src/libsyntax_ext/deriving/default.rs

   1 use crate::deriving::path_std;
   2 use crate::deriving::generic::*;
   3 use crate::deriving::generic::ty::*;
   4
   5 use syntax::ast::{Expr, MetaItem};
   6 use syntax::ext::base::{Annotatable, DummyResult, ExtCtxt};
   7 use syntax::ptr::P;
   8 use syntax::symbol::{kw, sym};
   9 use syntax::span_err;
  10 use syntax_pos::Span;
  11
  12 pub fn expand_deriving_default(cx: &mut ExtCtxt<'_>,
  13                                span: Span,
  14                                mitem: &MetaItem,
  15                                item: &Annotatable,
  16                                push: &mut dyn FnMut(Annotatable)) {
  17     let inline = cx.meta_word(span, sym::inline);
  18     let attrs = vec![cx.attribute(inline)];
  19     let trait_def = TraitDef {
  20         span,
  21         attributes: Vec::new(),
  22         path: path_std!(cx, default::Default),
  23         additional_bounds: Vec::new(),
  24         generics: LifetimeBounds::empty(),
  25         is_unsafe: false,
  26         supports_unions: false,
  27         methods: vec![MethodDef {
  28                           name: "default",
  29                           generics: LifetimeBounds::empty(),
  30                           explicit_self: None,
  31                           args: Vec::new(),
  32                           ret_ty: Self_,
  33                           attributes: attrs,
  34                           is_unsafe: false,
  35                           unify_fieldless_variants: false,
  36                           combine_substructure: combine_substructure(Box::new(|a, b, c| {
  37                               default_substructure(a, b, c)
  38                           })),
  39                       }],
  40         associated_types: Vec::new(),
  41     };
  42     trait_def.expand(cx, mitem, item, push)
  43 }
  44
  45 fn default_substructure(cx: &mut ExtCtxt<'_>,
  46                         trait_span: Span,
  47                         substr: &Substructure<'_>)
  48                         -> P<Expr> {
  49     // Note that `kw::Default` is "default" and `sym::Default` is "Default"!
  50     let default_ident = cx.std_path(&[kw::Default, sym::Default, kw::Default]);
  51     let default_call = |span| cx.expr_call_global(span, default_ident.clone(), Vec::new());
  52
  53     return match *substr.fields {
  54         StaticStruct(_, ref summary) => {
  55             match *summary {
  56                 Unnamed(ref fields, is_tuple) => {
  57                     if !is_tuple {
  58                         cx.expr_ident(trait_span, substr.type_ident)
  59                     } else {
  60                         let exprs = fields.iter().map(|sp| default_call(*sp)).collect();
  61                         cx.expr_call_ident(trait_span, substr.type_ident, exprs)
  62                     }
  63                 }
  64                 Named(ref fields) => {
  65                     let default_fields = fields.iter()
  66                         .map(|&(ident, span)| cx.field_imm(span, ident, default_call(span)))
  67                         .collect();
  68                     cx.expr_struct_ident(trait_span, substr.type_ident, default_fields)
  69                 }
  70             }
  71         }
  72         StaticEnum(..) => {
  73             span_err!(cx, trait_span, E0665,
  74                       "`Default` cannot be derived for enums, only structs");
  75             // let compilation continue
  76             DummyResult::raw_expr(trait_span, true)
  77         }
  78         _ => cx.span_bug(trait_span, "Non-static method in `derive(Default)`"),
  79     };
  80 }