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