clippy_lints/src/swap.rs

   1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution.
   3 //
   4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. This file may not be copied, modified, or distributed
   8 // except according to those terms.
   9
  10 use crate::utils::sugg::Sugg;
  11 use crate::utils::{
  12     differing_macro_contexts, match_type, paths, snippet, span_lint_and_then, walk_ptrs_ty, SpanlessEq,
  13 };
  14 use if_chain::if_chain;
  15 use matches::matches;
  16 use rustc::hir::*;
  17 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  18 use rustc::ty;
  19 use rustc::{declare_tool_lint, lint_array};
  20 use rustc_errors::Applicability;
  21
  22 /// **What it does:** Checks for manual swapping.
  23 ///
  24 /// **Why is this bad?** The `std::mem::swap` function exposes the intent better
  25 /// without deinitializing or copying either variable.
  26 ///
  27 /// **Known problems:** None.
  28 ///
  29 /// **Example:**
  30 /// ```rust,ignore
  31 /// let t = b;
  32 /// b = a;
  33 /// a = t;
  34 /// ```
  35 /// Use std::mem::swap():
  36 /// ```rust
  37 /// std::mem::swap(&mut a, &mut b);
  38 /// ```
  39 declare_clippy_lint! {
  40     pub MANUAL_SWAP,
  41     complexity,
  42     "manual swap of two variables"
  43 }
  44
  45 /// **What it does:** Checks for `foo = bar; bar = foo` sequences.
  46 ///
  47 /// **Why is this bad?** This looks like a failed attempt to swap.
  48 ///
  49 /// **Known problems:** None.
  50 ///
  51 /// **Example:**
  52 /// ```rust,ignore
  53 /// a = b;
  54 /// b = a;
  55 /// ```
  56 declare_clippy_lint! {
  57     pub ALMOST_SWAPPED,
  58     correctness,
  59     "`foo = bar; bar = foo` sequence"
  60 }
  61
  62 #[derive(Copy, Clone)]
  63 pub struct Swap;
  64
  65 impl LintPass for Swap {
  66     fn get_lints(&self) -> LintArray {
  67         lint_array![MANUAL_SWAP, ALMOST_SWAPPED]
  68     }
  69 }
  70
  71 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Swap {
  72     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx Block) {
  73         check_manual_swap(cx, block);
  74         check_suspicious_swap(cx, block);
  75     }
  76 }
  77
  78 /// Implementation of the `MANUAL_SWAP` lint.
  79 fn check_manual_swap(cx: &LateContext<'_, '_>, block: &Block) {
  80     for w in block.stmts.windows(3) {
  81         if_chain! {
  82             // let t = foo();
  83             if let StmtKind::Decl(ref tmp, _) = w[0].node;
  84             if let DeclKind::Local(ref tmp) = tmp.node;
  85             if let Some(ref tmp_init) = tmp.init;
  86             if let PatKind::Binding(_, _, ident, None) = tmp.pat.node;
  87
  88             // foo() = bar();
  89             if let StmtKind::Semi(ref first, _) = w[1].node;
  90             if let ExprKind::Assign(ref lhs1, ref rhs1) = first.node;
  91
  92             // bar() = t;
  93             if let StmtKind::Semi(ref second, _) = w[2].node;
  94             if let ExprKind::Assign(ref lhs2, ref rhs2) = second.node;
  95             if let ExprKind::Path(QPath::Resolved(None, ref rhs2)) = rhs2.node;
  96             if rhs2.segments.len() == 1;
  97
  98             if ident.as_str() == rhs2.segments[0].ident.as_str();
  99             if SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1);
 100             if SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2);
 101             then {
 102                 fn check_for_slice<'a>(
 103                     cx: &LateContext<'_, '_>,
 104                     lhs1: &'a Expr,
 105                     lhs2: &'a Expr,
 106                 ) -> Option<(&'a Expr, &'a Expr, &'a Expr)> {
 107                     if let ExprKind::Index(ref lhs1, ref idx1) = lhs1.node {
 108                         if let ExprKind::Index(ref lhs2, ref idx2) = lhs2.node {
 109                             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
 110                                 let ty = walk_ptrs_ty(cx.tables.expr_ty(lhs1));
 111
 112                                 if matches!(ty.sty, ty::Slice(_)) ||
 113                                     matches!(ty.sty, ty::Array(_, _)) ||
 114                                     match_type(cx, ty, &paths::VEC) ||
 115                                     match_type(cx, ty, &paths::VEC_DEQUE) {
 116                                         return Some((lhs1, idx1, idx2));
 117                                 }
 118                             }
 119                         }
 120                     }
 121
 122                     None
 123                 }
 124
 125                 let (replace, what, sugg) = if let Some((slice, idx1, idx2)) = check_for_slice(cx, lhs1, lhs2) {
 126                     if let Some(slice) = Sugg::hir_opt(cx, slice) {
 127                         (false,
 128                          format!(" elements of `{}`", slice),
 129                          format!("{}.swap({}, {})",
 130                                  slice.maybe_par(),
 131                                  snippet(cx, idx1.span, ".."),
 132                                  snippet(cx, idx2.span, "..")))
