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};
11 declare_clippy_lint! {
13 /// Finds nested `match` or `if let` expressions where the patterns may be "collapsed" together
14 /// without adding any branches.
16 /// Note that this lint is not intended to find _all_ cases where nested match patterns can be merged, but only
17 /// cases where merging would most likely make the code more readable.
19 /// ### Why is this bad?
20 /// It is unnecessarily verbose and complex.
24 /// fn func(opt: Option<Result<u64, String>>) {
25 /// let n = match opt {
26 /// Some(n) => match n {
36 /// fn func(opt: Option<Result<u64, String>>) {
37 /// let n = match opt {
43 pub COLLAPSIBLE_MATCH,
45 "Nested `match` or `if let` expressions where the patterns may be \"collapsed\" together."
48 declare_lint_pass!(CollapsibleMatch => [COLLAPSIBLE_MATCH]);
50 impl<'tcx> LateLintPass<'tcx> for CollapsibleMatch {
51 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
52 if let ExprKind::Match(_expr, arms, _source) = expr.kind {
53 if let Some(wild_arm) = arms.iter().rfind(|arm| arm_is_wild_like(cx, arm)) {
55 check_arm(arm, wild_arm, cx);
62 fn check_arm<'tcx>(arm: &Arm<'tcx>, wild_outer_arm: &Arm<'tcx>, cx: &LateContext<'tcx>) {
63 let expr = strip_singleton_blocks(arm.body);
65 if let ExprKind::Match(expr_in, arms_inner, _) = expr.kind;
66 // the outer arm pattern and the inner match
67 if expr_in.span.ctxt() == arm.pat.span.ctxt();
68 // there must be no more than two arms in the inner match for this lint
69 if arms_inner.len() == 2;
70 // no if guards on the inner match
71 if arms_inner.iter().all(|arm| arm.guard.is_none());
72 // match expression must be a local binding
73 // match <local> { .. }
74 if let Some(binding_id) = path_to_local(peel_ref_operators(cx, expr_in));
75 // one of the branches must be "wild-like"
76 if let Some(wild_inner_arm_idx) = arms_inner.iter().rposition(|arm_inner| arm_is_wild_like(cx, arm_inner));
77 let (wild_inner_arm, non_wild_inner_arm) =
78 (&arms_inner[wild_inner_arm_idx], &arms_inner[1 - wild_inner_arm_idx]);
79 if !pat_contains_or(non_wild_inner_arm.pat);
80 // the binding must come from the pattern of the containing match arm
81 // ..<local>.. => match <local> { .. }
82 if let Some(binding_span) = find_pat_binding(arm.pat, binding_id);
83 // the "wild-like" branches must be equal
84 if SpanlessEq::new(cx).eq_expr(wild_inner_arm.body, wild_outer_arm.body);
85 // the binding must not be used in the if guard
86 let mut used_visitor = LocalUsedVisitor::new(cx, binding_id);
89 Some(Guard::If(expr) | Guard::IfLet(_, expr)) => !used_visitor.check_expr(expr),
91 // ...or anywhere in the inner match
92 if !arms_inner.iter().any(|arm| used_visitor.check_arm(arm));
98 "unnecessary nested match",
100 let mut help_span = MultiSpan::from_spans(vec![binding_span, non_wild_inner_arm.pat.span]);
101 help_span.push_span_label(binding_span, "replace this binding".into());
102 help_span.push_span_label(non_wild_inner_arm.pat.span, "with this pattern".into());
103 diag.span_help(help_span, "the outer pattern can be modified to include the inner pattern");
110 fn strip_singleton_blocks<'hir>(mut expr: &'hir Expr<'hir>) -> &'hir Expr<'hir> {
111 while let ExprKind::Block(block, _) = expr.kind {
112 match (block.stmts, block.expr) {
113 ([stmt], None) => match stmt.kind {
114 StmtKind::Expr(e) | StmtKind::Semi(e) => expr = e,
117 ([], Some(e)) => expr = e,
124 /// A "wild-like" pattern is wild ("_") or `None`.
125 /// For this lint to apply, both the outer and inner match expressions
126 /// must have "wild-like" branches that can be combined.
127 fn arm_is_wild_like(cx: &LateContext<'_>, arm: &Arm<'_>) -> bool {
128 if arm.guard.is_some() {
132 PatKind::Binding(..) | PatKind::Wild => true,
133 PatKind::Path(ref qpath) => is_lang_ctor(cx, qpath, OptionNone),
138 fn find_pat_binding(pat: &Pat<'_>, hir_id: HirId) -> Option<Span> {
140 pat.walk_short(|p| match &p.kind {
141 // ignore OR patterns
142 PatKind::Or(_) => false,
143 PatKind::Binding(_bm, _, _ident, _) => {
144 let found = p.hir_id == hir_id;
155 fn pat_contains_or(pat: &Pat<'_>) -> bool {
156 let mut result = false;
158 let is_or = matches!(p.kind, PatKind::Or(_));