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::{snippet_opt, span_lint_and_then, 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 {
  38             maximum_size_difference_allowed: 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                     .map(|f| {
  65                         let ty = cx.tcx.type_of(f.did);
  66                         if ty.needs_subst() {
  67                             0 // we can't reason about generics, so we treat them as zero sized
  68                         } else {
  69                             type_size(cx, ty).expect("size should be computable for concrete type")
  70                         }
  71                     })
  72                     .sum();
  73
  74                 let grouped = (size, (i, variant));
  75
  76                 update_if(&mut smallest_variant, grouped, |a, b| b.0 <= a.0);
  77                 update_if(&mut largest_variant, grouped, |a, b| b.0 >= a.0);
  78             }
  79
  80             if let (Some(smallest), Some(largest)) = (smallest_variant, largest_variant) {
  81                 let difference = largest.0 - smallest.0;
  82
  83                 if difference > self.maximum_size_difference_allowed {
  84                     let (i, variant) = largest.1;
  85
  86                     span_lint_and_then(
  87                         cx,
  88                         LARGE_ENUM_VARIANT,
  89                         def.variants[i].span,
  90                         "large size difference between variants",
  91                         |db| {
  92                             if variant.fields.len() == 1 {
  93                                 let span = match def.variants[i].node.data {
  94                                     VariantData::Struct(ref fields, _) | VariantData::Tuple(ref fields, _) => {
  95                                         fields[0].ty.span
  96                                     },
  97                                     VariantData::Unit(_) => unreachable!(),
  98                                 };
  99                                 if let Some(snip) = snippet_opt(cx, span) {
 100                                     db.span_suggestion(
 101                                         span,
 102                                         "consider boxing the large fields to reduce the total size of the \
 103                                          enum",
 104                                         format!("Box<{}>", snip),
 105                                     );
 106                                     return;
 107                                 }
 108                             }
 109                             db.span_help(
 110                                 def.variants[i].span,
 111                                 "consider boxing the large fields to reduce the total size of the enum",
 112                             );
 113                         },
 114                     );
 115                 }
 116             }
 117         }
 118     }
 119 }
 120
 121 fn update_if<T, F>(old: &mut Option<T>, new: T, f: F)
 122 where
 123     F: Fn(&T, &T) -> bool,
 124 {
 125     if let Some(ref mut val) = *old {
 126         if f(val, &new) {
 127             *val = new;
 128         }
 129     } else {
 130         *old = Some(new);
 131     }
 132 }