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::LimitStack;
   7 use rustc_ast::ast::Attribute;
   8 use rustc_hir::intravisit::{walk_expr, FnKind, NestedVisitorMap, Visitor};
   9 use rustc_hir::{Body, Expr, ExprKind, FnDecl, HirId};
  10 use rustc_lint::{LateContext, LateLintPass, LintContext};
  11 use rustc_middle::hir::map::Map;
  12 use rustc_session::{declare_tool_lint, impl_lint_pass};
  13 use rustc_span::source_map::Span;
  14 use rustc_span::{sym, BytePos};
  15
  16 declare_clippy_lint! {
  17     /// ### What it does
  18     /// Checks for methods with high cognitive complexity.
  19     ///
  20     /// ### Why is this bad?
  21     /// Methods of high cognitive complexity tend to be hard to
  22     /// both read and maintain. Also LLVM will tend to optimize small methods better.
  23     ///
  24     /// ### Known problems
  25     /// Sometimes it's hard to find a way to reduce the
  26     /// complexity.
  27     ///
  28     /// ### Example
  29     /// No. You'll see it when you get the warning.
  30     #[clippy::version = "1.35.0"]
  31     pub COGNITIVE_COMPLEXITY,
  32     nursery,
  33     "functions that should be split up into multiple functions"
  34 }
  35
  36 pub struct CognitiveComplexity {
  37     limit: LimitStack,
  38 }
  39
  40 impl CognitiveComplexity {
  41     #[must_use]
  42     pub fn new(limit: u64) -> Self {
  43         Self {
  44             limit: LimitStack::new(limit),
  45         }
  46     }
  47 }
  48
  49 impl_lint_pass!(CognitiveComplexity => [COGNITIVE_COMPLEXITY]);
  50
  51 impl CognitiveComplexity {
  52     #[allow(clippy::cast_possible_truncation)]
  53     fn check<'tcx>(
  54         &mut self,
  55         cx: &LateContext<'tcx>,
  56         kind: FnKind<'tcx>,
  57         decl: &'tcx FnDecl<'_>,
  58         body: &'tcx Body<'_>,
  59         body_span: Span,
  60     ) {
  61         if body_span.from_expansion() {
  62             return;
  63         }
  64
  65         let expr = &body.value;
  66
  67         let mut helper = CcHelper { cc: 1, returns: 0 };
  68         helper.visit_expr(expr);
  69         let CcHelper { cc, returns } = helper;
  70         let ret_ty = cx.typeck_results().node_type(expr.hir_id);
  71         let ret_adjust = if is_type_diagnostic_item(cx, ret_ty, sym::Result) {
  72             returns
  73         } else {
  74             #[allow(clippy::integer_division)]
  75             (returns / 2)
  76         };
  77
  78         let mut rust_cc = cc;
  79         // prevent degenerate cases where unreachable code contains `return` statements
  80         if rust_cc >= ret_adjust {
  81             rust_cc -= ret_adjust;
  82         }
  83
  84         if rust_cc > self.limit.limit() {
  85             let fn_span = match kind {
  86                 FnKind::ItemFn(ident, _, _, _) | FnKind::Method(ident, _, _) => ident.span,
  87                 FnKind::Closure => {
  88                     let header_span = body_span.with_hi(decl.output.span().lo());
  89                     let pos = snippet_opt(cx, header_span).and_then(|snip| {
  90                         let low_offset = snip.find('|')?;
  91                         let high_offset = 1 + snip.get(low_offset + 1..)?.find('|')?;
  92                         let low = header_span.lo() + BytePos(low_offset as u32);
  93                         let high = low + BytePos(high_offset as u32 + 1);
  94
  95                         Some((low, high))
  96                     });
  97
  98                     if let Some((low, high)) = pos {
  99                         Span::new(low, high, header_span.ctxt(), header_span.parent())
 100                     } else {
 101                         return;
 102                     }
 103                 },
 104             };
 105
 106             span_lint_and_help(
 107                 cx,
 108                 COGNITIVE_COMPLEXITY,
 109                 fn_span,
 110                 &format!(
 111                     "the function has a cognitive complexity of ({}/{})",
 112                     rust_cc,
 113                     self.limit.limit()
 114                 ),
 115                 None,
 116                 "you could split it up into multiple smaller functions",
 117             );
 118         }
 119     }
 120 }
 121
 122 impl<'tcx> LateLintPass<'tcx> for CognitiveComplexity {
 123     fn check_fn(
 124         &mut self,
 125         cx: &LateContext<'tcx>,
 126         kind: FnKind<'tcx>,
 127         decl: &'tcx FnDecl<'_>,
 128         body: &'tcx Body<'_>,
 129         span: Span,
 130         hir_id: HirId,
 131     ) {
 132         let def_id = cx.tcx.hir().local_def_id(hir_id);
 133         if !cx.tcx.has_attr(def_id.to_def_id(), sym::test) {
 134             self.check(cx, kind, decl, body, span);
 135         }
 136     }
 137
 138     fn enter_lint_attrs(&mut self, cx: &LateContext<'tcx>, attrs: &'tcx [Attribute]) {
 139         self.limit.push_attrs(cx.sess(), attrs, "cognitive_complexity");
 140     }
 141     fn exit_lint_attrs(&mut self, cx: &LateContext<'tcx>, attrs: &'tcx [Attribute]) {
 142         self.limit.pop_attrs(cx.sess(), attrs, "cognitive_complexity");
 143     }
 144 }
 145
 146 struct CcHelper {
 147     cc: u64,
 148     returns: u64,
 149 }
 150
 151 impl<'tcx> Visitor<'tcx> for CcHelper {
 152     type Map = Map<'tcx>;
 153
 154     fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
 155         walk_expr(self, e);
 156         match e.kind {
 157             ExprKind::If(_, _, _) => {
 158                 self.cc += 1;
 159             },
 160             ExprKind::Match(_, arms, _) => {
 161                 if arms.len() > 1 {
 162                     self.cc += 1;
 163                 }
 164                 self.cc += arms.iter().filter(|arm| arm.guard.is_some()).count() as u64;
 165             },
 166             ExprKind::Ret(_) => self.returns += 1,
 167             _ => {},
 168         }
 169     }
 170     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 171         NestedVisitorMap::None
 172     }
 173 }