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 utils::{span_lint_and_then, snippet_opt, type_size};
   6 use rustc::ty::TypeFoldable;
   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_lint! {
  25     pub LARGE_ENUM_VARIANT,
  26     Warn,
  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 { maximum_size_difference_allowed: maximum_size_difference_allowed }
  38     }
  39 }
  40
  41 impl LintPass for LargeEnumVariant {
  42     fn get_lints(&self) -> LintArray {
  43         lint_array!(LARGE_ENUM_VARIANT)
  44     }
  45 }
  46
  47 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LargeEnumVariant {
  48     fn check_item(&mut self, cx: &LateContext, item: &Item) {
  49         let did = cx.tcx.hir.local_def_id(item.id);
  50         if let ItemEnum(ref def, _) = item.node {
  51             let ty = cx.tcx.type_of(did);
  52             let adt = ty.ty_adt_def().expect(
  53                 "already checked whether this is an enum",
  54             );
  55
  56             let mut smallest_variant: Option<(_, _)> = None;
  57             let mut largest_variant: Option<(_, _)> = None;
  58
  59             for (i, variant) in adt.variants.iter().enumerate() {
  60                 let size: u64 = variant
  61                     .fields
  62                     .iter()
  63                     .map(|f| {
  64                         let ty = cx.tcx.type_of(f.did);
  65                         if ty.needs_subst() {
  66                             0 // we can't reason about generics, so we treat them as zero sized
  67                         } else {
  68                             type_size(cx, ty).expect("size should be computable for concrete type")
  69                         }
  70                     })
  71                     .sum();
  72
  73                 let grouped = (size, (i, variant));
  74
  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);
  77             }
  78
  79             if let (Some(smallest), Some(largest)) = (smallest_variant, largest_variant) {
  80                 let difference = largest.0 - smallest.0;
  81
  82                 if difference > self.maximum_size_difference_allowed {
  83                     let (i, variant) = largest.1;
  84
  85                     span_lint_and_then(
  86                         cx,
  87                         LARGE_ENUM_VARIANT,
  88                         def.variants[i].span,
  89                         "large size difference between variants",
  90                         |db| {
  91                             if variant.fields.len() == 1 {
  92                                 let span = match def.variants[i].node.data {
  93                                     VariantData::Struct(ref fields, _) |
  94                                     VariantData::Tuple(ref fields, _) => fields[0].ty.span,
  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
 120 fn update_if<T, F>(old: &mut Option<T>, new: T, f: F)
 121 where
 122     F: Fn(&T, &T) -> bool,
 123 {
 124     if let Some(ref mut val) = *old {
 125         if f(val, &new) {
 126             *val = new;
 127         }
 128     } else {
 129         *old = Some(new);
 130     }
 131 }