src/tools/clippy/clippy_lints/src/collapsible_match.rs

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::visitors::LocalUsedVisitor;
   3 use clippy_utils::{is_lang_ctor, path_to_local, peel_ref_operators, SpanlessEq};
   4 use if_chain::if_chain;
   5 use rustc_hir::LangItem::OptionNone;
   6 use rustc_hir::{Arm, Expr, ExprKind, Guard, HirId, Pat, PatKind, StmtKind};
   7 use rustc_lint::{LateContext, LateLintPass};
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   9 use rustc_span::{MultiSpan, Span};
  10
  11 declare_clippy_lint! {
  12     /// **What it does:** Finds nested `match` or `if let` expressions where the patterns may be "collapsed" together
  13     /// without adding any branches.
  14     ///
  15     /// Note that this lint is not intended to find _all_ cases where nested match patterns can be merged, but only
  16     /// cases where merging would most likely make the code more readable.
  17     ///
  18     /// **Why is this bad?** It is unnecessarily verbose and complex.
  19     ///
  20     /// **Known problems:** None.
  21     ///
  22     /// **Example:**
  23     ///
  24     /// ```rust
  25     /// fn func(opt: Option<Result<u64, String>>) {
  26     ///     let n = match opt {
  27     ///         Some(n) => match n {
  28     ///             Ok(n) => n,
  29     ///             _ => return,
  30     ///         }
  31     ///         None => return,
  32     ///     };
  33     /// }
  34     /// ```
  35     /// Use instead:
  36     /// ```rust
  37     /// fn func(opt: Option<Result<u64, String>>) {
  38     ///     let n = match opt {
  39     ///         Some(Ok(n)) => n,
  40     ///         _ => return,
  41     ///     };
  42     /// }
  43     /// ```
  44     pub COLLAPSIBLE_MATCH,
  45     style,
  46     "Nested `match` or `if let` expressions where the patterns may be \"collapsed\" together."
  47 }
  48
  49 declare_lint_pass!(CollapsibleMatch => [COLLAPSIBLE_MATCH]);
  50
  51 impl<'tcx> LateLintPass<'tcx> for CollapsibleMatch {
  52     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
  53         if let ExprKind::Match(_expr, arms, _source) = expr.kind {
  54             if let Some(wild_arm) = arms.iter().rfind(|arm| arm_is_wild_like(cx, arm)) {
  55                 for arm in arms {
  56                     check_arm(arm, wild_arm, cx);
  57                 }
  58             }
  59         }
  60     }
  61 }
  62
  63 fn check_arm<'tcx>(arm: &Arm<'tcx>, wild_outer_arm: &Arm<'tcx>, cx: &LateContext<'tcx>) {
  64     let expr = strip_singleton_blocks(arm.body);
  65     if_chain! {
  66         if let ExprKind::Match(expr_in, arms_inner, _) = expr.kind;
  67         // the outer arm pattern and the inner match
  68         if expr_in.span.ctxt() == arm.pat.span.ctxt();
  69         // there must be no more than two arms in the inner match for this lint
  70         if arms_inner.len() == 2;
  71         // no if guards on the inner match
  72         if arms_inner.iter().all(|arm| arm.guard.is_none());
  73         // match expression must be a local binding
  74         // match <local> { .. }
  75         if let Some(binding_id) = path_to_local(peel_ref_operators(cx, expr_in));
  76         // one of the branches must be "wild-like"
  77         if let Some(wild_inner_arm_idx) = arms_inner.iter().rposition(|arm_inner| arm_is_wild_like(cx, arm_inner));
  78         let (wild_inner_arm, non_wild_inner_arm) =
  79             (&arms_inner[wild_inner_arm_idx], &arms_inner[1 - wild_inner_arm_idx]);
  80         if !pat_contains_or(non_wild_inner_arm.pat);
  81         // the binding must come from the pattern of the containing match arm
  82         // ..<local>.. => match <local> { .. }
  83         if let Some(binding_span) = find_pat_binding(arm.pat, binding_id);
  84         // the "wild-like" branches must be equal
  85         if SpanlessEq::new(cx).eq_expr(wild_inner_arm.body, wild_outer_arm.body);
  86         // the binding must not be used in the if guard
  87         let mut used_visitor = LocalUsedVisitor::new(cx, binding_id);
  88         if match arm.guard {
  89             None => true,
  90             Some(Guard::If(expr) | Guard::IfLet(_, expr)) => !used_visitor.check_expr(expr),
  91         };
  92         // ...or anywhere in the inner match
  93         if !arms_inner.iter().any(|arm| used_visitor.check_arm(arm));
  94         then {
  95             span_lint_and_then(
  96                 cx,
  97                 COLLAPSIBLE_MATCH,
  98                 expr.span,
  99                 "unnecessary nested match",
 100                 |diag| {
 101                     let mut help_span = MultiSpan::from_spans(vec![binding_span, non_wild_inner_arm.pat.span]);
 102                     help_span.push_span_label(binding_span, "replace this binding".into());
 103                     help_span.push_span_label(non_wild_inner_arm.pat.span, "with this pattern".into());
 104                     diag.span_help(help_span, "the outer pattern can be modified to include the inner pattern");
 105                 },
 106             );
 107         }
 108     }
 109 }
 110
 111 fn strip_singleton_blocks<'hir>(mut expr: &'hir Expr<'hir>) -> &'hir Expr<'hir> {
 112     while let ExprKind::Block(block, _) = expr.kind {
 113         match (block.stmts, block.expr) {
 114             ([stmt], None) => match stmt.kind {
 115                 StmtKind::Expr(e) | StmtKind::Semi(e) => expr = e,
 116                 _ => break,
 117             },
 118             ([], Some(e)) => expr = e,
 119             _ => break,
 120         }
 121     }
 122     expr
 123 }
 124
 125 /// A "wild-like" pattern is wild ("_") or `None`.
 126 /// For this lint to apply, both the outer and inner match expressions
 127 /// must have "wild-like" branches that can be combined.
 128 fn arm_is_wild_like(cx: &LateContext<'_>, arm: &Arm<'_>) -> bool {
 129     if arm.guard.is_some() {
 130         return false;
 131     }
 132     match arm.pat.kind {
 133         PatKind::Binding(..) | PatKind::Wild => true,
 134         PatKind::Path(ref qpath) => is_lang_ctor(cx, qpath, OptionNone),
 135         _ => false,
 136     }
 137 }
 138
 139 fn find_pat_binding(pat: &Pat<'_>, hir_id: HirId) -> Option<Span> {
 140     let mut span = None;
 141     pat.walk_short(|p| match &p.kind {
 142         // ignore OR patterns
 143         PatKind::Or(_) => false,
 144         PatKind::Binding(_bm, _, _ident, _) => {
 145             let found = p.hir_id == hir_id;
 146             if found {
 147                 span = Some(p.span);
 148             }
 149             !found
 150         },
 151         _ => true,
 152     });
 153     span
 154 }
 155
 156 fn pat_contains_or(pat: &Pat<'_>) -> bool {
 157     let mut result = false;
 158     pat.walk(|p| {
 159         let is_or = matches!(p.kind, PatKind::Or(_));
 160         result |= is_or;
 161         !is_or
 162     });
 163     result
 164 }