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     pub COGNITIVE_COMPLEXITY,
  31     nursery,
  32     "functions that should be split up into multiple functions"
  33 }
  34
  35 pub struct CognitiveComplexity {
  36     limit: LimitStack,
  37 }
  38
  39 impl CognitiveComplexity {
  40     #[must_use]
  41     pub fn new(limit: u64) -> Self {
  42         Self {
  43             limit: LimitStack::new(limit),
  44         }
  45     }
  46 }
  47
  48 impl_lint_pass!(CognitiveComplexity => [COGNITIVE_COMPLEXITY]);
  49
  50 impl CognitiveComplexity {
  51     #[allow(clippy::cast_possible_truncation)]
  52     fn check<'tcx>(
  53         &mut self,
  54         cx: &LateContext<'tcx>,
  55         kind: FnKind<'tcx>,
  56         decl: &'tcx FnDecl<'_>,
  57         body: &'tcx Body<'_>,
  58         body_span: Span,
  59     ) {
  60         if body_span.from_expansion() {
  61             return;
  62         }
  63
  64         let expr = &body.value;
  65
  66         let mut helper = CcHelper { cc: 1, returns: 0 };
  67         helper.visit_expr(expr);
  68         let CcHelper { cc, returns } = helper;
  69         let ret_ty = cx.typeck_results().node_type(expr.hir_id);
  70         let ret_adjust = if is_type_diagnostic_item(cx, ret_ty, sym::Result) {
  71             returns
  72         } else {
  73             #[allow(clippy::integer_division)]
  74             (returns / 2)
  75         };
  76
  77         let mut rust_cc = cc;
  78         // prevent degenerate cases where unreachable code contains `return` statements
  79         if rust_cc >= ret_adjust {
  80             rust_cc -= ret_adjust;
  81         }
  82
  83         if rust_cc > self.limit.limit() {
  84             let fn_span = match kind {
  85                 FnKind::ItemFn(ident, _, _, _) | FnKind::Method(ident, _, _) => ident.span,
  86                 FnKind::Closure => {
  87                     let header_span = body_span.with_hi(decl.output.span().lo());
  88                     let pos = snippet_opt(cx, header_span).and_then(|snip| {
  89                         let low_offset = snip.find('|')?;
  90                         let high_offset = 1 + snip.get(low_offset + 1..)?.find('|')?;
  91                         let low = header_span.lo() + BytePos(low_offset as u32);
  92                         let high = low + BytePos(high_offset as u32 + 1);
  93
  94                         Some((low, high))
  95                     });
  96
  97                     if let Some((low, high)) = pos {
  98                         Span::new(low, high, header_span.ctxt(), header_span.parent())
  99                     } else {
 100                         return;
 101                     }
 102                 },
 103             };
 104
 105             span_lint_and_help(
 106                 cx,
 107                 COGNITIVE_COMPLEXITY,
 108                 fn_span,
 109                 &format!(
 110                     "the function has a cognitive complexity of ({}/{})",
 111                     rust_cc,
 112                     self.limit.limit()
 113                 ),
 114                 None,
 115                 "you could split it up into multiple smaller functions",
 116             );
 117         }
 118     }
 119 }
 120
 121 impl<'tcx> LateLintPass<'tcx> for CognitiveComplexity {
 122     fn check_fn(
 123         &mut self,
 124         cx: &LateContext<'tcx>,
 125         kind: FnKind<'tcx>,
 126         decl: &'tcx FnDecl<'_>,
 127         body: &'tcx Body<'_>,
 128         span: Span,
 129         hir_id: HirId,
 130     ) {
 131         let def_id = cx.tcx.hir().local_def_id(hir_id);
 132         if !cx.tcx.has_attr(def_id.to_def_id(), sym::test) {
 133             self.check(cx, kind, decl, body, span);
 134         }
 135     }
 136
 137     fn enter_lint_attrs(&mut self, cx: &LateContext<'tcx>, attrs: &'tcx [Attribute]) {
 138         self.limit.push_attrs(cx.sess(), attrs, "cognitive_complexity");
 139     }
 140     fn exit_lint_attrs(&mut self, cx: &LateContext<'tcx>, attrs: &'tcx [Attribute]) {
 141         self.limit.pop_attrs(cx.sess(), attrs, "cognitive_complexity");
 142     }
 143 }
 144
 145 struct CcHelper {
 146     cc: u64,
 147     returns: u64,
 148 }
 149
 150 impl<'tcx> Visitor<'tcx> for CcHelper {
 151     type Map = Map<'tcx>;
 152
 153     fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
 154         walk_expr(self, e);
 155         match e.kind {
 156             ExprKind::If(_, _, _) => {
 157                 self.cc += 1;
 158             },
 159             ExprKind::Match(_, arms, _) => {
 160                 if arms.len() > 1 {
 161                     self.cc += 1;
 162                 }
 163                 self.cc += arms.iter().filter(|arm| arm.guard.is_some()).count() as u64;
 164             },
 165             ExprKind::Ret(_) => self.returns += 1,
 166             _ => {},
 167         }
 168     }
 169     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 170         NestedVisitorMap::None
 171     }
 172 }