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".to_owned(), 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".to_owned(), 0, || decode(d)),
  77                     span: d.read_field("span".to_owned(), 1, || decode(d)),
  78                 }
  79             })
  80         }
  81     }
  82 ```
  83 */
  84
  85 use ast;
  86 use ast::{MetaItem, Item, Expr, ExprRet, MutMutable, LitNil};
  87 use codemap::Span;
  88 use ext::base::ExtCtxt;
  89 use ext::build::AstBuilder;
  90 use ext::deriving::generic::*;
  91 use parse::token;
  92
  93 pub fn expand_deriving_encodable(cx: &mut ExtCtxt,
  94                                  span: Span,
  95                                  mitem: @MetaItem,
  96                                  item: @Item,
  97                                  push: |@Item|) {
  98     let trait_def = TraitDef {
  99         span: span,
 100         attributes: Vec::new(),
 101         path: Path::new_(vec!("serialize", "Encodable"), None,
 102                          vec!(~Literal(Path::new_local("__S")),
 103                               ~Literal(Path::new_local("__E"))), true),
 104         additional_bounds: Vec::new(),
 105         generics: LifetimeBounds {
 106             lifetimes: Vec::new(),
 107             bounds: vec!(("__S", ast::StaticSize, vec!(Path::new_(
 108                             vec!("serialize", "Encoder"), None,
 109                             vec!(~Literal(Path::new_local("__E"))), true))),
 110                          ("__E", ast::StaticSize, vec!()))
 111         },
 112         methods: vec!(
 113             MethodDef {
 114                 name: "encode",
 115                 generics: LifetimeBounds::empty(),
 116                 explicit_self: borrowed_explicit_self(),
 117                 args: vec!(Ptr(~Literal(Path::new_local("__S")),
 118                             Borrowed(None, MutMutable))),
 119                 ret_ty: Literal(Path::new_(vec!("std", "result", "Result"),
 120                                            None,
 121                                            vec!(~Tuple(Vec::new()),
 122                                                 ~Literal(Path::new_local("__E"))),
 123                                            true)),
 124                 attributes: Vec::new(),
 125                 const_nonmatching: true,
 126                 combine_substructure: encodable_substructure,
 127             })
 128     };
 129
 130     trait_def.expand(cx, mitem, item, push)
 131 }
 132
 133 fn encodable_substructure(cx: &mut ExtCtxt, trait_span: Span,
 134                           substr: &Substructure) -> @Expr {
 135     let encoder = substr.nonself_args[0];
 136     // throw an underscore in front to suppress unused variable warnings
 137     let blkarg = cx.ident_of("_e");
 138     let blkencoder = cx.expr_ident(trait_span, blkarg);
 139     let encode = cx.ident_of("encode");
 140
 141     return match *substr.fields {
 142         Struct(ref fields) => {
 143             let emit_struct_field = cx.ident_of("emit_struct_field");
 144             let mut stmts = Vec::new();
 145             let last = fields.len() - 1;
 146             for (i, &FieldInfo {
 147                     name,
 148                     self_,
 149                     span,
 150                     ..
 151                 }) in fields.iter().enumerate() {
 152                 let name = match name {
 153                     Some(id) => token::get_ident(id),
 154                     None => {
 155                         token::intern_and_get_ident(format!("_field{}", i))
 156                     }
 157                 };
 158                 let enc = cx.expr_method_call(span, self_, encode, vec!(blkencoder));
 159                 let lambda = cx.lambda_expr_1(span, enc, blkarg);
 160                 let call = cx.expr_method_call(span, blkencoder,
 161                                                emit_struct_field,
 162                                                vec!(cx.expr_str(span, name),
 163                                                  cx.expr_uint(span, i),
 164                                                  lambda));
 165
 166                 // last call doesn't need a try!
 167                 let call = if i != last {
 168                     cx.expr_try(span, call)
 169                 } else {
 170                     cx.expr(span, ExprRet(Some(call)))
 171                 };
 172                 stmts.push(cx.stmt_expr(call));
 173             }
 174
 175             let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
 176             cx.expr_method_call(trait_span,
 177                                 encoder,
 178                                 cx.ident_of("emit_struct"),
 179                                 vec!(
 180                 cx.expr_str(trait_span, token::get_ident(substr.type_ident)),
 181                 cx.expr_uint(trait_span, fields.len()),
 182                 blk
 183             ))
 184         }
 185
 186         EnumMatching(idx, variant, ref fields) => {
 187             // We're not generating an AST that the borrow checker is expecting,
 188             // so we need to generate a unique local variable to take the
 189             // mutable loan out on, otherwise we get conflicts which don't
 190             // actually exist.
 191             let me = cx.stmt_let(trait_span, false, blkarg, encoder);
 192             let encoder = cx.expr_ident(trait_span, blkarg);
 193             let emit_variant_arg = cx.ident_of("emit_enum_variant_arg");
 194             let mut stmts = Vec::new();
 195             let last = fields.len() - 1;
 196             for (i, &FieldInfo { self_, span, .. }) in fields.iter().enumerate() {
 197                 let enc = cx.expr_method_call(span, self_, encode, vec!(blkencoder));
 198                 let lambda = cx.lambda_expr_1(span, enc, blkarg);
 199                 let call = cx.expr_method_call(span, blkencoder,
 200                                                emit_variant_arg,
 201                                                vec!(cx.expr_uint(span, i),
 202                                                  lambda));
 203                 let call = if i != last {
 204                     cx.expr_try(span, call)
 205                 } else {
 206                     cx.expr(span, ExprRet(Some(call)))
 207                 };
 208                 stmts.push(cx.stmt_expr(call));
 209             }
 210
 211             // enums with no fields need to return Ok()
 212             if stmts.len() == 0 {
 213                 let ret_ok = cx.expr(trait_span,
 214                                      ExprRet(Some(cx.expr_ok(trait_span,
 215                                                              cx.expr_lit(trait_span, LitNil)))));
 216                 stmts.push(cx.stmt_expr(ret_ok));
 217             }
 218
 219             let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
 220             let name = cx.expr_str(trait_span, token::get_ident(variant.node.name));
 221             let call = cx.expr_method_call(trait_span, blkencoder,
 222                                            cx.ident_of("emit_enum_variant"),
 223                                            vec!(name,
 224                                              cx.expr_uint(trait_span, idx),
 225                                              cx.expr_uint(trait_span, fields.len()),
 226                                              blk));
 227             let blk = cx.lambda_expr_1(trait_span, call, blkarg);
 228             let ret = cx.expr_method_call(trait_span,
 229                                           encoder,
 230                                           cx.ident_of("emit_enum"),
 231                                           vec!(
 232                 cx.expr_str(trait_span, token::get_ident(substr.type_ident)),
 233                 blk
 234             ));
 235             cx.expr_block(cx.block(trait_span, vec!(me), Some(ret)))
 236         }
 237
 238         _ => cx.bug("expected Struct or EnumMatching in deriving(Encodable)")
 239     };
 240 }