clippy_lints/src/large_enum_variant.rs

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