clippy_lints/src/swap.rs

   1 use crate::utils::sugg::Sugg;
   2 use crate::utils::{
   3     differing_macro_contexts, is_type_diagnostic_item, match_type, paths, snippet, span_lint_and_then, walk_ptrs_ty,
   4     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].kind;
  83             if let Some(ref tmp_init) = tmp.init;
  84             if let PatKind::Binding(.., ident, None) = tmp.pat.kind;
  85
  86             // foo() = bar();
  87             if let StmtKind::Semi(ref first) = w[1].kind;
  88             if let ExprKind::Assign(ref lhs1, ref rhs1) = first.kind;
  89
  90             // bar() = t;
  91             if let StmtKind::Semi(ref second) = w[2].kind;
  92             if let ExprKind::Assign(ref lhs2, ref rhs2) = second.kind;
  93             if let ExprKind::Path(QPath::Resolved(None, ref rhs2)) = rhs2.kind;
  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.kind {
 106                         if let ExprKind::Index(ref lhs2, ref idx2) = lhs2.kind {
 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.kind, ty::Slice(_)) ||
 111                                     matches!(ty.kind, 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                 if let ExprKind::Field(ref lhs1, _) = lhs1.kind {
 124                     if let ExprKind::Field(ref lhs2, _) = lhs2.kind {
 125                         if lhs1.hir_id.owner_def_id() == lhs2.hir_id.owner_def_id() {
 126                             return;
 127                         }
 128                     }
 129                 }
 130
 131                 let (replace, what, sugg) = if let Some((slice, idx1, idx2)) = check_for_slice(cx, lhs1, lhs2) {
 132                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 133                         (false,
 134                          format!(" elements of `{}`", slice),
 135                          format!("{}.swap({}, {})",
 136                                  slice.maybe_par(),
 137                                  snippet(cx, idx1.span, ".."),
 138                                  snippet(cx, idx2.span, "..")))
 139                     } else {
 140                         (false, String::new(), String::new())
 141                     }
 142                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 143                     (true, format!(" `{}` and `{}`", first, second),
 144                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()))
 145                 } else {
 146                     (true, String::new(), String::new())
 147                 };
 148
 149                 let span = w[0].span.to(second.span);
 150
 151                 span_lint_and_then(cx,
 152                                    MANUAL_SWAP,
 153                                    span,
 154                                    &format!("this looks like you are swapping{} manually", what),
 155                                    |db| {
 156                                        if !sugg.is_empty() {
 157                                            db.span_suggestion(
 158                                                span,
 159                                                "try",
 160                                                sugg,
 161                                                Applicability::Unspecified,
 162                                            );
 163
 164                                            if replace {
 165                                                db.note("or maybe you should use `std::mem::replace`?");
 166                                            }
 167                                        }
 168                                    });
 169             }
 170         }
 171     }
 172 }
 173
 174 /// Implementation of the `ALMOST_SWAPPED` lint.
 175 fn check_suspicious_swap(cx: &LateContext<'_, '_>, block: &Block) {
 176     for w in block.stmts.windows(2) {
 177         if_chain! {
 178             if let StmtKind::Semi(ref first) = w[0].kind;
 179             if let StmtKind::Semi(ref second) = w[1].kind;
 180             if !differing_macro_contexts(first.span, second.span);
 181             if let ExprKind::Assign(ref lhs0, ref rhs0) = first.kind;
 182             if let ExprKind::Assign(ref lhs1, ref rhs1) = second.kind;
 183             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1);
 184             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0);
 185             then {
 186                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 187                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 188                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 189                     (
 190                         format!(" `{}` and `{}`", first, second),
 191                         first.mut_addr().to_string(),
 192                         second.mut_addr().to_string(),
 193                     )
 194                 } else {
 195                     (String::new(), String::new(), String::new())
 196                 };
 197
 198                 let span = first.span.to(second.span);
 199
 200                 span_lint_and_then(cx,
 201                                    ALMOST_SWAPPED,
 202                                    span,
 203                                    &format!("this looks like you are trying to swap{}", what),
 204                                    |db| {
 205                                        if !what.is_empty() {
 206                                            db.span_suggestion(
 207                                                span,
 208                                                "try",
 209                                                format!(
 210                                                    "std::mem::swap({}, {})",
 211                                                    lhs,
 212                                                    rhs,
 213                                                ),
 214                                                Applicability::MaybeIncorrect,
 215                                            );
 216                                            db.note("or maybe you should use `std::mem::replace`?");
 217                                        }
 218                                    });
 219             }
 220         }
 221     }
 222 }