src/libsyntax/ext/deriving/encodable.rs

   1 // Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 /*!
  12
  13 The compiler code necessary to implement the `#[deriving(Encodable)]`
  14 (and `Decodable`, in decodable.rs) extension.  The idea here is that
  15 type-defining items may be tagged with `#[deriving(Encodable, Decodable)]`.
  16
  17 For example, a type like:
  18
  19 ```ignore
  20 #[deriving(Encodable, Decodable)]
  21 struct Node { id: uint }
  22 ```
  23
  24 would generate two implementations like:
  25
  26 ```ignore
  27 impl<S:serialize::Encoder> Encodable<S> for Node {
  28     fn encode(&self, s: &S) {
  29         s.emit_struct("Node", 1, || {
  30             s.emit_field("id", 0, || s.emit_uint(self.id))
  31         })
  32     }
  33 }
  34
  35 impl<D:Decoder> Decodable for node_id {
  36     fn decode(d: &D) -> Node {
  37         d.read_struct("Node", 1, || {
  38             Node {
  39                 id: d.read_field(~"x", 0, || decode(d))
  40             }
  41         })
  42     }
  43 }
  44 ```
  45
  46 Other interesting scenarios are whe the item has type parameters or
  47 references other non-built-in types.  A type definition like:
  48
  49 ```ignore
  50 #[deriving(Encodable, Decodable)]
  51 struct spanned<T> { node: T, span: Span }
  52 ```
  53
  54 would yield functions like:
  55
  56 ```ignore
  57     impl<
  58         S: Encoder,
  59         T: Encodable<S>
  60     > spanned<T>: Encodable<S> {
  61         fn encode<S:Encoder>(s: &S) {
  62             s.emit_rec(|| {
  63                 s.emit_field("node", 0, || self.node.encode(s));
  64                 s.emit_field("span", 1, || self.span.encode(s));
  65             })
  66         }
  67     }
  68
  69     impl<
  70         D: Decoder,
  71         T: Decodable<D>
  72     > spanned<T>: Decodable<D> {
  73         fn decode(d: &D) -> spanned<T> {
  74             d.read_rec(|| {
  75                 {
  76                     node: d.read_field(~"node", 0, || decode(d)),
  77                     span: d.read_field(~"span", 1, || decode(d)),
  78                 }
  79             })
  80         }
  81     }
  82 ```
  83 */
  84
  85 use ast::{MetaItem, Item, Expr, MutMutable};
  86 use codemap::Span;
  87 use ext::base::ExtCtxt;
  88 use ext::build::AstBuilder;
  89 use ext::deriving::generic::*;
  90 use parse::token;
  91
  92 pub fn expand_deriving_encodable(cx: &mut ExtCtxt,
  93                                  span: Span,
  94                                  mitem: @MetaItem,
  95                                  item: @Item,
  96                                  push: |@Item|) {
  97     let trait_def = TraitDef {
  98         span: span,
  99         attributes: ~[],
 100         path: Path::new_(~["serialize", "Encodable"], None,
 101                          ~[~Literal(Path::new_local("__E"))], true),
 102         additional_bounds: ~[],
 103         generics: LifetimeBounds {
 104             lifetimes: ~[],
 105             bounds: ~[("__E", ~[Path::new(~["serialize", "Encoder"])])],
 106         },
 107         methods: ~[
 108             MethodDef {
 109                 name: "encode",
 110                 generics: LifetimeBounds::empty(),
 111                 explicit_self: borrowed_explicit_self(),
 112                 args: ~[Ptr(~Literal(Path::new_local("__E")),
 113                             Borrowed(None, MutMutable))],
 114                 ret_ty: nil_ty(),
 115                 inline: false,
 116                 const_nonmatching: true,
 117                 combine_substructure: encodable_substructure,
 118             },
 119         ]
 120     };
 121
 122     trait_def.expand(cx, mitem, item, push)
 123 }
 124
 125 fn encodable_substructure(cx: &mut ExtCtxt, trait_span: Span,
 126                           substr: &Substructure) -> @Expr {
 127     let encoder = substr.nonself_args[0];
 128     // throw an underscore in front to suppress unused variable warnings
 129     let blkarg = cx.ident_of("_e");
 130     let blkencoder = cx.expr_ident(trait_span, blkarg);
 131     let encode = cx.ident_of("encode");
 132
 133     return match *substr.fields {
 134         Struct(ref fields) => {
 135             let emit_struct_field = cx.ident_of("emit_struct_field");
 136             let mut stmts = ~[];
 137             for (i, &FieldInfo {
 138                     name,
 139                     self_,
 140                     span,
 141                     ..
