rust-clippy/src/cyclomatic_complexity.rs

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//! calculate cyclomatic complexity and warn about overly complex functions
use rustc::cfg::CFG;
use rustc::lint::*;
use rustc::ty;
use rustc::hir::*;
use rustc::hir::intravisit::{Visitor, walk_expr};
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use syntax::ast::Attribute;
use syntax::attr;
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use syntax::codemap::Span;
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use utils::{in_macro, LimitStack, span_help_and_lint};
/// **What it does:** This lint checks for methods with high cyclomatic complexity
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///
/// **Why is this bad?** Methods of high cyclomatic complexity tend to be badly readable. Also LLVM will usually optimize small methods better.
///
/// **Known problems:** Sometimes it's hard to find a way to reduce the complexity
///
/// **Example:** No. You'll see it when you get the warning.
declare_lint! {
pub CYCLOMATIC_COMPLEXITY, Warn,
"finds functions that should be split up into multiple functions"
}
pub struct CyclomaticComplexity {
limit: LimitStack,
}
impl CyclomaticComplexity {
pub fn new(limit: u64) -> Self {
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CyclomaticComplexity { limit: LimitStack::new(limit) }
}
}
impl LintPass for CyclomaticComplexity {
fn get_lints(&self) -> LintArray {
lint_array!(CYCLOMATIC_COMPLEXITY)
}
}
impl CyclomaticComplexity {
fn check<'a, 'tcx>(&mut self, cx: &'a LateContext<'a, 'tcx>, block: &Block, span: Span) {
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if in_macro(cx, span) {
return;
}
let cfg = CFG::new(cx.tcx, block);
let n = cfg.graph.len_nodes() as u64;
let e = cfg.graph.len_edges() as u64;
if e + 2 < n {
// the function has unreachable code, other lints should catch this
return;
}
let cc = e + 2 - n;
let mut helper = CCHelper {
match_arms: 0,
divergence: 0,
short_circuits: 0,
tcx: &cx.tcx,
};
helper.visit_block(block);
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let CCHelper { match_arms, divergence, short_circuits, .. } = helper;
if cc + divergence < match_arms + short_circuits {
report_cc_bug(cx, cc, match_arms, divergence, short_circuits, span);
} else {
let rust_cc = cc + divergence - match_arms - short_circuits;
if rust_cc > self.limit.limit() {
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span_help_and_lint(cx,
CYCLOMATIC_COMPLEXITY,
span,
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&format!("the function has a cyclomatic complexity of {}", rust_cc),
"you could split it up into multiple smaller functions");
}
}
}
}
impl LateLintPass for CyclomaticComplexity {
fn check_item(&mut self, cx: &LateContext, item: &Item) {
if let ItemFn(_, _, _, _, _, ref block) = item.node {
if !attr::contains_name(&item.attrs, "test") {
self.check(cx, block, item.span);
}
}
}
fn check_impl_item(&mut self, cx: &LateContext, item: &ImplItem) {
if let ImplItemKind::Method(_, ref block) = item.node {
self.check(cx, block, item.span);
}
}
fn check_trait_item(&mut self, cx: &LateContext, item: &TraitItem) {
if let MethodTraitItem(_, Some(ref block)) = item.node {
self.check(cx, block, item.span);
}
}
fn enter_lint_attrs(&mut self, cx: &LateContext, attrs: &[Attribute]) {
self.limit.push_attrs(cx.sess(), attrs, "cyclomatic_complexity");
}
fn exit_lint_attrs(&mut self, cx: &LateContext, attrs: &[Attribute]) {
self.limit.pop_attrs(cx.sess(), attrs, "cyclomatic_complexity");
}
}
struct CCHelper<'a, 'tcx: 'a> {
match_arms: u64,
divergence: u64,
short_circuits: u64, // && and ||
tcx: &'a ty::TyCtxt<'tcx>,
}
impl<'a, 'b, 'tcx> Visitor<'a> for CCHelper<'b, 'tcx> {
fn visit_expr(&mut self, e: &'a Expr) {
match e.node {
ExprMatch(_, ref arms, _) => {
walk_expr(self, e);
let arms_n: u64 = arms.iter().map(|arm| arm.pats.len() as u64).sum();
if arms_n > 1 {
self.match_arms += arms_n - 2;
}
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}
ExprCall(ref callee, _) => {
walk_expr(self, e);
let ty = self.tcx.node_id_to_type(callee.id);
match ty.sty {
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ty::TyFnDef(_, _, ty) |
ty::TyFnPtr(ty) if ty.sig.skip_binder().output.diverges() => {
self.divergence += 1;
}
_ => (),
}
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}
ExprClosure(..) => (),
ExprBinary(op, _, _) => {
walk_expr(self, e);
match op.node {
BiAnd | BiOr => self.short_circuits += 1,
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_ => (),
}
}
_ => walk_expr(self, e),
}
}
}
#[cfg(feature="debugging")]
fn report_cc_bug(_: &LateContext, cc: u64, narms: u64, div: u64, shorts: u64, span: Span) {
span_bug!(span,
"Clippy encountered a bug calculating cyclomatic complexity: cc = {}, arms = {}, \
div = {}, shorts = {}. Please file a bug report.",
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cc,
narms,
div,
shorts);
}
#[cfg(not(feature="debugging"))]
fn report_cc_bug(cx: &LateContext, cc: u64, narms: u64, div: u64, shorts: u64, span: Span) {
if cx.current_level(CYCLOMATIC_COMPLEXITY) != Level::Allow {
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cx.sess().span_note_without_error(span,
&format!("Clippy encountered a bug calculating cyclomatic complexity \
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(hide this message with `#[allow(cyclomatic_complexity)]`): cc \
= {}, arms = {}, div = {}, shorts = {}. Please file a bug report.",
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cc,
narms,
div,
shorts));
}
}