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