 142                 }) in fields.iter().enumerate() {
 143                 let name = match name {
 144                     Some(id) => token::get_ident(id),
 145                     None => {
 146                         token::intern_and_get_ident(format!("_field{}", i))
 147                     }
 148                 };
 149                 let enc = cx.expr_method_call(span, self_, encode, ~[blkencoder]);
 150                 let lambda = cx.lambda_expr_1(span, enc, blkarg);
 151                 let call = cx.expr_method_call(span, blkencoder,
 152                                                emit_struct_field,
 153                                                ~[cx.expr_str(span, name),
 154                                                  cx.expr_uint(span, i),
 155                                                  lambda]);
 156                 stmts.push(cx.stmt_expr(call));
 157             }
 158
 159             let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
 160             cx.expr_method_call(trait_span,
 161                                 encoder,
 162                                 cx.ident_of("emit_struct"),
 163                                 ~[
 164                 cx.expr_str(trait_span, token::get_ident(substr.type_ident)),
 165                 cx.expr_uint(trait_span, fields.len()),
 166                 blk
 167             ])
 168         }
 169
 170         EnumMatching(idx, variant, ref fields) => {
 171             // We're not generating an AST that the borrow checker is expecting,
 172             // so we need to generate a unique local variable to take the
 173             // mutable loan out on, otherwise we get conflicts which don't
 174             // actually exist.
 175             let me = cx.stmt_let(trait_span, false, blkarg, encoder);
 176             let encoder = cx.expr_ident(trait_span, blkarg);
 177             let emit_variant_arg = cx.ident_of("emit_enum_variant_arg");
 178             let mut stmts = ~[];
 179             for (i, &FieldInfo { self_, span, .. }) in fields.iter().enumerate() {
 180                 let enc = cx.expr_method_call(span, self_, encode, ~[blkencoder]);
 181                 let lambda = cx.lambda_expr_1(span, enc, blkarg);
 182                 let call = cx.expr_method_call(span, blkencoder,
 183                                                emit_variant_arg,
 184                                                ~[cx.expr_uint(span, i),
 185                                                  lambda]);
 186                 stmts.push(cx.stmt_expr(call));
 187             }
 188
 189             let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
 190             let name = cx.expr_str(trait_span, token::get_ident(variant.node.name));
 191             let call = cx.expr_method_call(trait_span, blkencoder,
 192                                            cx.ident_of("emit_enum_variant"),
 193                                            ~[name,
 194                                              cx.expr_uint(trait_span, idx),
 195                                              cx.expr_uint(trait_span, fields.len()),
 196                                              blk]);
 197             let blk = cx.lambda_expr_1(trait_span, call, blkarg);
 198             let ret = cx.expr_method_call(trait_span,
 199                                           encoder,
 200                                           cx.ident_of("emit_enum"),
 201                                           ~[
 202                 cx.expr_str(trait_span, token::get_ident(substr.type_ident)),
 203                 blk
 204             ]);
 205             cx.expr_block(cx.block(trait_span, ~[me], Some(ret)))
 206         }
 207
 208         _ => cx.bug("expected Struct or EnumMatching in deriving(Encodable)")
 209     };
 210 }