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_session::{declare_tool_lint, impl_lint_pass};
10 use rustc_target::abi::LayoutOf;
12 declare_clippy_lint! {
13 /// **What it does:** Checks for large size differences between variants on
16 /// **Why is this bad?** Enum size is bounded by the largest variant. Having a
17 /// large variant can penalize the memory layout of that enum.
19 /// **Known problems:** This lint obviously cannot take the distribution of
20 /// variants in your running program into account. It is possible that the
21 /// smaller variants make up less than 1% of all instances, in which case
22 /// the overhead is negligible and the boxing is counter-productive. Always
23 /// measure the change this lint suggests.
34 /// // Possibly better
37 /// B(Box<[i32; 8000]>),
40 pub LARGE_ENUM_VARIANT,
42 "large size difference between variants on an enum"
45 #[derive(Copy, Clone)]
46 pub struct LargeEnumVariant {
47 maximum_size_difference_allowed: u64,
50 impl LargeEnumVariant {
52 pub fn new(maximum_size_difference_allowed: u64) -> Self {
54 maximum_size_difference_allowed,
59 impl_lint_pass!(LargeEnumVariant => [LARGE_ENUM_VARIANT]);
61 impl<'tcx> LateLintPass<'tcx> for LargeEnumVariant {
62 fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
63 if in_external_macro(cx.tcx.sess, item.span) {
66 if let ItemKind::Enum(ref def, _) = item.kind {
67 let ty = cx.tcx.type_of(item.def_id);
68 let adt = ty.ty_adt_def().expect("already checked whether this is an enum");
70 let mut largest_variant: Option<(_, _)> = None;
71 let mut second_variant: Option<(_, _)> = None;
73 for (i, variant) in adt.variants.iter().enumerate() {
74 let size: u64 = variant
78 let ty = cx.tcx.type_of(f.did);
79 // don't count generics by filtering out everything
80 // that does not have a layout
81 cx.layout_of(ty).ok().map(|l| l.size.bytes())
85 let grouped = (size, (i, variant));
87 if grouped.0 >= largest_variant.map_or(0, |x| x.0) {
88 second_variant = largest_variant;
89 largest_variant = Some(grouped);
93 if let (Some(largest), Some(second)) = (largest_variant, second_variant) {
94 let difference = largest.0 - second.0;
96 if difference > self.maximum_size_difference_allowed {
97 let (i, variant) = largest.1;
99 let help_text = "consider boxing the large fields to reduce the total size of the enum";
103 def.variants[i].span,
104 "large size difference between variants",
107 def.variants[(largest.1).0].span,
108 &format!("this variant is {} bytes", largest.0),
111 def.variants[(second.1).0].span,
112 &format!("and the second-largest variant is {} bytes:", second.0),
114 if variant.fields.len() == 1 {
115 let span = match def.variants[i].data {
116 VariantData::Struct(fields, ..) | VariantData::Tuple(fields, ..) => {
119 VariantData::Unit(..) => unreachable!(),
121 if let Some(snip) = snippet_opt(cx, span) {
122 diag.span_suggestion(
125 format!("Box<{}>", snip),
126 Applicability::MaybeIncorrect,
131 diag.span_help(def.variants[i].span, help_text);