src/librustc_builtin_macros/deriving/decodable.rs

   1 //! The compiler code necessary for `#[derive(RustcDecodable)]`. See encodable.rs for more.
   2
   3 use crate::deriving::generic::ty::*;
   4 use crate::deriving::generic::*;
   5 use crate::deriving::pathvec_std;
   6
   7 use rustc_ast::ast;
   8 use rustc_ast::ast::{Expr, MetaItem, Mutability};
   9 use rustc_ast::ptr::P;
  10 use rustc_expand::base::{Annotatable, ExtCtxt};
  11 use rustc_span::symbol::{sym, Symbol};
  12 use rustc_span::Span;
  13
  14 pub fn expand_deriving_rustc_decodable(
  15     cx: &mut ExtCtxt<'_>,
  16     span: Span,
  17     mitem: &MetaItem,
  18     item: &Annotatable,
  19     push: &mut dyn FnMut(Annotatable),
  20 ) {
  21     let krate = "rustc_serialize";
  22     let typaram = "__D";
  23
  24     let trait_def = TraitDef {
  25         span,
  26         attributes: Vec::new(),
  27         path: Path::new_(vec![krate, "Decodable"], None, vec![], PathKind::Global),
  28         additional_bounds: Vec::new(),
  29         generics: LifetimeBounds::empty(),
  30         is_unsafe: false,
  31         supports_unions: false,
  32         methods: vec![MethodDef {
  33             name: sym::decode,
  34             generics: LifetimeBounds {
  35                 lifetimes: Vec::new(),
  36                 bounds: vec![(
  37                     typaram,
  38                     vec![Path::new_(vec![krate, "Decoder"], None, vec![], PathKind::Global)],
  39                 )],
  40             },
  41             explicit_self: None,
  42             args: vec![(
  43                 Ptr(Box::new(Literal(Path::new_local(typaram))), Borrowed(None, Mutability::Mut)),
  44                 "d",
  45             )],
  46             ret_ty: Literal(Path::new_(
  47                 pathvec_std!(cx, result::Result),
  48                 None,
  49                 vec![
  50                     Box::new(Self_),
  51                     Box::new(Literal(Path::new_(
  52                         vec![typaram, "Error"],
  53                         None,
  54                         vec![],
  55                         PathKind::Local,
  56                     ))),
  57                 ],
  58                 PathKind::Std,
  59             )),
  60             attributes: Vec::new(),
  61             is_unsafe: false,
  62             unify_fieldless_variants: false,
  63             combine_substructure: combine_substructure(Box::new(|a, b, c| {
  64                 decodable_substructure(a, b, c, krate)
  65             })),
  66         }],
  67         associated_types: Vec::new(),
  68     };
  69
  70     trait_def.expand(cx, mitem, item, push)
  71 }
  72
  73 fn decodable_substructure(
  74     cx: &mut ExtCtxt<'_>,
  75     trait_span: Span,
  76     substr: &Substructure<'_>,
  77     krate: &str,
  78 ) -> P<Expr> {
  79     let decoder = substr.nonself_args[0].clone();
  80     let recurse = vec![
  81         cx.ident_of(krate, trait_span),
  82         cx.ident_of("Decodable", trait_span),
  83         cx.ident_of("decode", trait_span),
  84     ];
  85     let exprdecode = cx.expr_path(cx.path_global(trait_span, recurse));
  86     // throw an underscore in front to suppress unused variable warnings
  87     let blkarg = cx.ident_of("_d", trait_span);
  88     let blkdecoder = cx.expr_ident(trait_span, blkarg);
  89
  90     match *substr.fields {
  91         StaticStruct(_, ref summary) => {
  92             let nfields = match *summary {
  93                 Unnamed(ref fields, _) => fields.len(),
  94                 Named(ref fields) => fields.len(),
  95             };
  96             let read_struct_field = cx.ident_of("read_struct_field", trait_span);
  97
  98             let path = cx.path_ident(trait_span, substr.type_ident);
  99             let result =
 100                 decode_static_fields(cx, trait_span, path, summary, |cx, span, name, field| {
 101                     cx.expr_try(
 102                         span,
 103                         cx.expr_method_call(
 104                             span,
 105                             blkdecoder.clone(),
 106                             read_struct_field,
 107                             vec![
 108                                 cx.expr_str(span, name),
 109                                 cx.expr_usize(span, field),
 110                                 exprdecode.clone(),
 111                             ],
 112                         ),
 113                     )
 114                 });
 115             let result = cx.expr_ok(trait_span, result);
 116             cx.expr_method_call(
