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, ExprRet, MutMutable, LitNil};
  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: Vec::new(),
 100         path: Path::new_(vec!("serialize", "Encodable"), None,
 101                          vec!(~Literal(Path::new_local("__S")),
 102                               ~Literal(Path::new_local("__E"))), true),
 103         additional_bounds: Vec::new(),
 104         generics: LifetimeBounds {
 105             lifetimes: Vec::new(),
 106             bounds: vec!(("__S", vec!(Path::new_(
 107                             vec!("serialize", "Encoder"), None,
 108                             vec!(~Literal(Path::new_local("__E"))), true))),
 109                          ("__E", vec!()))
 110         },
 111         methods: vec!(
 112             MethodDef {
 113                 name: "encode",
 114                 generics: LifetimeBounds::empty(),
 115                 explicit_self: borrowed_explicit_self(),
 116                 args: vec!(Ptr(~Literal(Path::new_local("__S")),
 117                             Borrowed(None, MutMutable))),
 118                 ret_ty: Literal(Path::new_(vec!("std", "result", "Result"),
 119                                            None,
 120                                            vec!(~Tuple(Vec::new()),
 121                                                 ~Literal(Path::new_local("__E"))),
 122                                            true)),
 123                 inline: false,
 124                 const_nonmatching: true,
 125                 combine_substructure: encodable_substructure,
 126             })
 127     };
 128
 129     trait_def.expand(cx, mitem, item, push)
 130 }
 131
 132 fn encodable_substructure(cx: &mut ExtCtxt, trait_span: Span,
 133                           substr: &Substructure) -> @Expr {
 134     let encoder = substr.nonself_args[0];
 135     // throw an underscore in front to suppress unused variable warnings
 136     let blkarg = cx.ident_of("_e");
 137     let blkencoder = cx.expr_ident(trait_span, blkarg);
 138     let encode = cx.ident_of("encode");
 139
 140     return match *substr.fields {
 141         Struct(ref fields) => {
 142             let emit_struct_field = cx.ident_of("emit_struct_field");
 143             let mut stmts = Vec::new();
 144             let last = fields.len() - 1;
 145             for (i, &FieldInfo {
 146                     name,
 147                     self_,
 148                     span,
 149                     ..
 150                 }) in fields.iter().enumerate() {
 151                 let name = match name {
 152                     Some(id) => token::get_ident(id),
 153                     None => {
 154                         token::intern_and_get_ident(format!("_field{}", i))
 155                     }
 156                 };
 157                 let enc = cx.expr_method_call(span, self_, encode, vec!(blkencoder));
 158                 let lambda = cx.lambda_expr_1(span, enc, blkarg);
 159                 let call = cx.expr_method_call(span, blkencoder,
 160                                                emit_struct_field,
 161                                                vec!(cx.expr_str(span, name),
 162                                                  cx.expr_uint(span, i),
 163                                                  lambda));
 164
 165                 // last call doesn't need a try!
 166                 let call = if i != last {
 167                     cx.expr_try(span, call)
 168                 } else {
 169                     cx.expr(span, ExprRet(Some(call)))
 170                 };
 171                 stmts.push(cx.stmt_expr(call));
 172             }
 173
 174             let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
 175             cx.expr_method_call(trait_span,
 176                                 encoder,
 177                                 cx.ident_of("emit_struct"),
 178                                 vec!(
 179                 cx.expr_str(trait_span, token::get_ident(substr.type_ident)),
 180                 cx.expr_uint(trait_span, fields.len()),
 181                 blk
 182             ))
 183         }
 184
 185         EnumMatching(idx, variant, ref fields) => {
 186             // We're not generating an AST that the borrow checker is expecting,
 187             // so we need to generate a unique local variable to take the
 188             // mutable loan out on, otherwise we get conflicts which don't
 189             // actually exist.
 190             let me = cx.stmt_let(trait_span, false, blkarg, encoder);
 191             let encoder = cx.expr_ident(trait_span, blkarg);
 192             let emit_variant_arg = cx.ident_of("emit_enum_variant_arg");
 193             let mut stmts = Vec::new();
 194             let last = fields.len() - 1;
 195             for (i, &FieldInfo { self_, span, .. }) in fields.iter().enumerate() {
 196                 let enc = cx.expr_method_call(span, self_, encode, vec!(blkencoder));
 197                 let lambda = cx.lambda_expr_1(span, enc, blkarg);
 198                 let call = cx.expr_method_call(span, blkencoder,
 199                                                emit_variant_arg,
 200                                                vec!(cx.expr_uint(span, i),
 201                                                  lambda));
 202                 let call = if i != last {
 203                     cx.expr_try(span, call)
 204                 } else {
 205                     cx.expr(span, ExprRet(Some(call)))
 206                 };
 207                 stmts.push(cx.stmt_expr(call));
 208             }
 209
 210             // enums with no fields need to return Ok()
 211             if stmts.len() == 0 {
 212                 let ret_ok = cx.expr(trait_span,
 213                                      ExprRet(Some(cx.expr_ok(trait_span,
 214                                                              cx.expr_lit(trait_span, LitNil)))));
 215                 stmts.push(cx.stmt_expr(ret_ok));
 216             }
 217
 218             let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
 219             let name = cx.expr_str(trait_span, token::get_ident(variant.node.name));
 220             let call = cx.expr_method_call(trait_span, blkencoder,
 221                                            cx.ident_of("emit_enum_variant"),
 222                                            vec!(name,
 223                                              cx.expr_uint(trait_span, idx),
 224                                              cx.expr_uint(trait_span, fields.len()),
 225                                              blk));
 226             let blk = cx.lambda_expr_1(trait_span, call, blkarg);
 227             let ret = cx.expr_method_call(trait_span,
 228                                           encoder,
 229                                           cx.ident_of("emit_enum"),
 230                                           vec!(
 231                 cx.expr_str(trait_span, token::get_ident(substr.type_ident)),
 232                 blk
 233             ));
 234             cx.expr_block(cx.block(trait_span, vec!(me), Some(ret)))
 235         }
 236
 237         _ => cx.bug("expected Struct or EnumMatching in deriving(Encodable)")
 238     };
 239 }