135 lines
5.1 KiB
Rust
135 lines
5.1 KiB
Rust
//! lint when there is a large size difference between variants on an enum
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use crate::utils::{snippet_opt, span_lint_and_then};
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use rustc_errors::Applicability;
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use rustc_hir::{Item, ItemKind, VariantData};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_session::{declare_tool_lint, impl_lint_pass};
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use rustc_target::abi::LayoutOf;
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declare_clippy_lint! {
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/// **What it does:** Checks for large size differences between variants on
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/// `enum`s.
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///
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/// **Why is this bad?** Enum size is bounded by the largest variant. Having a
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/// large variant can penalize the memory layout of that enum.
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///
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/// **Known problems:** This lint obviously cannot take the distribution of
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/// variants in your running program into account. It is possible that the
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/// smaller variants make up less than 1% of all instances, in which case
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/// the overhead is negligible and the boxing is counter-productive. Always
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/// measure the change this lint suggests.
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///
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/// **Example:**
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///
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/// ```rust
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/// // Bad
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/// enum Test {
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/// A(i32),
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/// B([i32; 8000]),
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/// }
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///
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/// // Possibly better
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/// enum Test2 {
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/// A(i32),
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/// B(Box<[i32; 8000]>),
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/// }
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/// ```
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pub LARGE_ENUM_VARIANT,
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perf,
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"large size difference between variants on an enum"
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}
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#[derive(Copy, Clone)]
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pub struct LargeEnumVariant {
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maximum_size_difference_allowed: u64,
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}
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impl LargeEnumVariant {
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#[must_use]
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pub fn new(maximum_size_difference_allowed: u64) -> Self {
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Self {
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maximum_size_difference_allowed,
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}
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}
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}
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impl_lint_pass!(LargeEnumVariant => [LARGE_ENUM_VARIANT]);
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impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LargeEnumVariant {
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fn check_item(&mut self, cx: &LateContext<'_, '_>, item: &Item<'_>) {
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let did = cx.tcx.hir().local_def_id(item.hir_id);
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if let ItemKind::Enum(ref def, _) = item.kind {
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let ty = cx.tcx.type_of(did);
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let adt = ty.ty_adt_def().expect("already checked whether this is an enum");
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let mut largest_variant: Option<(_, _)> = None;
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let mut second_variant: Option<(_, _)> = None;
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for (i, variant) in adt.variants.iter().enumerate() {
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let size: u64 = variant
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.fields
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.iter()
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.filter_map(|f| {
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let ty = cx.tcx.type_of(f.did);
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// don't count generics by filtering out everything
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// that does not have a layout
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cx.layout_of(ty).ok().map(|l| l.size.bytes())
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})
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.sum();
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let grouped = (size, (i, variant));
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if grouped.0 >= largest_variant.map_or(0, |x| x.0) {
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second_variant = largest_variant;
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largest_variant = Some(grouped);
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}
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}
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if let (Some(largest), Some(second)) = (largest_variant, second_variant) {
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let difference = largest.0 - second.0;
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if difference > self.maximum_size_difference_allowed {
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let (i, variant) = largest.1;
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let help_text = "consider boxing the large fields to reduce the total size of the enum";
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span_lint_and_then(
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cx,
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LARGE_ENUM_VARIANT,
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def.variants[i].span,
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"large size difference between variants",
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|diag| {
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diag.span_label(
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def.variants[(largest.1).0].span,
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&format!("this variant is {} bytes", largest.0),
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);
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diag.span_note(
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def.variants[(second.1).0].span,
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&format!("and the second-largest variant is {} bytes:", second.0),
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);
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if variant.fields.len() == 1 {
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let span = match def.variants[i].data {
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VariantData::Struct(ref fields, ..) | VariantData::Tuple(ref fields, ..) => {
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fields[0].ty.span
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},
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VariantData::Unit(..) => unreachable!(),
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};
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if let Some(snip) = snippet_opt(cx, span) {
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diag.span_suggestion(
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span,
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help_text,
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format!("Box<{}>", snip),
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Applicability::MaybeIncorrect,
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);
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return;
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}
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}
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diag.span_help(def.variants[i].span, help_text);
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},
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);
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}
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}
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}
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}
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}
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