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1 | //! lint when there is a large size difference between variants on an enum |
2 | ||
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; | |
10 | ||
11 | declare_clippy_lint! { | |
12 | /// **What it does:** Checks for large size differences between variants on | |
13 | /// `enum`s. | |
14 | /// | |
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. | |
17 | /// | |
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. | |
23 | /// | |
24 | /// **Example:** | |
25 | /// | |
26 | /// ```rust | |
27 | /// // Bad | |
28 | /// enum Test { | |
29 | /// A(i32), | |
30 | /// B([i32; 8000]), | |
31 | /// } | |
32 | /// | |
33 | /// // Possibly better | |
34 | /// enum Test2 { | |
35 | /// A(i32), | |
36 | /// B(Box<[i32; 8000]>), | |
37 | /// } | |
38 | /// ``` | |
39 | pub LARGE_ENUM_VARIANT, | |
40 | perf, | |
41 | "large size difference between variants on an enum" | |
42 | } | |
43 | ||
44 | #[derive(Copy, Clone)] | |
45 | pub struct LargeEnumVariant { | |
46 | maximum_size_difference_allowed: u64, | |
47 | } | |
48 | ||
49 | impl LargeEnumVariant { | |
50 | #[must_use] | |
51 | pub fn new(maximum_size_difference_allowed: u64) -> Self { | |
52 | Self { | |
53 | maximum_size_difference_allowed, | |
54 | } | |
55 | } | |
56 | } | |
57 | ||
58 | impl_lint_pass!(LargeEnumVariant => [LARGE_ENUM_VARIANT]); | |
59 | ||
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) { | |
63 | return; | |
64 | } | |
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"); | |
68 | ||
69 | let mut largest_variant: Option<(_, _)> = None; | |
70 | let mut second_variant: Option<(_, _)> = None; | |
71 | ||
72 | for (i, variant) in adt.variants.iter().enumerate() { | |
73 | let size: u64 = variant | |
74 | .fields | |
75 | .iter() | |
76 | .filter_map(|f| { | |
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()) | |
81 | }) | |
82 | .sum(); | |
83 | ||
84 | let grouped = (size, (i, variant)); | |
85 | ||
86 | if grouped.0 >= largest_variant.map_or(0, |x| x.0) { | |
87 | second_variant = largest_variant; | |
88 | largest_variant = Some(grouped); | |
89 | } | |
90 | } | |
91 | ||
92 | if let (Some(largest), Some(second)) = (largest_variant, second_variant) { | |
93 | let difference = largest.0 - second.0; | |
94 | ||
95 | if difference > self.maximum_size_difference_allowed { | |
96 | let (i, variant) = largest.1; | |
97 | ||
98 | let help_text = "consider boxing the large fields to reduce the total size of the enum"; | |
99 | span_lint_and_then( | |
100 | cx, | |
101 | LARGE_ENUM_VARIANT, | |
102 | def.variants[i].span, | |
103 | "large size difference between variants", | |
104 | |diag| { | |
105 | diag.span_label( | |
106 | def.variants[(largest.1).0].span, | |
107 | &format!("this variant is {} bytes", largest.0), | |
108 | ); | |
109 | diag.span_note( | |
110 | def.variants[(second.1).0].span, | |
111 | &format!("and the second-largest variant is {} bytes:", second.0), | |
112 | ); | |
113 | if variant.fields.len() == 1 { | |
114 | let span = match def.variants[i].data { | |
115 | VariantData::Struct(ref fields, ..) | VariantData::Tuple(ref fields, ..) => { | |
116 | fields[0].ty.span | |
117 | }, | |
118 | VariantData::Unit(..) => unreachable!(), | |
119 | }; | |
120 | if let Some(snip) = snippet_opt(cx, span) { | |
121 | diag.span_suggestion( | |
122 | span, | |
123 | help_text, | |
124 | format!("Box<{}>", snip), | |
125 | Applicability::MaybeIncorrect, | |
126 | ); | |
127 | return; | |
128 | } | |
129 | } | |
130 | diag.span_help(def.variants[i].span, help_text); | |
131 | }, | |
132 | ); | |
133 | } | |
134 | } | |
135 | } | |
136 | } | |
137 | } |