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::{declare_lint, lint_array};
   5 use rustc::hir::*;
   6 use crate::utils::{snippet_opt, span_lint_and_then};
   7 use rustc::ty::layout::LayoutOf;
   8
   9 /// **What it does:** Checks for large size differences between variants on
  10 /// `enum`s.
  11 ///
  12 /// **Why is this bad?** Enum size is bounded by the largest variant. Having a
  13 /// large variant
  14 /// can penalize the memory layout of that enum.
  15 ///
  16 /// **Known problems:** None.
  17 ///
  18 /// **Example:**
  19 /// ```rust
  20 /// enum Test {
  21 ///    A(i32),
  22 ///    B([i32; 8000]),
  23 /// }
  24 /// ```
  25 declare_clippy_lint! {
  26     pub LARGE_ENUM_VARIANT,
  27     perf,
  28     "large size difference between variants on an enum"
  29 }
  30
  31 #[derive(Copy, Clone)]
  32 pub struct LargeEnumVariant {
  33     maximum_size_difference_allowed: u64,
  34 }
  35
  36 impl LargeEnumVariant {
  37     pub fn new(maximum_size_difference_allowed: u64) -> Self {
  38         Self {
  39             maximum_size_difference_allowed,
  40         }
  41     }
  42 }
  43
  44 impl LintPass for LargeEnumVariant {
  45     fn get_lints(&self) -> LintArray {
  46         lint_array!(LARGE_ENUM_VARIANT)
  47     }
  48 }
  49
  50 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LargeEnumVariant {
  51     fn check_item(&mut self, cx: &LateContext, item: &Item) {
  52         let did = cx.tcx.hir.local_def_id(item.id);
  53         if let ItemKind::Enum(ref def, _) = item.node {
  54             let ty = cx.tcx.type_of(did);
  55             let adt = ty.ty_adt_def()
  56                 .expect("already checked whether this is an enum");
  57
  58             let mut smallest_variant: Option<(_, _)> = None;
  59             let mut largest_variant: Option<(_, _)> = None;
  60
  61             for (i, variant) in adt.variants.iter().enumerate() {
  62                 let size: u64 = variant
  63                     .fields
  64                     .iter()
  65                     .filter_map(|f| {
  66                         let ty = cx.tcx.type_of(f.did);
  67                         // don't count generics by filtering out everything
  68                         // that does not have a layout
  69                         cx.layout_of(ty).ok().map(|l| l.size.bytes())
  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, _) | VariantData::Tuple(ref fields, _) => {
  94                                         fields[0].ty.span
  95                                     },
  96                                     VariantData::Unit(_) => unreachable!(),
  97                                 };
  98                                 if let Some(snip) = snippet_opt(cx, span) {
  99                                     db.span_suggestion(
 100                                         span,
 101                                         "consider boxing the large fields to reduce the total size of the \
 102                                          enum",
 103                                         format!("Box<{}>", snip),
 104                                     );
 105                                     return;
 106                                 }
 107                             }
 108                             db.span_help(
 109                                 def.variants[i].span,
 110                                 "consider boxing the large fields to reduce the total size of the enum",
 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 }