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use clippy_utils::source::snippet;
use clippy_utils::{diagnostics::span_lint_and_sugg, ty::implements_trait};
use rustc_errors::Applicability;
use rustc_hir::{Expr, ExprKind, LangItem, MatchSource, QPath};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_target::abi::Size;
declare_clippy_lint! {
/// ### What it does
/// It checks for the size of a `Future` created by `async fn` or `async {}`.
///
/// ### Why is this bad?
/// Due to the current [unideal implemention](https://github.com/rust-lang/rust/issues/69826) of `Generator`,
/// large size of a `Future` may cause stack overflows.
///
/// ### Example
/// ```rust
/// async fn wait(f: impl std::future::Future<Output = ()>) {}
///
/// async fn big_fut(arg: [u8; 1024]) {}
///
/// pub async fn test() {
/// let fut = big_fut([0u8; 1024]);
/// wait(fut).await;
/// }
/// ```
///
/// `Box::pin` the big future instead.
///
/// ```rust
/// async fn wait(f: impl std::future::Future<Output = ()>) {}
///
/// async fn big_fut(arg: [u8; 1024]) {}
///
/// pub async fn test() {
/// let fut = Box::pin(big_fut([0u8; 1024]));
/// wait(fut).await;
/// }
/// ```
#[clippy::version = "1.68.0"]
pub LARGE_FUTURES,
pedantic,
"large future may lead to unexpected stack overflows"
}
#[derive(Copy, Clone)]
pub struct LargeFuture {
future_size_threshold: u64,
}
impl LargeFuture {
pub fn new(future_size_threshold: u64) -> Self {
Self { future_size_threshold }
}
}
impl_lint_pass!(LargeFuture => [LARGE_FUTURES]);
impl<'tcx> LateLintPass<'tcx> for LargeFuture {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
if matches!(expr.span.ctxt().outer_expn_data().kind, rustc_span::ExpnKind::Macro(..)) {
return;
}
if let ExprKind::Match(expr, _, MatchSource::AwaitDesugar) = expr.kind {
if let ExprKind::Call(func, [expr, ..]) = expr.kind
&& let ExprKind::Path(QPath::LangItem(LangItem::IntoFutureIntoFuture, ..)) = func.kind
&& let ty = cx.typeck_results().expr_ty(expr)
&& let Some(future_trait_def_id) = cx.tcx.lang_items().future_trait()
&& implements_trait(cx, ty, future_trait_def_id, &[])
&& let Ok(layout) = cx.tcx.layout_of(cx.param_env.and(ty))
&& let size = layout.layout.size()
&& size >= Size::from_bytes(self.future_size_threshold)
{
span_lint_and_sugg(
cx,
LARGE_FUTURES,
expr.span,
&format!("large future with a size of {} bytes", size.bytes()),
"consider `Box::pin` on it",
format!("Box::pin({})", snippet(cx, expr.span, "..")),
Applicability::Unspecified,
);
}
}
}
}
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