src/tools/clippy/clippy_lints/src/cognitive_complexity.rs

   1 //! calculate cognitive complexity and warn about overly complex functions
   2
   3 use clippy_utils::diagnostics::span_lint_and_help;
   4 use clippy_utils::source::snippet_opt;
   5 use clippy_utils::ty::is_type_diagnostic_item;
   6 use clippy_utils::visitors::for_each_expr;
   7 use clippy_utils::LimitStack;
   8 use core::ops::ControlFlow;
   9 use rustc_ast::ast::Attribute;
  10 use rustc_hir::intravisit::FnKind;
  11 use rustc_hir::{Body, ExprKind, FnDecl, HirId};
  12 use rustc_lint::{LateContext, LateLintPass, LintContext};
  13 use rustc_session::{declare_tool_lint, impl_lint_pass};
  14 use rustc_span::source_map::Span;
  15 use rustc_span::{sym, BytePos};
  16
  17 declare_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
  37 pub struct CognitiveComplexity {
  38     limit: LimitStack,
  39 }
  40
  41 impl CognitiveComplexity {
  42     #[must_use]
  43     pub fn new(limit: u64) -> Self {
  44         Self {
  45             limit: LimitStack::new(limit),
  46         }
  47     }
  48 }
  49
  50 impl_lint_pass!(CognitiveComplexity => [COGNITIVE_COMPLEXITY]);
  51
  52 impl 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         body: &'tcx Body<'_>,
  60         body_span: Span,
  61     ) {
  62         if body_span.from_expansion() {
  63             return;
  64         }
  65
  66         let expr = body.value;
  67
  68         let mut cc = 1u64;
  69         let mut returns = 0u64;
  70         let _: Option<!> = for_each_expr(expr, |e| {
  71             match e.kind {
  72                 ExprKind::If(_, _, _) => {
  73                     cc += 1;
  74                 },
  75                 ExprKind::Match(_, arms, _) => {
  76                     if arms.len() > 1 {
  77                         cc += 1;
  78                     }
  79                     cc += arms.iter().filter(|arm| arm.guard.is_some()).count() as u64;
  80                 },
  81                 ExprKind::Ret(_) => returns += 1,
  82                 _ => {},
  83             }
  84             ControlFlow::Continue(())
  85         });
  86
  87         let ret_ty = cx.typeck_results().node_type(expr.hir_id);
  88         let ret_adjust = if is_type_diagnostic_item(cx, ret_ty, sym::Result) {
  89             returns
  90         } else {
  91             #[expect(clippy::integer_division)]
  92             (returns / 2)
  93         };
  94
  95         // prevent degenerate cases where unreachable code contains `return` statements
  96         if cc >= ret_adjust {
  97             cc -= ret_adjust;
  98         }
  99
 100         if cc > self.limit.limit() {
 101             let fn_span = match kind {
 102                 FnKind::ItemFn(ident, _, _) | FnKind::Method(ident, _) => ident.span,
 103                 FnKind::Closure => {
 104                     let header_span = body_span.with_hi(decl.output.span().lo());
 105                     let pos = snippet_opt(cx, header_span).and_then(|snip| {
 106                         let low_offset = snip.find('|')?;
 107                         let high_offset = 1 + snip.get(low_offset + 1..)?.find('|')?;
 108                         let low = header_span.lo() + BytePos(low_offset as u32);
 109                         let high = low + BytePos(high_offset as u32 + 1);
 110
 111                         Some((low, high))
 112                     });
 113
 114                     if let Some((low, high)) = pos {
 115                         Span::new(low, high, header_span.ctxt(), header_span.parent())
 116                     } else {
 117                         return;
 118                     }
 119                 },
 120             };
 121
 122             span_lint_and_help(
 123                 cx,
 124                 COGNITIVE_COMPLEXITY,
 125                 fn_span,
 126                 &format!(
 127                     "the function has a cognitive complexity of ({cc}/{})",
 128                     self.limit.limit()
 129                 ),
 130                 None,
 131                 "you could split it up into multiple smaller functions",
 132             );
 133         }
 134     }
 135 }
 136
 137 impl<'tcx> LateLintPass<'tcx> for CognitiveComplexity {
 138     fn check_fn(
 139         &mut self,
 140         cx: &LateContext<'tcx>,
 141         kind: FnKind<'tcx>,
 142         decl: &'tcx FnDecl<'_>,
 143         body: &'tcx Body<'_>,
 144         span: Span,
 145         hir_id: HirId,
 146     ) {
 147         let def_id = cx.tcx.hir().local_def_id(hir_id);
 148         if !cx.tcx.has_attr(def_id.to_def_id(), sym::test) {
 149             self.check(cx, kind, decl, body, span);
 150         }
 151     }
 152
 153     fn enter_lint_attrs(&mut self, cx: &LateContext<'tcx>, attrs: &'tcx [Attribute]) {
 154         self.limit.push_attrs(cx.sess(), attrs, "cognitive_complexity");
 155     }
 156     fn exit_lint_attrs(&mut self, cx: &LateContext<'tcx>, attrs: &'tcx [Attribute]) {
 157         self.limit.pop_attrs(cx.sess(), attrs, "cognitive_complexity");
 158     }
 159 }