1 //! lint when there is a large size difference between variants on an enum
3 use clippy_utils::diagnostics::span_lint_and_then;
4 use clippy_utils::source::snippet_opt;
5 use rustc_errors::Applicability;
6 use rustc_hir::{Item, ItemKind, VariantData};
7 use rustc_lint::{LateContext, LateLintPass};
8 use rustc_middle::lint::in_external_macro;
9 use rustc_middle::ty::layout::LayoutOf;
10 use rustc_session::{declare_tool_lint, impl_lint_pass};
12 declare_clippy_lint! {
14 /// Checks for large size differences between variants on
17 /// ### Why is this bad?
18 /// Enum size is bounded by the largest variant. Having a
19 /// large variant can penalize the memory layout of that enum.
21 /// ### Known problems
22 /// This lint obviously cannot take the distribution of
23 /// variants in your running program into account. It is possible that the
24 /// smaller variants make up less than 1% of all instances, in which case
25 /// the overhead is negligible and the boxing is counter-productive. Always
26 /// measure the change this lint suggests.
36 /// // Possibly better
39 /// B(Box<[i32; 8000]>),
42 pub LARGE_ENUM_VARIANT,
44 "large size difference between variants on an enum"
47 #[derive(Copy, Clone)]
48 pub struct LargeEnumVariant {
49 maximum_size_difference_allowed: u64,
52 impl LargeEnumVariant {
54 pub fn new(maximum_size_difference_allowed: u64) -> Self {
56 maximum_size_difference_allowed,
61 impl_lint_pass!(LargeEnumVariant => [LARGE_ENUM_VARIANT]);
63 impl<'tcx> LateLintPass<'tcx> for LargeEnumVariant {
64 fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
65 if in_external_macro(cx.tcx.sess, item.span) {
68 if let ItemKind::Enum(ref def, _) = item.kind {
69 let ty = cx.tcx.type_of(item.def_id);
70 let adt = ty.ty_adt_def().expect("already checked whether this is an enum");
72 let mut largest_variant: Option<(_, _)> = None;
73 let mut second_variant: Option<(_, _)> = None;
75 for (i, variant) in adt.variants.iter().enumerate() {
76 let size: u64 = variant
80 let ty = cx.tcx.type_of(f.did);
81 // don't count generics by filtering out everything
82 // that does not have a layout
83 cx.layout_of(ty).ok().map(|l| l.size.bytes())
87 let grouped = (size, (i, variant));
89 if grouped.0 >= largest_variant.map_or(0, |x| x.0) {
90 second_variant = largest_variant;
91 largest_variant = Some(grouped);
95 if let (Some(largest), Some(second)) = (largest_variant, second_variant) {
96 let difference = largest.0 - second.0;
98 if difference > self.maximum_size_difference_allowed {
99 let (i, variant) = largest.1;
101 let help_text = "consider boxing the large fields to reduce the total size of the enum";
105 def.variants[i].span,
106 "large size difference between variants",
109 def.variants[(largest.1).0].span,
110 &format!("this variant is {} bytes", largest.0),
113 def.variants[(second.1).0].span,
114 &format!("and the second-largest variant is {} bytes:", second.0),
116 if variant.fields.len() == 1 {
117 let span = match def.variants[i].data {
118 VariantData::Struct(fields, ..) | VariantData::Tuple(fields, ..) => {
121 VariantData::Unit(..) => unreachable!(),
123 if let Some(snip) = snippet_opt(cx, span) {
124 diag.span_suggestion(
127 format!("Box<{}>", snip),
128 Applicability::MaybeIncorrect,
133 diag.span_help(def.variants[i].span, help_text);