1//! calculate cognitive complexity and warn about overly complex functions
2
3use clippy_utils::diagnostics::span_lint_and_help;
4use clippy_utils::source::snippet_opt;
5use clippy_utils::ty::is_type_diagnostic_item;
6use clippy_utils::visitors::for_each_expr;
7use clippy_utils::{get_async_fn_body, is_async_fn, LimitStack};
8use core::ops::ControlFlow;
9use rustc_ast::ast::Attribute;
10use rustc_hir::intravisit::FnKind;
11use rustc_hir::{Body, Expr, ExprKind, FnDecl};
12use rustc_lint::{LateContext, LateLintPass, LintContext};
13use rustc_session::impl_lint_pass;
14use rustc_span::def_id::LocalDefId;
15use rustc_span::{sym, BytePos, Span};
16
17declare_clippy_lint! {
18 /// ### What it does
19 /// Checks for methods with high cognitive complexity.
20 ///
21 /// ### Why is this bad?
22 /// Methods of high cognitive complexity tend to be hard to
23 /// both read and maintain. Also LLVM will tend to optimize small methods better.
24 ///
25 /// ### Known problems
26 /// Sometimes it's hard to find a way to reduce the
27 /// complexity.
28 ///
29 /// ### Example
30 /// You'll see it when you get the warning.
31 #[clippy::version = "1.35.0"]
32 pub COGNITIVE_COMPLEXITY,
33 nursery,
34 "functions that should be split up into multiple functions"
35}
36
37pub struct CognitiveComplexity {
38 limit: LimitStack,
39}
40
41impl CognitiveComplexity {
42 #[must_use]
43 pub fn new(limit: u64) -> Self {
44 Self {
45 limit: LimitStack::new(limit),
46 }
47 }
48}
49
50impl_lint_pass!(CognitiveComplexity => [COGNITIVE_COMPLEXITY]);
51
52impl CognitiveComplexity {
53 #[expect(clippy::cast_possible_truncation)]
54 fn check<'tcx>(
55 &mut self,
56 cx: &LateContext<'tcx>,
57 kind: FnKind<'tcx>,
58 decl: &'tcx FnDecl<'_>,
59 expr: &'tcx Expr<'_>,
60 body_span: Span,
61 ) {
62 if body_span.from_expansion() {
63 return;
64 }
65
66 let mut cc = 1u64;
67 let mut returns = 0u64;
68 let _: Option<!> = for_each_expr(expr, |e| {
69 match e.kind {
70 ExprKind::If(_, _, _) => {
71 cc += 1;
72 },
73 ExprKind::Match(_, arms, _) => {
74 if arms.len() > 1 {
75 cc += 1;
76 }
77 cc += arms.iter().filter(|arm| arm.guard.is_some()).count() as u64;
78 },
79 ExprKind::Ret(_) => returns += 1,
80 _ => {},
81 }
82 ControlFlow::Continue(())
83 });
84
85 let ret_ty = cx.typeck_results().node_type(expr.hir_id);
86 let ret_adjust = if is_type_diagnostic_item(cx, ret_ty, sym::Result) {
87 returns
88 } else {
89 #[expect(clippy::integer_division)]
90 (returns / 2)
91 };
92
93 // prevent degenerate cases where unreachable code contains `return` statements
94 if cc >= ret_adjust {
95 cc -= ret_adjust;
96 }
97
98 if cc > self.limit.limit() {
99 let fn_span = match kind {
100 FnKind::ItemFn(ident, _, _) | FnKind::Method(ident, _) => ident.span,
101 FnKind::Closure => {
102 let header_span = body_span.with_hi(decl.output.span().lo());
103 let pos = snippet_opt(cx, header_span).and_then(|snip| {
104 let low_offset = snip.find('|')?;
105 let high_offset = 1 + snip.get(low_offset + 1..)?.find('|')?;
106 let low = header_span.lo() + BytePos(low_offset as u32);
107 let high = low + BytePos(high_offset as u32 + 1);
108
109 Some((low, high))
110 });
111
112 if let Some((low, high)) = pos {
113 Span::new(low, high, header_span.ctxt(), header_span.parent())
114 } else {
115 return;
116 }
117 },
118 };
119
120 span_lint_and_help(
121 cx,
122 COGNITIVE_COMPLEXITY,
123 fn_span,
124 &format!(
125 "the function has a cognitive complexity of ({cc}/{})",
126 self.limit.limit()
127 ),
128 None,
129 "you could split it up into multiple smaller functions",
130 );
131 }
132 }
133}
134
135impl<'tcx> LateLintPass<'tcx> for CognitiveComplexity {
136 fn check_fn(
137 &mut self,
138 cx: &LateContext<'tcx>,
139 kind: FnKind<'tcx>,
140 decl: &'tcx FnDecl<'_>,
141 body: &'tcx Body<'_>,
142 span: Span,
143 def_id: LocalDefId,
144 ) {
145 if !cx.tcx.has_attr(def_id, sym::test) {
146 let expr = if is_async_fn(kind) {
147 match get_async_fn_body(cx.tcx, body) {
148 Some(b) => b,
149 None => {
150 return;
151 },
152 }
153 } else {
154 body.value
155 };
156
157 self.check(cx, kind, decl, expr, span);
158 }
159 }
160
161 fn enter_lint_attrs(&mut self, cx: &LateContext<'tcx>, attrs: &'tcx [Attribute]) {
162 self.limit.push_attrs(cx.sess(), attrs, "cognitive_complexity");
163 }
164 fn exit_lint_attrs(&mut self, cx: &LateContext<'tcx>, attrs: &'tcx [Attribute]) {
165 self.limit.pop_attrs(cx.sess(), attrs, "cognitive_complexity");
166 }
167}
168

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