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_middle::lint::in_external_macro;
8 use rustc_session::{declare_tool_lint, impl_lint_pass};
9 use rustc_target::abi::LayoutOf;
11 declare_clippy_lint! {
12 /// **What it does:** Checks for large size differences between variants on
15 /// **Why is this bad?** Enum size is bounded by the largest variant. Having a
16 /// large variant can penalize the memory layout of that enum.
18 /// **Known problems:** This lint obviously cannot take the distribution of
19 /// variants in your running program into account. It is possible that the
20 /// smaller variants make up less than 1% of all instances, in which case
21 /// the overhead is negligible and the boxing is counter-productive. Always
22 /// measure the change this lint suggests.
33 /// // Possibly better
36 /// B(Box<[i32; 8000]>),
39 pub LARGE_ENUM_VARIANT,
41 "large size difference between variants on an enum"
44 #[derive(Copy, Clone)]
45 pub struct LargeEnumVariant {
46 maximum_size_difference_allowed: u64,
49 impl LargeEnumVariant {
51 pub fn new(maximum_size_difference_allowed: u64) -> Self {
53 maximum_size_difference_allowed,
58 impl_lint_pass!(LargeEnumVariant => [LARGE_ENUM_VARIANT]);
60 impl<'tcx> LateLintPass<'tcx> for LargeEnumVariant {
61 fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
62 if in_external_macro(cx.tcx.sess, item.span) {
65 if let ItemKind::Enum(ref def, _) = item.kind {
66 let ty = cx.tcx.type_of(item.def_id);
67 let adt = ty.ty_adt_def().expect("already checked whether this is an enum");
69 let mut largest_variant: Option<(_, _)> = None;
70 let mut second_variant: Option<(_, _)> = None;
72 for (i, variant) in adt.variants.iter().enumerate() {
73 let size: u64 = variant
77 let ty = cx.tcx.type_of(f.did);
78 // don't count generics by filtering out everything
79 // that does not have a layout
80 cx.layout_of(ty).ok().map(|l| l.size.bytes())
84 let grouped = (size, (i, variant));
86 if grouped.0 >= largest_variant.map_or(0, |x| x.0) {
87 second_variant = largest_variant;
88 largest_variant = Some(grouped);
92 if let (Some(largest), Some(second)) = (largest_variant, second_variant) {
93 let difference = largest.0 - second.0;
95 if difference > self.maximum_size_difference_allowed {
96 let (i, variant) = largest.1;
98 let help_text = "consider boxing the large fields to reduce the total size of the enum";
102 def.variants[i].span,
103 "large size difference between variants",
106 def.variants[(largest.1).0].span,
107 &format!("this variant is {} bytes", largest.0),
110 def.variants[(second.1).0].span,
111 &format!("and the second-largest variant is {} bytes:", second.0),
113 if variant.fields.len() == 1 {
114 let span = match def.variants[i].data {
115 VariantData::Struct(ref fields, ..) | VariantData::Tuple(ref fields, ..) => {
118 VariantData::Unit(..) => unreachable!(),
120 if let Some(snip) = snippet_opt(cx, span) {
121 diag.span_suggestion(
124 format!("Box<{}>", snip),
125 Applicability::MaybeIncorrect,
130 diag.span_help(def.variants[i].span, help_text);