src/tools/clippy/clippy_lints/src/cyclomatic_complexity.rs

   1 //! calculate cyclomatic complexity and warn about overly complex functions
   2
   3 use rustc::cfg::CFG;
   4 use rustc::lint::*;
   5 use rustc::hir::*;
   6 use rustc::ty;
   7 use rustc::hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
   8 use syntax::ast::{Attribute, NodeId};
   9 use syntax::codemap::Span;
  10
  11 use utils::{in_macro, is_allowed, match_type, paths, span_help_and_lint, LimitStack};
  12
  13 /// **What it does:** Checks for methods with high cyclomatic complexity.
  14 ///
  15 /// **Why is this bad?** Methods of high cyclomatic complexity tend to be badly
  16 /// readable. Also LLVM will usually optimize small methods better.
  17 ///
  18 /// **Known problems:** Sometimes it's hard to find a way to reduce the
  19 /// complexity.
  20 ///
  21 /// **Example:** No. You'll see it when you get the warning.
  22 declare_lint! {
  23     pub CYCLOMATIC_COMPLEXITY,
  24     Warn,
  25     "functions that should be split up into multiple functions"
  26 }
  27
  28 pub struct CyclomaticComplexity {
  29     limit: LimitStack,
  30 }
  31
  32 impl CyclomaticComplexity {
  33     pub fn new(limit: u64) -> Self {
  34         Self {
  35             limit: LimitStack::new(limit),
  36         }
  37     }
  38 }
  39
  40 impl LintPass for CyclomaticComplexity {
  41     fn get_lints(&self) -> LintArray {
  42         lint_array!(CYCLOMATIC_COMPLEXITY)
  43     }
  44 }
  45
  46 impl CyclomaticComplexity {
  47     fn check<'a, 'tcx: 'a>(&mut self, cx: &'a LateContext<'a, 'tcx>, body: &'tcx Body, span: Span) {
  48         if in_macro(span) {
  49             return;
  50         }
  51
  52         let cfg = CFG::new(cx.tcx, body);
  53         let expr = &body.value;
  54         let n = cfg.graph.len_nodes() as u64;
  55         let e = cfg.graph.len_edges() as u64;
  56         if e + 2 < n {
  57             // the function has unreachable code, other lints should catch this
  58             return;
  59         }
  60         let cc = e + 2 - n;
  61         let mut helper = CCHelper {
  62             match_arms: 0,
  63             divergence: 0,
  64             short_circuits: 0,
  65             returns: 0,
  66             cx: cx,
  67         };
  68         helper.visit_expr(expr);
  69         let CCHelper {
  70             match_arms,
  71             divergence,
  72             short_circuits,
  73             returns,
  74             ..
  75         } = helper;
  76         let ret_ty = cx.tables.node_id_to_type(expr.hir_id);
  77         let ret_adjust = if match_type(cx, ret_ty, &paths::RESULT) {
  78             returns
  79         } else {
  80             returns / 2
  81         };
  82
  83         if cc + divergence < match_arms + short_circuits {
  84             report_cc_bug(
  85                 cx,
  86                 cc,
  87                 match_arms,
  88                 divergence,
  89                 short_circuits,
  90                 ret_adjust,
  91                 span,
  92                 body.id().node_id,
  93             );
  94         } else {
  95             let mut rust_cc = cc + divergence - match_arms - short_circuits;
  96             // prevent degenerate cases where unreachable code contains `return` statements
  97             if rust_cc >= ret_adjust {
  98                 rust_cc -= ret_adjust;
  99             }
 100             if rust_cc > self.limit.limit() {
 101                 span_help_and_lint(
 102                     cx,
 103                     CYCLOMATIC_COMPLEXITY,
 104                     span,
 105                     &format!("the function has a cyclomatic complexity of {}", rust_cc),
 106                     "you could split it up into multiple smaller functions",
 107                 );
 108             }
 109         }
 110     }
 111 }
 112
 113 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CyclomaticComplexity {
 114     fn check_fn(
