1 //! lint when there is a large size difference between variants on an enum
3 use crate::utils::{snippet_opt, span_lint_and_then};
4 use rustc_errors::Applicability;
5 use rustc_hir::{Item, ItemKind, VariantData};
6 use rustc_lint::{LateContext, LateLintPass};
7 use rustc_session::{declare_tool_lint, impl_lint_pass};
8 use rustc_target::abi::LayoutOf;
10 declare_clippy_lint! {
11 /// **What it does:** Checks for large size differences between variants on
14 /// **Why is this bad?** Enum size is bounded by the largest variant. Having a
15 /// large variant can penalize the memory layout of that enum.
17 /// **Known problems:** This lint obviously cannot take the distribution of
18 /// variants in your running program into account. It is possible that the
19 /// smaller variants make up less than 1% of all instances, in which case
20 /// the overhead is negligible and the boxing is counter-productive. Always
21 /// measure the change this lint suggests.
32 /// // Possibly better
35 /// B(Box<[i32; 8000]>),
38 pub LARGE_ENUM_VARIANT,
40 "large size difference between variants on an enum"
43 #[derive(Copy, Clone)]
44 pub struct LargeEnumVariant {
45 maximum_size_difference_allowed: u64,
48 impl LargeEnumVariant {
50 pub fn new(maximum_size_difference_allowed: u64) -> Self {
52 maximum_size_difference_allowed,
57 impl_lint_pass!(LargeEnumVariant => [LARGE_ENUM_VARIANT]);
59 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LargeEnumVariant {
60 fn check_item(&mut self, cx: &LateContext<'_, '_>, item: &Item<'_>) {
61 let did = cx.tcx.hir().local_def_id(item.hir_id);
62 if let ItemKind::Enum(ref def, _) = item.kind {
63 let ty = cx.tcx.type_of(did);
64 let adt = ty.ty_adt_def().expect("already checked whether this is an enum");
66 let mut largest_variant: Option<(_, _)> = None;
67 let mut second_variant: Option<(_, _)> = None;
69 for (i, variant) in adt.variants.iter().enumerate() {
70 let size: u64 = variant
74 let ty = cx.tcx.type_of(f.did);
75 // don't count generics by filtering out everything
76 // that does not have a layout
77 cx.layout_of(ty).ok().map(|l| l.size.bytes())
81 let grouped = (size, (i, variant));
83 if grouped.0 >= largest_variant.map_or(0, |x| x.0) {
84 second_variant = largest_variant;
85 largest_variant = Some(grouped);
89 if let (Some(largest), Some(second)) = (largest_variant, second_variant) {
90 let difference = largest.0 - second.0;
92 if difference > self.maximum_size_difference_allowed {
93 let (i, variant) = largest.1;
95 let help_text = "consider boxing the large fields to reduce the total size of the enum";
100 "large size difference between variants",
103 def.variants[(largest.1).0].span,
104 &format!("this variant is {} bytes", largest.0),
107 def.variants[(second.1).0].span,
108 &format!("and the second-largest variant is {} bytes:", second.0),
110 if variant.fields.len() == 1 {
111 let span = match def.variants[i].data {
112 VariantData::Struct(ref fields, ..) | VariantData::Tuple(ref fields, ..) => {
115 VariantData::Unit(..) => unreachable!(),
117 if let Some(snip) = snippet_opt(cx, span) {
118 diag.span_suggestion(
121 format!("Box<{}>", snip),
122 Applicability::MaybeIncorrect,
127 diag.span_help(def.variants[i].span, help_text);