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

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