 115         &mut self,
 116         cx: &LateContext<'a, 'tcx>,
 117         _: intravisit::FnKind<'tcx>,
 118         _: &'tcx FnDecl,
 119         body: &'tcx Body,
 120         span: Span,
 121         node_id: NodeId,
 122     ) {
 123         let def_id = cx.tcx.hir.local_def_id(node_id);
 124         if !cx.tcx.has_attr(def_id, "test") {
 125             self.check(cx, body, span);
 126         }
 127     }
 128
 129     fn enter_lint_attrs(&mut self, cx: &LateContext<'a, 'tcx>, attrs: &'tcx [Attribute]) {
 130         self.limit
 131             .push_attrs(cx.sess(), attrs, "cyclomatic_complexity");
 132     }
 133     fn exit_lint_attrs(&mut self, cx: &LateContext<'a, 'tcx>, attrs: &'tcx [Attribute]) {
 134         self.limit
 135             .pop_attrs(cx.sess(), attrs, "cyclomatic_complexity");
 136     }
 137 }
 138
 139 struct CCHelper<'a, 'tcx: 'a> {
 140     match_arms: u64,
 141     divergence: u64,
 142     returns: u64,
 143     short_circuits: u64, // && and ||
 144     cx: &'a LateContext<'a, 'tcx>,
 145 }
 146
 147 impl<'a, 'tcx> Visitor<'tcx> for CCHelper<'a, 'tcx> {
 148     fn visit_expr(&mut self, e: &'tcx Expr) {
 149         match e.node {
 150             ExprMatch(_, ref arms, _) => {
 151                 walk_expr(self, e);
 152                 let arms_n: u64 = arms.iter().map(|arm| arm.pats.len() as u64).sum();
 153                 if arms_n > 1 {
 154                     self.match_arms += arms_n - 2;
 155                 }
 156             },
 157             ExprCall(ref callee, _) => {
 158                 walk_expr(self, e);
 159                 let ty = self.cx.tables.node_id_to_type(callee.hir_id);
 160                 match ty.sty {
 161                     ty::TyFnDef(..) | ty::TyFnPtr(_) => {
 162                         let sig = ty.fn_sig(self.cx.tcx);
 163                         if sig.skip_binder().output().sty == ty::TyNever {
 164                             self.divergence += 1;
 165                         }
 166                     },
 167                     _ => (),
 168                 }
 169             },
 170             ExprClosure(.., _) => (),
 171             ExprBinary(op, _, _) => {
 172                 walk_expr(self, e);
 173                 match op.node {
 174                     BiAnd | BiOr => self.short_circuits += 1,
 175                     _ => (),
 176                 }
 177             },
 178             ExprRet(_) => self.returns += 1,
 179             _ => walk_expr(self, e),
 180         }
 181     }
 182     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
 183         NestedVisitorMap::None
 184     }
 185 }
 186
 187 #[cfg(feature = "debugging")]
 188 #[allow(too_many_arguments)]
 189 fn report_cc_bug(_: &LateContext, cc: u64, narms: u64, div: u64, shorts: u64, returns: u64, span: Span, _: NodeId) {
 190     span_bug!(
 191         span,
 192         "Clippy encountered a bug calculating cyclomatic complexity: cc = {}, arms = {}, \
 193          div = {}, shorts = {}, returns = {}. Please file a bug report.",
 194         cc,
 195         narms,
 196         div,
 197         shorts,
 198         returns
 199     );
 200 }
 201 #[cfg(not(feature = "debugging"))]
 202 #[allow(too_many_arguments)]
 203 fn report_cc_bug(cx: &LateContext, cc: u64, narms: u64, div: u64, shorts: u64, returns: u64, span: Span, id: NodeId) {
 204     if !is_allowed(cx, CYCLOMATIC_COMPLEXITY, id) {
 205         cx.sess().span_note_without_error(
 206             span,
 207             &format!(
 208                 "Clippy encountered a bug calculating cyclomatic complexity \
 209                  (hide this message with `#[allow(cyclomatic_complexity)]`): \
 210                  cc = {}, arms = {}, div = {}, shorts = {}, returns = {}. \
 211                  Please file a bug report.",
 212                 cc,
 213                 narms,
 214                 div,
 215                 shorts,
 216                 returns
 217             ),
 218         );
 219     }
 220 }