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| 1 | +use crate::utils::visitors::LocalUsedVisitor; |
| 2 | +use crate::utils::{span_lint_and_then, SpanlessEq}; |
| 3 | +use if_chain::if_chain; |
| 4 | +use rustc_hir::def::{CtorKind, CtorOf, DefKind, Res}; |
| 5 | +use rustc_hir::{Arm, Expr, ExprKind, Guard, HirId, Pat, PatKind, QPath, StmtKind}; |
| 6 | +use rustc_lint::{LateContext, LateLintPass}; |
| 7 | +use rustc_middle::ty::{DefIdTree, TyCtxt}; |
| 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(arm, cx.tcx)) { |
| 55 | + for arm in arms { |
| 56 | + check_arm(arm, wild_arm, cx); |
| 57 | + } |
| 58 | + } |
| 59 | + } |
| 60 | + } |
| 61 | +} |
| 62 | + |
| 63 | +fn check_arm(arm: &Arm<'_>, wild_outer_arm: &Arm<'_>, cx: &LateContext<'_>) { |
| 64 | + if_chain! { |
| 65 | + let expr = strip_singleton_blocks(arm.body); |
| 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 ExprKind::Path(QPath::Resolved(None, path)) = expr_in.kind; |
| 76 | + if let Res::Local(binding_id) = path.res; |
| 77 | + // one of the branches must be "wild-like" |
| 78 | + if let Some(wild_inner_arm_idx) = arms_inner.iter().rposition(|arm_inner| arm_is_wild_like(arm_inner, cx.tcx)); |
| 79 | + let (wild_inner_arm, non_wild_inner_arm) = |
| 80 | + (&arms_inner[wild_inner_arm_idx], &arms_inner[1 - wild_inner_arm_idx]); |
| 81 | + if !pat_contains_or(non_wild_inner_arm.pat); |
| 82 | + // the binding must come from the pattern of the containing match arm |
| 83 | + // ..<local>.. => match <local> { .. } |
| 84 | + if let Some(binding_span) = find_pat_binding(arm.pat, binding_id); |
| 85 | + // the "wild-like" branches must be equal |
| 86 | + if SpanlessEq::new(cx).eq_expr(wild_inner_arm.body, wild_outer_arm.body); |
| 87 | + // the binding must not be used in the if guard |
| 88 | + if !matches!(arm.guard, Some(Guard::If(guard)) if LocalUsedVisitor::new(binding_id).check_expr(guard)); |
| 89 | + // ...or anywhere in the inner match |
| 90 | + if !arms_inner.iter().any(|arm| LocalUsedVisitor::new(binding_id).check_arm(arm)); |
| 91 | + then { |
| 92 | + span_lint_and_then( |
| 93 | + cx, |
| 94 | + COLLAPSIBLE_MATCH, |
| 95 | + expr.span, |
| 96 | + "Unnecessary nested match", |
| 97 | + |diag| { |
| 98 | + let mut help_span = MultiSpan::from_spans(vec![binding_span, non_wild_inner_arm.pat.span]); |
| 99 | + help_span.push_span_label(binding_span, "Replace this binding".into()); |
| 100 | + help_span.push_span_label(non_wild_inner_arm.pat.span, "with this pattern".into()); |
| 101 | + diag.span_help(help_span, "The outer pattern can be modified to include the inner pattern."); |
| 102 | + }, |
| 103 | + ); |
| 104 | + } |
| 105 | + } |
| 106 | +} |
| 107 | + |
| 108 | +fn strip_singleton_blocks<'hir>(mut expr: &'hir Expr<'hir>) -> &'hir Expr<'hir> { |
| 109 | + while let ExprKind::Block(block, _) = expr.kind { |
| 110 | + match (block.stmts, block.expr) { |
| 111 | + ([stmt], None) => match stmt.kind { |
| 112 | + StmtKind::Expr(e) | StmtKind::Semi(e) => expr = e, |
| 113 | + _ => break, |
| 114 | + }, |
| 115 | + ([], Some(e)) => expr = e, |
| 116 | + _ => break, |
| 117 | + } |
| 118 | + } |
| 119 | + expr |
| 120 | +} |
| 121 | + |
| 122 | +/// A "wild-like" pattern is wild ("_") or `None`. |
| 123 | +/// For this lint to apply, both the outer and inner match expressions |
| 124 | +/// must have "wild-like" branches that can be combined. |
| 125 | +fn arm_is_wild_like(arm: &Arm<'_>, tcx: TyCtxt<'_>) -> bool { |
| 126 | + if arm.guard.is_some() { |
| 127 | + return false; |
| 128 | + } |
| 129 | + match arm.pat.kind { |
| 130 | + PatKind::Binding(..) | PatKind::Wild => true, |
| 131 | + PatKind::Path(QPath::Resolved(None, path)) if is_none_ctor(path.res, tcx) => true, |
| 132 | + _ => false, |
| 133 | + } |
| 134 | +} |
| 135 | + |
| 136 | +fn find_pat_binding(pat: &Pat<'_>, hir_id: HirId) -> Option<Span> { |
| 137 | + let mut span = None; |
| 138 | + pat.walk_short(|p| match &p.kind { |
| 139 | + // ignore OR patterns |
| 140 | + PatKind::Or(_) => false, |
| 141 | + PatKind::Binding(_bm, _, _ident, _) => { |
| 142 | + let found = p.hir_id == hir_id; |
| 143 | + if found { |
| 144 | + span = Some(p.span); |
| 145 | + } |
| 146 | + !found |
| 147 | + }, |
| 148 | + _ => true, |
| 149 | + }); |
| 150 | + span |
| 151 | +} |
| 152 | + |
| 153 | +fn pat_contains_or(pat: &Pat<'_>) -> bool { |
| 154 | + let mut result = false; |
| 155 | + pat.walk(|p| { |
| 156 | + let is_or = matches!(p.kind, PatKind::Or(_)); |
| 157 | + result |= is_or; |
| 158 | + !is_or |
| 159 | + }); |
| 160 | + result |
| 161 | +} |
| 162 | + |
| 163 | +fn is_none_ctor(res: Res, tcx: TyCtxt<'_>) -> bool { |
| 164 | + if let Some(none_id) = tcx.lang_items().option_none_variant() { |
| 165 | + if let Res::Def(DefKind::Ctor(CtorOf::Variant, CtorKind::Const), id) = res { |
| 166 | + if let Some(variant_id) = tcx.parent(id) { |
| 167 | + return variant_id == none_id; |
| 168 | + } |
| 169 | + } |
| 170 | + } |
| 171 | + false |
| 172 | +} |
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