(),
 129                                  snippet(cx, idx1.span, ".."),
 130                                  snippet(cx, idx2.span, "..")))
 131                     } else {
 132                         (false, String::new(), String::new())
 133                     }
 134                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 135                     (true, format!(" `{}` and `{}`", first, second),
 136                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()))
 137                 } else {
 138                     (true, String::new(), String::new())
 139                 };
 140
 141                 let span = w[0].span.to(second.span);
 142
 143                 span_lint_and_then(cx,
 144                                    MANUAL_SWAP,
 145                                    span,
 146                                    &format!("this looks like you are swapping{} manually", what),
 147                                    |db| {
 148                                        if !sugg.is_empty() {
 149                                            db.span_suggestion(
 150                                                span,
 151                                                "try",
 152                                                sugg,
 153                                                Applicability::Unspecified,
 154                                            );
 155
 156                                            if replace {
 157                                                db.note("or maybe you should use `std::mem::replace`?");
 158                                            }
 159                                        }
 160                                    });
 161             }
 162         }
 163     }
 164 }
 165
 166 /// Implementation of the `ALMOST_SWAPPED` lint.
 167 fn check_suspicious_swap(cx: &LateContext<'_, '_>, block: &Block) {
 168     for w in block.stmts.windows(2) {
 169         if_chain! {
 170             if let StmtKind::Semi(ref first) = w[0].node;
 171             if let StmtKind::Semi(ref second) = w[1].node;
 172             if !differing_macro_contexts(first.span, second.span);
 173             if let ExprKind::Assign(ref lhs0, ref rhs0) = first.node;
 174             if let ExprKind::Assign(ref lhs1, ref rhs1) = second.node;
 175             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1);
 176             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0);
 177             then {
 178                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 179                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 180                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 181                     (
 182                         format!(" `{}` and `{}`", first, second),
 183                         first.mut_addr().to_string(),
 184                         second.mut_addr().to_string(),
 185                     )
 186                 } else {
 187                     (String::new(), String::new(), String::new())
 188                 };
 189
 190                 let span = first.span.to(second.span);
 191
 192                 span_lint_and_then(cx,
 193                                    ALMOST_SWAPPED,
 194                                    span,
 195                                    &format!("this looks like you are trying to swap{}", what),
 196                                    |db| {
 197                                        if !what.is_empty() {
 198                                            db.span_suggestion(
 199                                                span,
 200                                                "try",
 201                                                format!(
 202                                                    "std::mem::swap({}, {})",
 203                                                    lhs,
 204                                                    rhs,
 205                                                ),
 206                                                Applicability::MaybeIncorrect,
 207                                            );
 208                                            db.note("or maybe you should use `std::mem::replace`?");
 209                                        }
 210                                    });
 211             }
 212         }
 213     }
 214 }