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use clippy_utils::diagnostics::span_lint_and_then;
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::{Item, ItemKind};
use rustc_hir_analysis::hir_ty_to_ty;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::layout::LayoutOf;
use rustc_middle::ty::{self, ConstKind};
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::{BytePos, Pos, Span};
declare_clippy_lint! {
/// ### What it does
/// Checks for large `const` arrays that should
/// be defined as `static` instead.
///
/// ### Why is this bad?
/// Performance: const variables are inlined upon use.
/// Static items result in only one instance and has a fixed location in memory.
///
/// ### Example
/// ```rust,ignore
/// pub const a = [0u32; 1_000_000];
/// ```
///
/// Use instead:
/// ```rust,ignore
/// pub static a = [0u32; 1_000_000];
/// ```
#[clippy::version = "1.44.0"]
pub LARGE_CONST_ARRAYS,
perf,
"large non-scalar const array may cause performance overhead"
}
pub struct LargeConstArrays {
maximum_allowed_size: u128,
}
impl LargeConstArrays {
#[must_use]
pub fn new(maximum_allowed_size: u128) -> Self {
Self { maximum_allowed_size }
}
}
impl_lint_pass!(LargeConstArrays => [LARGE_CONST_ARRAYS]);
impl<'tcx> LateLintPass<'tcx> for LargeConstArrays {
fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
if_chain! {
if !item.span.from_expansion();
if let ItemKind::Const(hir_ty, _) = &item.kind;
let ty = hir_ty_to_ty(cx.tcx, hir_ty);
if let ty::Array(element_type, cst) = ty.kind();
if let ConstKind::Value(ty::ValTree::Leaf(element_count)) = cst.kind();
if let Ok(element_count) = element_count.try_to_target_usize(cx.tcx);
if let Ok(element_size) = cx.layout_of(*element_type).map(|l| l.size.bytes());
if self.maximum_allowed_size < u128::from(element_count) * u128::from(element_size);
then {
let hi_pos = item.ident.span.lo() - BytePos::from_usize(1);
let sugg_span = Span::new(
hi_pos - BytePos::from_usize("const".len()),
hi_pos,
item.span.ctxt(),
item.span.parent(),
);
span_lint_and_then(
cx,
LARGE_CONST_ARRAYS,
item.span,
"large array defined as const",
|diag| {
diag.span_suggestion(
sugg_span,
"make this a static item",
"static",
Applicability::MachineApplicable,
);
}
);
}
}
}
}
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