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