1 //! lint when there is a large size difference between variants on an enum
4 use rustc::{declare_lint, lint_array};
6 use crate::utils::{snippet_opt, span_lint_and_then};
7 use rustc::ty::layout::LayoutOf;
9 /// **What it does:** Checks for large size differences between variants on
12 /// **Why is this bad?** Enum size is bounded by the largest variant. Having a
14 /// can penalize the memory layout of that enum.
16 /// **Known problems:** None.
25 declare_clippy_lint! {
26 pub LARGE_ENUM_VARIANT,
28 "large size difference between variants on an enum"
31 #[derive(Copy, Clone)]
32 pub struct LargeEnumVariant {
33 maximum_size_difference_allowed: u64,
36 impl LargeEnumVariant {
37 pub fn new(maximum_size_difference_allowed: u64) -> Self {
39 maximum_size_difference_allowed,
44 impl LintPass for LargeEnumVariant {
45 fn get_lints(&self) -> LintArray {
46 lint_array!(LARGE_ENUM_VARIANT)
50 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LargeEnumVariant {
51 fn check_item(&mut self, cx: &LateContext<'_, '_>, item: &Item) {
52 let did = cx.tcx.hir.local_def_id(item.id);
53 if let ItemKind::Enum(ref def, _) = item.node {
54 let ty = cx.tcx.type_of(did);
55 let adt = ty.ty_adt_def()
56 .expect("already checked whether this is an enum");
58 let mut smallest_variant: Option<(_, _)> = None;
59 let mut largest_variant: Option<(_, _)> = None;
61 for (i, variant) in adt.variants.iter().enumerate() {
62 let size: u64 = variant
66 let ty = cx.tcx.type_of(f.did);
67 // don't count generics by filtering out everything
68 // that does not have a layout
69 cx.layout_of(ty).ok().map(|l| l.size.bytes())
73 let grouped = (size, (i, variant));
75 update_if(&mut smallest_variant, grouped, |a, b| b.0 <= a.0);
76 update_if(&mut largest_variant, grouped, |a, b| b.0 >= a.0);
79 if let (Some(smallest), Some(largest)) = (smallest_variant, largest_variant) {
80 let difference = largest.0 - smallest.0;
82 if difference > self.maximum_size_difference_allowed {
83 let (i, variant) = largest.1;
89 "large size difference between variants",
91 if variant.fields.len() == 1 {
92 let span = match def.variants[i].node.data {
93 VariantData::Struct(ref fields, _) | VariantData::Tuple(ref fields, _) => {
96 VariantData::Unit(_) => unreachable!(),
98 if let Some(snip) = snippet_opt(cx, span) {
101 "consider boxing the large fields to reduce the total size of the \
103 format!("Box<{}>", snip),
109 def.variants[i].span,
110 "consider boxing the large fields to reduce the total size of the enum",
120 fn update_if<T, F>(old: &mut Option<T>, new: T, f: F)
122 F: Fn(&T, &T) -> bool,
124 if let Some(ref mut val) = *old {