 117                 trait_span,
 118                 decoder,
 119                 cx.ident_of("read_struct", trait_span),
 120                 vec![
 121                     cx.expr_str(trait_span, substr.type_ident.name),
 122                     cx.expr_usize(trait_span, nfields),
 123                     cx.lambda1(trait_span, result, blkarg),
 124                 ],
 125             )
 126         }
 127         StaticEnum(_, ref fields) => {
 128             let variant = cx.ident_of("i", trait_span);
 129
 130             let mut arms = Vec::with_capacity(fields.len() + 1);
 131             let mut variants = Vec::with_capacity(fields.len());
 132             let rvariant_arg = cx.ident_of("read_enum_variant_arg", trait_span);
 133
 134             for (i, &(ident, v_span, ref parts)) in fields.iter().enumerate() {
 135                 variants.push(cx.expr_str(v_span, ident.name));
 136
 137                 let path = cx.path(trait_span, vec![substr.type_ident, ident]);
 138                 let decoded =
 139                     decode_static_fields(cx, v_span, path, parts, |cx, span, _, field| {
 140                         let idx = cx.expr_usize(span, field);
 141                         cx.expr_try(
 142                             span,
 143                             cx.expr_method_call(
 144                                 span,
 145                                 blkdecoder.clone(),
 146                                 rvariant_arg,
 147                                 vec![idx, exprdecode.clone()],
 148                             ),
 149                         )
 150                     });
 151
 152                 arms.push(cx.arm(v_span, cx.pat_lit(v_span, cx.expr_usize(v_span, i)), decoded));
 153             }
 154
 155             arms.push(cx.arm_unreachable(trait_span));
 156
 157             let result = cx.expr_ok(
 158                 trait_span,
 159                 cx.expr_match(trait_span, cx.expr_ident(trait_span, variant), arms),
 160             );
 161             let lambda = cx.lambda(trait_span, vec![blkarg, variant], result);
 162             let variant_vec = cx.expr_vec(trait_span, variants);
 163             let variant_vec = cx.expr_addr_of(trait_span, variant_vec);
 164             let result = cx.expr_method_call(
 165                 trait_span,
 166                 blkdecoder,
 167                 cx.ident_of("read_enum_variant", trait_span),
 168                 vec![variant_vec, lambda],
 169             );
 170             cx.expr_method_call(
 171                 trait_span,
 172                 decoder,
 173                 cx.ident_of("read_enum", trait_span),
 174                 vec![
 175                     cx.expr_str(trait_span, substr.type_ident.name),
 176                     cx.lambda1(trait_span, result, blkarg),
 177                 ],
 178             )
 179         }
 180         _ => cx.bug("expected StaticEnum or StaticStruct in derive(Decodable)"),
 181     }
 182 }
 183
 184 /// Creates a decoder for a single enum variant/struct:
 185 /// - `outer_pat_path` is the path to this enum variant/struct
 186 /// - `getarg` should retrieve the `usize`-th field with name `@str`.
 187 fn decode_static_fields<F>(
 188     cx: &mut ExtCtxt<'_>,
 189     trait_span: Span,
 190     outer_pat_path: ast::Path,
 191     fields: &StaticFields,
 192     mut getarg: F,
 193 ) -> P<Expr>
 194 where
 195     F: FnMut(&mut ExtCtxt<'_>, Span, Symbol, usize) -> P<Expr>,
 196 {
 197     match *fields {
 198         Unnamed(ref fields, is_tuple) => {
 199             let path_expr = cx.expr_path(outer_pat_path);
 200             if !is_tuple {
 201                 path_expr
 202             } else {
 203                 let fields = fields
 204                     .iter()
 205                     .enumerate()
 206                     .map(|(i, &span)| getarg(cx, span, Symbol::intern(&format!("_field{}", i)), i))
 207                     .collect();
 208
 209                 cx.expr_call(trait_span, path_expr, fields)
 210             }
 211         }
 212         Named(ref fields) => {
 213             // use the field's span to get nicer error messages.
 214             let fields = fields
 215                 .iter()
 216                 .enumerate()
 217                 .map(|(i, &(ident, span))| {
 218                     let arg = getarg(cx, span, ident.name, i);
 219                     cx.field_imm(span, ident, arg)
 220                 })
 221                 .collect();
 222             cx.expr_struct(trait_span, outer_pat_path, fields)
 223         }
 224     }
 225 }