clippy_lints/src/empty_enum.rs

   1 //! lint when there is an enum with no variants
   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 `enum`s with no variants.
  11 ///
  12 /// **Why is this bad?** Enum's with no variants should be replaced with `!`.
  13 ///
  14 /// **Known problems:** None.
  15 ///
  16 /// **Example:**
  17 /// ```rust
  18 /// enum Test {}
  19 /// ```
  20 declare_lint! {
  21     pub EMPTY_ENUM,
  22     Warn,
  23     "enum with no variants"
  24 }
  25
  26 #[derive(Copy,Clone)]
  27 pub struct EmptyEnum;
  28
  29 impl LintPass for EmptyEnum {
  30     fn get_lints(&self) -> LintArray {
  31         lint_array!(EMPTY_ENUM)
  32     }
  33 }
  34
  35 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for EmptyEnum {
  36     fn check_item(&mut self, cx: &LateContext, item: &Item) {
  37         let did = cx.tcx.hir.local_def_id(item.id);
  38         if let ItemEnum(ref def, _) = item.node {
  39             let ty = cx.tcx.item_type(did);
  40             let adt = ty.ty_adt_def().expect("already checked whether this is an enum");
  41             for (i, variant) in adt.variants.iter().enumerate() {
  42                 let data_layout = TargetDataLayout::parse(cx.sess());
  43                 cx.tcx.infer_ctxt((), Reveal::All).enter(|infcx| {
  44                     let size: u64 = variant.fields
  45                         .iter()
  46                         .map(|f| {
  47                             let ty = cx.tcx.item_type(f.did);
  48                             if ty.needs_subst() {
  49                                 0 // we can't reason about generics, so we treat them as zero sized
  50                             } else {
  51                                 ty.layout(&infcx)
  52                                     .expect("layout should be computable for concrete type")
  53                                     .size(&data_layout)
  54                                     .bytes()
  55                             }
  56                         })
  57                         .sum();
  58                     if size > 0 {
  59                         span_lint_and_then(cx, EMPTY_ENUM, def.variants[i].span, "large enum variant found", |db| {
  60                             if variant.fields.len() == 1 {
  61                                 let span = match def.variants[i].node.data {
  62                                     VariantData::Struct(ref fields, _) |
  63                                     VariantData::Tuple(ref fields, _) => fields[0].ty.span,
  64                                     VariantData::Unit(_) => unreachable!(),
  65                                 };
  66                                 if let Some(snip) = snippet_opt(cx, span) {
  67                                     db.span_suggestion(span,
  68                                                        "consider boxing the large fields to reduce the total size of \
  69                                                         the enum",
  70                                                        format!("Box<{}>", snip));
  71                                     return;
  72                                 }
  73                             }
  74                             db.span_help(def.variants[i].span,
  75                                          "consider boxing the large fields to reduce the total size of the enum");
  76                         });
  77                     }
  78                 });
  79             }
  80         }
  81     }
  82 }