src/tools/clippy/clippy_lints/src/equatable_if_let.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::source::snippet_with_context;
   3 use clippy_utils::ty::implements_trait;
   4 use if_chain::if_chain;
   5 use rustc_errors::Applicability;
   6 use rustc_hir::{Expr, ExprKind, Pat, PatKind};
   7 use rustc_lint::{LateContext, LateLintPass};
   8 use rustc_middle::ty::Ty;
   9 use rustc_session::{declare_lint_pass, declare_tool_lint};
  10
  11 declare_clippy_lint! {
  12     /// ### What it does
  13     /// Checks for pattern matchings that can be expressed using equality.
  14     ///
  15     /// ### Why is this bad?
  16     ///
  17     /// * It reads better and has less cognitive load because equality won't cause binding.
  18     /// * It is a [Yoda condition](https://en.wikipedia.org/wiki/Yoda_conditions). Yoda conditions are widely
  19     /// criticized for increasing the cognitive load of reading the code.
  20     /// * Equality is a simple bool expression and can be merged with `&&` and `||` and
  21     /// reuse if blocks
  22     ///
  23     /// ### Example
  24     /// ```rust,ignore
  25     /// if let Some(2) = x {
  26     ///     do_thing();
  27     /// }
  28     /// ```
  29     /// Should be written
  30     /// ```rust,ignore
  31     /// if x == Some(2) {
  32     ///     do_thing();
  33     /// }
  34     /// ```
  35     pub EQUATABLE_IF_LET,
  36     nursery,
  37     "using pattern matching instead of equality"
  38 }
  39
  40 declare_lint_pass!(PatternEquality => [EQUATABLE_IF_LET]);
  41
  42 /// detects if pattern matches just one thing
  43 fn unary_pattern(pat: &Pat<'_>) -> bool {
  44     fn array_rec(pats: &[Pat<'_>]) -> bool {
  45         pats.iter().all(unary_pattern)
  46     }
  47     match &pat.kind {
  48         PatKind::Slice(_, _, _) | PatKind::Range(_, _, _) | PatKind::Binding(..) | PatKind::Wild | PatKind::Or(_) => {
  49             false
  50         },
  51         PatKind::Struct(_, a, etc) => !etc && a.iter().all(|x| unary_pattern(x.pat)),
  52         PatKind::Tuple(a, etc) | PatKind::TupleStruct(_, a, etc) => !etc.is_some() && array_rec(a),
  53         PatKind::Ref(x, _) | PatKind::Box(x) => unary_pattern(x),
  54         PatKind::Path(_) | PatKind::Lit(_) => true,
  55     }
  56 }
  57
  58 fn is_structural_partial_eq(cx: &LateContext<'tcx>, ty: Ty<'tcx>, other: Ty<'tcx>) -> bool {
  59     if let Some(def_id) = cx.tcx.lang_items().eq_trait() {
  60         implements_trait(cx, ty, def_id, &[other.into()])
  61     } else {
  62         false
  63     }
  64 }
  65
  66 impl<'tcx> LateLintPass<'tcx> for PatternEquality {
  67     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
  68         if_chain! {
  69             if let ExprKind::Let(pat, exp, _) = expr.kind;
  70             if unary_pattern(pat);
  71             let exp_ty = cx.typeck_results().expr_ty(exp);
  72             let pat_ty = cx.typeck_results().pat_ty(pat);
  73             if is_structural_partial_eq(cx, exp_ty, pat_ty);
  74             then {
  75
  76                 let mut applicability = Applicability::MachineApplicable;
  77                 let pat_str = match pat.kind {
  78                     PatKind::Struct(..) => format!(
  79                         "({})",
  80                         snippet_with_context(cx, pat.span, expr.span.ctxt(), "..", &mut applicability).0,
  81                     ),
  82                     _ => snippet_with_context(cx, pat.span, expr.span.ctxt(), "..", &mut applicability).0.to_string(),
  83                 };
  84                 span_lint_and_sugg(
  85                     cx,
  86                     EQUATABLE_IF_LET,
  87                     expr.span,
  88                     "this pattern matching can be expressed using equality",
  89                     "try",
  90                     format!(
  91                         "{} == {}",
  92                         snippet_with_context(cx, exp.span, expr.span.ctxt(), "..", &mut applicability).0,
  93                         pat_str,
  94                     ),
  95                     applicability,
  96                 );
  97             }
  98         }
  99     }
 100 }