clippy_lints/src/large_enum_variant.rs

   1 //! lint when there are large 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 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 variants on an enum"
  28 }
  29
  30 #[derive(Copy,Clone)]
  31 pub struct LargeEnumVariant {
  32     maximum_variant_size_allowed: u64,
  33 }
  34
  35 impl LargeEnumVariant {
  36     pub fn new(maximum_variant_size_allowed: u64) -> Self {
  37         LargeEnumVariant { maximum_variant_size_allowed: maximum_variant_size_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             for (i, variant) in adt.variants.iter().enumerate() {
  54                 let data_layout = TargetDataLayout::parse(cx.sess());
  55                 cx.tcx.infer_ctxt((), Reveal::All).enter(|infcx| {
  56                     let size: u64 = variant.fields
  57                         .iter()
  58                         .map(|f| {
  59                             let ty = cx.tcx.item_type(f.did);
  60                             if ty.needs_subst() {
  61                                 0 // we can't reason about generics, so we treat them as zero sized
  62                             } else {
  63                                 ty.layout(&infcx)
  64                                     .expect("layout should be computable for concrete type")
  65                                     .size(&data_layout)
  66                                     .bytes()
  67                             }
  68                         })
  69                         .sum();
  70                     if size > self.maximum_variant_size_allowed {
  71                         span_lint_and_then(cx,
  72                                            LARGE_ENUM_VARIANT,
  73                                            def.variants[i].span,
  74                                            "large enum variant found",
  75                                            |db| {
  76                             if variant.fields.len() == 1 {
  77                                 let span = match def.variants[i].node.data {
  78                                     VariantData::Struct(ref fields, _) |
  79                                     VariantData::Tuple(ref fields, _) => fields[0].ty.span,
  80                                     VariantData::Unit(_) => unreachable!(),
  81                                 };
  82                                 if let Some(snip) = snippet_opt(cx, span) {
  83                                     db.span_suggestion(span,
  84                                                        "consider boxing the large fields to reduce the total size of \
  85                                                         the enum",
  86                                                        format!("Box<{}>", snip));
  87                                     return;
  88                                 }
  89                             }
  90                             db.span_help(def.variants[i].span,
  91                                          "consider boxing the large fields to reduce the total size of the enum");
  92                         });
  93                     }
  94                 });
  95             }
  96         }
  97     }
  98 }