clippy_lints/src/cognitive_complexity.rs

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