rust-clippy/clippy_lints/src/cognitive_complexity.rs

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//! calculate cognitive complexity and warn about overly complex functions
use rustc::hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
use rustc::hir::*;
use rustc::lint::{LateContext, LateLintPass, LintArray, LintContext, LintPass};
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use rustc::{declare_tool_lint, impl_lint_pass};
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use syntax::ast::Attribute;
use syntax::source_map::Span;
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use crate::utils::{match_type, paths, span_help_and_lint, LimitStack};
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declare_clippy_lint! {
/// **What it does:** Checks for methods with high cognitive complexity.
///
/// **Why is this bad?** Methods of high cognitive complexity tend to be hard to
/// both read and maintain. Also LLVM will tend to 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.
pub COGNITIVE_COMPLEXITY,
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complexity,
"functions that should be split up into multiple functions"
}
pub struct CognitiveComplexity {
limit: LimitStack,
}
impl CognitiveComplexity {
pub fn new(limit: u64) -> Self {
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Self {
limit: LimitStack::new(limit),
}
}
}
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impl_lint_pass!(CognitiveComplexity => [COGNITIVE_COMPLEXITY]);
impl CognitiveComplexity {
fn check<'a, 'tcx>(&mut self, cx: &'a LateContext<'a, 'tcx>, body: &'tcx Body, span: Span) {
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if span.from_expansion() {
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return;
}
let expr = &body.value;
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let mut helper = CCHelper { cc: 1, returns: 0 };
helper.visit_expr(expr);
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let CCHelper { cc, returns } = helper;
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let ret_ty = cx.tables.node_type(expr.hir_id);
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let ret_adjust = if match_type(cx, ret_ty, &paths::RESULT) {
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returns
} else {
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#[allow(clippy::integer_division)]
(returns / 2)
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};
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let mut rust_cc = cc;
// prevent degenerate cases where unreachable code contains `return` statements
if rust_cc >= ret_adjust {
rust_cc -= ret_adjust;
}
if rust_cc > self.limit.limit() {
span_help_and_lint(
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cx,
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COGNITIVE_COMPLEXITY,
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span,
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&format!(
"the function has a cognitive complexity of ({}/{})",
rust_cc,
self.limit.limit()
),
"you could split it up into multiple smaller functions",
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);
}
}
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CognitiveComplexity {
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fn check_fn(
&mut self,
cx: &LateContext<'a, 'tcx>,
_: intravisit::FnKind<'tcx>,
_: &'tcx FnDecl,
body: &'tcx Body,
span: Span,
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hir_id: HirId,
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) {
let def_id = cx.tcx.hir().local_def_id(hir_id);
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if !cx.tcx.has_attr(def_id, sym!(test)) {
self.check(cx, body, span);
}
}
fn enter_lint_attrs(&mut self, cx: &LateContext<'a, 'tcx>, attrs: &'tcx [Attribute]) {
self.limit.push_attrs(cx.sess(), attrs, "cognitive_complexity");
}
fn exit_lint_attrs(&mut self, cx: &LateContext<'a, 'tcx>, attrs: &'tcx [Attribute]) {
self.limit.pop_attrs(cx.sess(), attrs, "cognitive_complexity");
}
}
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struct CCHelper {
cc: u64,
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returns: u64,
}
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impl<'tcx> Visitor<'tcx> for CCHelper {
fn visit_expr(&mut self, e: &'tcx Expr) {
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walk_expr(self, e);
match e.node {
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ExprKind::Match(_, ref arms, _) => {
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if arms.len() > 1 {
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self.cc += 1;
}
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self.cc += arms.iter().filter(|arm| arm.guard.is_some()).count() as u64;
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},
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ExprKind::Ret(_) => self.returns += 1,
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_ => {},
}
}
fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
NestedVisitorMap::None
}
}