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::{span_lint_and_then, snippet_opt};
   6 use rustc::ty::layout::TargetDataLayout;
   7 use rustc::ty::TypeFoldable;
   8 use rustc::traits::Reveal;
   9
  10 /// **What it does:** Checks for large size differences between variants on `enum`s.
  11 ///
  12 /// **Why is this bad?** Enum size is bounded by the largest variant. Having a 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         LargeEnumVariant { maximum_size_difference_allowed: maximum_size_difference_allowed }
  38     }
  39 }
  40
  41 impl LintPass for LargeEnumVariant {
  42     fn get_lints(&self) -> LintArray {
  43         lint_array!(LARGE_ENUM_VARIANT)
  44     }
  45 }
  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.id);
  50         if let ItemEnum(ref def, _) = item.node {
  51             let ty = cx.tcx.item_type(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 data_layout = TargetDataLayout::parse(cx.sess());
  59                 cx.tcx.infer_ctxt((), Reveal::All).enter(|infcx| {
  60                     let size: u64 = variant.fields
  61                         .iter()
  62                         .map(|f| {
  63                             let ty = cx.tcx.item_type(f.did);
  64                             if ty.needs_subst() {
  65                                 0 // we can't reason about generics, so we treat them as zero sized
  66                             } else {
  67                                 ty.layout(&infcx)
  68                                     .expect("layout should be computable for concrete type")
  69                                     .size(&data_layout)
  70                                     .bytes()
  71                             }
  72                         })
  73                         .sum();
  74
  75                     let grouped = (size, (i, variant));
  76
  77                     update_if(&mut smallest_variant, grouped, |a, b| b.0 <= a.0);
  78                     update_if(&mut largest_variant, grouped, |a, b| b.0 >= a.0);
  79                 });
  80             }
  81
  82             if let (Some(smallest), Some(largest)) = (smallest_variant, largest_variant) {
  83                 let difference = largest.0 - smallest.0;
  84
  85                 if difference > self.maximum_size_difference_allowed {
  86                     let (i, variant) = largest.1;
  87
  88                     span_lint_and_then(cx,
  89                                        LARGE_ENUM_VARIANT,
  90                                        def.variants[i].span,
  91                                        "large size difference between variants",
  92                                        |db| {
  93                         if variant.fields.len() == 1 {
  94                             let span = match def.variants[i].node.data {
  95                                 VariantData::Struct(ref fields, _) |
  96                                 VariantData::Tuple(ref fields, _) => fields[0].ty.span,
  97                                 VariantData::Unit(_) => unreachable!(),
  98                             };
  99                             if let Some(snip) = snippet_opt(cx, span) {
 100                                 db.span_suggestion(span,
 101                                                    "consider boxing the large fields to reduce the total size of the \
 102                                                     enum",
 103                                                    format!("Box<{}>", snip));
 104                                 return;
 105                             }
 106                         }
 107                         db.span_help(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 fn update_if<T, F>(old: &mut Option<T>, new: T, f: F)
 118     where F: Fn(&T, &T) -> bool
 119 {
 120     if let Some(ref mut val) = *old {
 121         if f(val, &new) {
 122             *val = new;
 123         }
 124     } else {
 125         *old = Some(new);
 126     }
 127 }