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::hir::*;
   5 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   6 use rustc::ty::layout::LayoutOf;
   7 use rustc::{declare_tool_lint, impl_lint_pass};
   8 use rustc_errors::Applicability;
   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     pub fn new(maximum_size_difference_allowed: u64) -> Self {
  39         Self {
  40             maximum_size_difference_allowed,
  41         }
  42     }
  43 }
  44
  45 impl_lint_pass!(LargeEnumVariant => [LARGE_ENUM_VARIANT]);
  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.hir_id);
  50         if let ItemKind::Enum(ref def, _) = item.node {
  51             let ty = cx.tcx.type_of(did);
  52             let adt = ty.ty_adt_def().expect("already checked whether this is an enum");
  53
  54             let mut smallest_variant: Option<(_, _)> = None;
  55             let mut largest_variant: Option<(_, _)> = None;
  56
  57             for (i, variant) in adt.variants.iter().enumerate() {
  58                 let size: u64 = variant
  59                     .fields
  60                     .iter()
  61                     .filter_map(|f| {
  62                         let ty = cx.tcx.type_of(f.did);
  63                         // don't count generics by filtering out everything
  64                         // that does not have a layout
  65                         cx.layout_of(ty).ok().map(|l| l.size.bytes())
  66                     })
  67                     .sum();
  68
  69                 let grouped = (size, (i, variant));
  70
  71                 update_if(&mut smallest_variant, grouped, |a, b| b.0 <= a.0);
  72                 update_if(&mut largest_variant, grouped, |a, b| b.0 >= a.0);
  73             }
  74
  75             if let (Some(smallest), Some(largest)) = (smallest_variant, largest_variant) {
  76                 let difference = largest.0 - smallest.0;
  77
  78                 if difference > self.maximum_size_difference_allowed {
  79                     let (i, variant) = largest.1;
  80
  81                     span_lint_and_then(
  82                         cx,
  83                         LARGE_ENUM_VARIANT,
  84                         def.variants[i].span,
  85                         "large size difference between variants",
  86                         |db| {
  87                             if variant.fields.len() == 1 {
  88                                 let span = match def.variants[i].data {
  89                                     VariantData::Struct(ref fields, ..) | VariantData::Tuple(ref fields, ..) => {
  90                                         fields[0].ty.span
  91                                     },
  92                                     VariantData::Unit(..) => unreachable!(),
  93                                 };
  94                                 if let Some(snip) = snippet_opt(cx, span) {
  95                                     db.span_suggestion(
  96                                         span,
  97                                         "consider boxing the large fields to reduce the total size of the \
  98                                          enum",
  99                                         format!("Box<{}>", snip),
 100                                         Applicability::MaybeIncorrect,
 101                                     );
 102                                     return;
 103                                 }
 104                             }
 105                             db.span_help(
 106                                 def.variants[i].span,
 107                                 "consider boxing the large fields to reduce the total size of the enum",
 108                             );
 109                         },
 110                     );
 111                 }
 112             }
 113         }
 114     }
 115 }
 116
 117 fn update_if<T, F>(old: &mut Option<T>, new: T, f: F)
 118 where
 119     F: Fn(&T, &T) -> bool,
 120 {
 121     if let Some(ref mut val) = *old {
 122         if f(val, &new) {
 123             *val = new;
 124         }
 125     } else {
 126         *old = Some(new);
 127     }
 128 }