 133                     } else {
 134                         (false, String::new(), String::new())
 135                     }
 136                 } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
 137                     (true, format!(" `{}` and `{}`", first, second),
 138                         format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()))
 139                 } else {
 140                     (true, String::new(), String::new())
 141                 };
 142
 143                 let span = w[0].span.to(second.span);
 144
 145                 span_lint_and_then(cx,
 146                                    MANUAL_SWAP,
 147                                    span,
 148                                    &format!("this looks like you are swapping{} manually", what),
 149                                    |db| {
 150                                        if !sugg.is_empty() {
 151                                            db.span_suggestion_with_applicability(
 152                                                span,
 153                                                "try",
 154                                                sugg,
 155                                                Applicability::Unspecified,
 156                                            );
 157
 158                                            if replace {
 159                                                db.note("or maybe you should use `std::mem::replace`?");
 160                                            }
 161                                        }
 162                                    });
 163             }
 164         }
 165     }
 166 }
 167
 168 /// Implementation of the `ALMOST_SWAPPED` lint.
 169 fn check_suspicious_swap(cx: &LateContext<'_, '_>, block: &Block) {
 170     for w in block.stmts.windows(2) {
 171         if_chain! {
 172             if let StmtKind::Semi(ref first, _) = w[0].node;
 173             if let StmtKind::Semi(ref second, _) = w[1].node;
 174             if !differing_macro_contexts(first.span, second.span);
 175             if let ExprKind::Assign(ref lhs0, ref rhs0) = first.node;
 176             if let ExprKind::Assign(ref lhs1, ref rhs1) = second.node;
 177             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1);
 178             if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, rhs0);
 179             then {
 180                 let lhs0 = Sugg::hir_opt(cx, lhs0);
 181                 let rhs0 = Sugg::hir_opt(cx, rhs0);
 182                 let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
 183                     (
 184                         format!(" `{}` and `{}`", first, second),
 185                         first.mut_addr().to_string(),
 186                         second.mut_addr().to_string(),
 187                     )
 188                 } else {
 189                     (String::new(), String::new(), String::new())
 190                 };
 191
 192                 let span = first.span.to(second.span);
 193
 194                 span_lint_and_then(cx,
 195                                    ALMOST_SWAPPED,
 196                                    span,
 197                                    &format!("this looks like you are trying to swap{}", what),
 198                                    |db| {
 199                                        if !what.is_empty() {
 200                                            db.span_suggestion_with_applicability(
 201                                                span,
 202                                                "try",
 203                                                format!(
 204                                                    "std::mem::swap({}, {})",
 205                                                    lhs,
 206                                                    rhs,
 207                                                ),
 208                                                Applicability::MaybeIncorrect,
 209                                            );
 210                                            db.note("or maybe you should use `std::mem::replace`?");
 211                                        }
 212                                    });
 213             }
 214         }
 215     }
 216 }