src/tools/clippy/clippy_lints/src/comparison_chain.rs

   1 use clippy_utils::diagnostics::span_lint_and_help;
   2 use clippy_utils::ty::implements_trait;
   3 use clippy_utils::{get_trait_def_id, if_sequence, in_constant, is_else_clause, paths, SpanlessEq};
   4 use rustc_hir::{BinOpKind, Expr, ExprKind};
   5 use rustc_lint::{LateContext, LateLintPass};
   6 use rustc_session::{declare_lint_pass, declare_tool_lint};
   7
   8 declare_clippy_lint! {
   9     /// ### What it does
  10     /// Checks comparison chains written with `if` that can be
  11     /// rewritten with `match` and `cmp`.
  12     ///
  13     /// ### Why is this bad?
  14     /// `if` is not guaranteed to be exhaustive and conditionals can get
  15     /// repetitive
  16     ///
  17     /// ### Known problems
  18     /// The match statement may be slower due to the compiler
  19     /// not inlining the call to cmp. See issue [#5354](https://github.com/rust-lang/rust-clippy/issues/5354)
  20     ///
  21     /// ### Example
  22     /// ```rust,ignore
  23     /// # fn a() {}
  24     /// # fn b() {}
  25     /// # fn c() {}
  26     /// fn f(x: u8, y: u8) {
  27     ///     if x > y {
  28     ///         a()
  29     ///     } else if x < y {
  30     ///         b()
  31     ///     } else {
  32     ///         c()
  33     ///     }
  34     /// }
  35     /// ```
  36     ///
  37     /// Use instead:
  38     /// ```rust,ignore
  39     /// use std::cmp::Ordering;
  40     /// # fn a() {}
  41     /// # fn b() {}
  42     /// # fn c() {}
  43     /// fn f(x: u8, y: u8) {
  44     ///      match x.cmp(&y) {
  45     ///          Ordering::Greater => a(),
  46     ///          Ordering::Less => b(),
  47     ///          Ordering::Equal => c()
  48     ///      }
  49     /// }
  50     /// ```
  51     #[clippy::version = "1.40.0"]
  52     pub COMPARISON_CHAIN,
  53     style,
  54     "`if`s that can be rewritten with `match` and `cmp`"
  55 }
  56
  57 declare_lint_pass!(ComparisonChain => [COMPARISON_CHAIN]);
  58
  59 impl<'tcx> LateLintPass<'tcx> for ComparisonChain {
  60     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  61         if expr.span.from_expansion() {
  62             return;
  63         }
  64
  65         // We only care about the top-most `if` in the chain
  66         if is_else_clause(cx.tcx, expr) {
  67             return;
  68         }
  69
  70         if in_constant(cx, expr.hir_id) {
  71             return;
  72         }
  73
  74         // Check that there exists at least one explicit else condition
  75         let (conds, _) = if_sequence(expr);
  76         if conds.len() < 2 {
  77             return;
  78         }
  79
  80         for cond in conds.windows(2) {
  81             if let (&ExprKind::Binary(ref kind1, lhs1, rhs1), &ExprKind::Binary(ref kind2, lhs2, rhs2)) =
  82                 (&cond[0].kind, &cond[1].kind)
  83             {
  84                 if !kind_is_cmp(kind1.node) || !kind_is_cmp(kind2.node) {
  85                     return;
  86                 }
  87
  88                 // Check that both sets of operands are equal
  89                 let mut spanless_eq = SpanlessEq::new(cx);
  90                 let same_fixed_operands = spanless_eq.eq_expr(lhs1, lhs2) && spanless_eq.eq_expr(rhs1, rhs2);
  91                 let same_transposed_operands = spanless_eq.eq_expr(lhs1, rhs2) && spanless_eq.eq_expr(rhs1, lhs2);
  92
  93                 if !same_fixed_operands && !same_transposed_operands {
  94                     return;
  95                 }
  96
  97                 // Check that if the operation is the same, either it's not `==` or the operands are transposed
  98                 if kind1.node == kind2.node {
  99                     if kind1.node == BinOpKind::Eq {
 100                         return;
 101                     }
 102                     if !same_transposed_operands {
 103                         return;
 104                     }
 105                 }
 106
 107                 // Check that the type being compared implements `core::cmp::Ord`
 108                 let ty = cx.typeck_results().expr_ty(lhs1);
 109                 let is_ord = get_trait_def_id(cx, &paths::ORD).map_or(false, |id| implements_trait(cx, ty, id, &[]));
 110
 111                 if !is_ord {
 112                     return;
 113                 }
 114             } else {
 115                 // We only care about comparison chains
 116                 return;
 117             }
 118         }
 119         span_lint_and_help(
 120             cx,
 121             COMPARISON_CHAIN,
 122             expr.span,
 123             "`if` chain can be rewritten with `match`",
 124             None,
 125             "consider rewriting the `if` chain to use `cmp` and `match`",
 126         );
 127     }
 128 }
 129
 130 fn kind_is_cmp(kind: BinOpKind) -> bool {
 131     matches!(kind, BinOpKind::Lt | BinOpKind::Gt | BinOpKind::Eq)
 132 }