rust-clippy/clippy_lints/src/large_const_arrays.rs
2020-04-17 08:08:00 +02:00

85 lines
2.8 KiB
Rust

use crate::rustc_target::abi::LayoutOf;
use crate::utils::span_lint_and_then;
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::{Item, ItemKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::mir::interpret::ConstValue;
use rustc_middle::ty::{self, ConstKind};
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::{BytePos, Pos, Span};
use rustc_typeck::hir_ty_to_ty;
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.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust,ignore
/// // Bad
/// pub const a = [0u32; 1_000_000];
///
/// // Good
/// pub static a = [0u32; 1_000_000];
/// ```
pub LARGE_CONST_ARRAYS,
perf,
"large non-scalar const array may cause performance overhead"
}
pub struct LargeConstArrays {
maximum_allowed_size: u64,
}
impl LargeConstArrays {
#[must_use]
pub fn new(maximum_allowed_size: u64) -> Self {
Self { maximum_allowed_size }
}
}
impl_lint_pass!(LargeConstArrays => [LARGE_CONST_ARRAYS]);
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LargeConstArrays {
fn check_item(&mut self, cx: &LateContext<'a, '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(val) = cst.val;
if let ConstValue::Scalar(element_count) = val;
if let Ok(element_count) = element_count.to_machine_usize(&cx.tcx);
if let Ok(element_size) = cx.layout_of(element_type).map(|l| l.size.bytes());
if self.maximum_allowed_size < element_count * 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(),
);
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".to_string(),
Applicability::MachineApplicable,
);
}
);
}
}
}
}