clippy_lints/src/large_enum_variant.rs

   1 //! lint when there is a large size difference between variants on an enum
   2
   3 use rustc::lint::*;
   4 use rustc::hir::*;
   5 use crate::utils::{snippet_opt, span_lint_and_then};
   6 use rustc::ty::layout::LayoutOf;
   7
   8 /// **What it does:** Checks for large size differences between variants on
   9 /// `enum`s.
  10 ///
  11 /// **Why is this bad?** Enum size is bounded by the largest variant. Having a
  12 /// large variant
  13 /// can penalize the memory layout of that enum.
  14 ///
  15 /// **Known problems:** None.
  16 ///
  17 /// **Example:**
  18 /// ```rust
  19 /// enum Test {
  20 ///    A(i32),
  21 ///    B([i32; 8000]),
  22 /// }
  23 /// ```
  24 declare_clippy_lint! {
  25     pub LARGE_ENUM_VARIANT,
  26     perf,
  27     "large size difference between variants on an enum"
  28 }
  29
  30 #[derive(Copy, Clone)]
  31 pub struct LargeEnumVariant {
  32     maximum_size_difference_allowed: u64,
  33 }
  34
  35 impl LargeEnumVariant {
  36     pub fn new(maximum_size_difference_allowed: u64) -> Self {
  37         Self {
  38             maximum_size_difference_allowed,
  39         }
  40     }
  41 }
  42
  43 impl LintPass for LargeEnumVariant {
  44     fn get_lints(&self) -> LintArray {
  45         lint_array!(LARGE_ENUM_VARIANT)
  46     }
  47 }
  48
  49 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LargeEnumVariant {
  50     fn check_item(&mut self, cx: &LateContext, item: &Item) {
  51         let did = cx.tcx.hir.local_def_id(item.id);
  52         if let ItemEnum(ref def, _) = item.node {
  53             let ty = cx.tcx.type_of(did);
  54             let adt = ty.ty_adt_def()
  55                 .expect("already checked whether this is an enum");
  56
  57             let mut smallest_variant: Option<(_, _)> = None;
  58             let mut largest_variant: Option<(_, _)> = None;
  59
  60             for (i, variant) in adt.variants.iter().enumerate() {
  61                 let size: u64 = variant
  62                     .fields
  63                     .iter()
  64                     .filter_map(|f| {
  65                         let ty = cx.tcx.type_of(f.did);
  66                         // don't count generics by filtering out everything
  67                         // that does not have a layout
  68                         cx.layout_of(ty).ok().map(|l| l.size.bytes())
  69                     })
  70                     .sum();
  71
  72                 let grouped = (size, (i, variant));
  73
  74                 update_if(&mut smallest_variant, grouped, |a, b| b.0 <= a.0);
  75                 update_if(&mut largest_variant, grouped, |a, b| b.0 >= a.0);
  76             }
  77
  78             if let (Some(smallest), Some(largest)) = (smallest_variant, largest_variant) {
  79                 let difference = largest.0 - smallest.0;
  80
  81                 if difference > self.maximum_size_difference_allowed {
  82                     let (i, variant) = largest.1;
  83
  84                     span_lint_and_then(
  85                         cx,
  86                         LARGE_ENUM_VARIANT,
  87                         def.variants[i].span,
  88                         "large size difference between variants",
  89                         |db| {
  90                             if variant.fields.len() == 1 {
  91                                 let span = match def.variants[i].node.data {
  92                                     VariantData::Struct(ref fields, _) | VariantData::Tuple(ref fields, _) => {
  93                                         fields[0].ty.span
  94                                     },
  95                                     VariantData::Unit(_) => unreachable!(),
  96                                 };
  97                                 if let Some(snip) = snippet_opt(cx, span) {
  98                                     db.span_suggestion(
  99                                         span,
 100                                         "consider boxing the large fields to reduce the total size of the \
 101                                          enum",
 102                                         format!("Box<{}>", snip),
 103                                     );
 104                                     return;
 105                                 }
 106                             }
 107                             db.span_help(
 108                                 def.variants[i].span,
 109                                 "consider boxing the large fields to reduce the total size of the enum",
 110                             );
 111                         },
 112                     );
 113                 }
 114             }
 115         }
 116     }
 117 }
 118
 119 fn update_if<T, F>(old: &mut Option<T>, new: T, f: F)
 120 where
 121     F: Fn(&T, &T) -> bool,
 122 {
 123     if let Some(ref mut val) = *old {
 124         if f(val, &new) {
 125             *val = new;
 126         }
 127     } else {
 128         *old = Some(new);
 129     }
 130 }