rust-clippy/src/cyclomatic_complexity.rs

//! calculate cyclomatic complexity and warn about overly complex functions

use rustc::lint::*;
use rustc_front::hir::*;
use rustc::middle::cfg::CFG;
use syntax::codemap::Span;
use syntax::attr::*;
use syntax::ast::Attribute;
use rustc_front::intravisit::{Visitor, walk_expr};

use utils::{in_macro, LimitStack};

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 {
        CyclomaticComplexity {
            limit: LimitStack::new(limit),
        }
    }
}

impl LintPass for CyclomaticComplexity {
    fn get_lints(&self) -> LintArray {
        lint_array!(CYCLOMATIC_COMPLEXITY)
    }
}

impl CyclomaticComplexity {
    fn check(&mut self, cx: &LateContext, block: &Block, span: Span) {
        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;
        let cc = e + 2 - n;
        let mut arm_counter = MatchArmCounter(0);
        arm_counter.visit_block(block);
        let mut narms = arm_counter.0;
        if narms > 0 {
            narms = narms - 1;
        }
        
        if cc < narms {
            report_cc_bug(cx, cc, narms, span);
        } else {
            let rust_cc = cc - narms;
            if rust_cc > self.limit.limit() {
                cx.span_lint_help(CYCLOMATIC_COMPLEXITY, span,
                &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 {
            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 MatchArmCounter(u64);

impl<'a> Visitor<'a> for MatchArmCounter {
    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 > 0 {
                    self.0 += arms_n - 1;
                }
            },
            ExprClosure(..) => {},
            _ => walk_expr(self, e),
        }
    }
}

#[cfg(feature="debugging")]
fn report_cc_bug(cx: &LateContext, cc: u64, narms: u64, span: Span) {
    cx.sess().span_bug(span, &format!("Clippy encountered a bug calculating cyclomatic complexity: \
                                       cc = {}, arms = {}. Please file a bug report.", cc, narms));;
}
#[cfg(not(feature="debugging"))]
fn report_cc_bug(cx: &LateContext, cc: u64, narms: u64, span: Span) {
    if cx.current_level(CYCLOMATIC_COMPLEXITY) != Level::Allow {
        cx.sess().span_note(span, &format!("Clippy encountered a bug calculating cyclomatic complexity \
                                            (hide this message with `#[allow(cyclomatic_complexity)]`): \
                                            cc = {}, arms = {}. Please file a bug report.", cc, narms));
    }
}