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::{get_async_fn_body, is_async_fn, LimitStack};
   8 use core::ops::ControlFlow;
   9 use rustc_ast::ast::Attribute;
  10 use rustc_hir::intravisit::FnKind;
  11 use rustc_hir::{Body, Expr, 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         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
 135 impl<'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         hir_id: HirId,
 144     ) {
 145         let def_id = cx.tcx.hir().local_def_id(hir_id);
 146         if !cx.tcx.has_attr(def_id.to_def_id(), sym::test) {
 147             let expr = if is_async_fn(kind) {
 148                 match get_async_fn_body(cx.tcx, body) {
 149                     Some(b) => b,
 150                     None => {
 151                         return;
 152                     },
 153                 }
 154             } else {
 155                 body.value
 156             };
 157
 158             self.check(cx, kind, decl, expr, span);
 159         }
 160     }
 161
 162     fn enter_lint_attrs(&mut self, cx: &LateContext<'tcx>, attrs: &'tcx [Attribute]) {
 163         self.limit.push_attrs(cx.sess(), attrs, "cognitive_complexity");
 164     }
 165     fn exit_lint_attrs(&mut self, cx: &LateContext<'tcx>, attrs: &'tcx [Attribute]) {
 166         self.limit.pop_attrs(cx.sess(), attrs, "cognitive_complexity");
 167     }
 168 }