rust/src/libsyntax/ext/deriving/encodable.rs

// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

/*!

The compiler code necessary to implement the `#[deriving(Encodable)]`
(and `Decodable`, in decodable.rs) extension.  The idea here is that
type-defining items may be tagged with `#[deriving(Encodable, Decodable)]`.

For example, a type like:

```ignore
#[deriving(Encodable, Decodable)]
struct Node { id: uint }
```

would generate two implementations like:

```ignore
impl<S:serialize::Encoder> Encodable<S> for Node {
    fn encode(&self, s: &S) {
        s.emit_struct("Node", 1, || {
            s.emit_field("id", 0, || s.emit_uint(self.id))
        })
    }
}

impl<D:Decoder> Decodable for node_id {
    fn decode(d: &D) -> Node {
        d.read_struct("Node", 1, || {
            Node {
                id: d.read_field("x".to_owned(), 0, || decode(d))
            }
        })
    }
}
```

Other interesting scenarios are whe the item has type parameters or
references other non-built-in types.  A type definition like:

```ignore
#[deriving(Encodable, Decodable)]
struct spanned<T> { node: T, span: Span }
```

would yield functions like:

```ignore
    impl<
        S: Encoder,
        T: Encodable<S>
    > spanned<T>: Encodable<S> {
        fn encode<S:Encoder>(s: &S) {
            s.emit_rec(|| {
                s.emit_field("node", 0, || self.node.encode(s));
                s.emit_field("span", 1, || self.span.encode(s));
            })
        }
    }

    impl<
        D: Decoder,
        T: Decodable<D>
    > spanned<T>: Decodable<D> {
        fn decode(d: &D) -> spanned<T> {
            d.read_rec(|| {
                {
                    node: d.read_field("node".to_owned(), 0, || decode(d)),
                    span: d.read_field("span".to_owned(), 1, || decode(d)),
                }
            })
        }
    }
```
*/

use ast::{MetaItem, Item, Expr, ExprRet, MutMutable, LitNil};
use codemap::Span;
use ext::base::ExtCtxt;
use ext::build::AstBuilder;
use ext::deriving::generic::*;
use parse::token;

pub fn expand_deriving_encodable(cx: &mut ExtCtxt,
                                 span: Span,
                                 mitem: @MetaItem,
                                 item: @Item,
                                 push: |@Item|) {
    let trait_def = TraitDef {
        span: span,
        attributes: Vec::new(),
        path: Path::new_(vec!("serialize", "Encodable"), None,
                         vec!(~Literal(Path::new_local("__S")),
                              ~Literal(Path::new_local("__E"))), true),
        additional_bounds: Vec::new(),
        generics: LifetimeBounds {
            lifetimes: Vec::new(),
            bounds: vec!(("__S", vec!(Path::new_(
                            vec!("serialize", "Encoder"), None,
                            vec!(~Literal(Path::new_local("__E"))), true))),
                         ("__E", vec!()))
        },
        methods: vec!(
            MethodDef {
                name: "encode",
                generics: LifetimeBounds::empty(),
                explicit_self: borrowed_explicit_self(),
                args: vec!(Ptr(~Literal(Path::new_local("__S")),
                            Borrowed(None, MutMutable))),
                ret_ty: Literal(Path::new_(vec!("std", "result", "Result"),
                                           None,
                                           vec!(~Tuple(Vec::new()),
                                                ~Literal(Path::new_local("__E"))),
                                           true)),
                inline: false,
                const_nonmatching: true,
                combine_substructure: encodable_substructure,
            })
    };

    trait_def.expand(cx, mitem, item, push)
}

fn encodable_substructure(cx: &mut ExtCtxt, trait_span: Span,
                          substr: &Substructure) -> @Expr {
    let encoder = substr.nonself_args[0];
    // throw an underscore in front to suppress unused variable warnings
    let blkarg = cx.ident_of("_e");
    let blkencoder = cx.expr_ident(trait_span, blkarg);
    let encode = cx.ident_of("encode");

    return match *substr.fields {
        Struct(ref fields) => {
            let emit_struct_field = cx.ident_of("emit_struct_field");
            let mut stmts = Vec::new();
            let last = fields.len() - 1;
            for (i, &FieldInfo {
                    name,
                    self_,
                    span,
                    ..
                }) in fields.iter().enumerate() {
                let name = match name {
                    Some(id) => token::get_ident(id),
                    None => {
                        token::intern_and_get_ident(format!("_field{}", i))
                    }
                };
                let enc = cx.expr_method_call(span, self_, encode, vec!(blkencoder));
                let lambda = cx.lambda_expr_1(span, enc, blkarg);
                let call = cx.expr_method_call(span, blkencoder,
                                               emit_struct_field,
                                               vec!(cx.expr_str(span, name),
                                                 cx.expr_uint(span, i),
                                                 lambda));

                // last call doesn't need a try!
                let call = if i != last {
                    cx.expr_try(span, call)
                } else {
                    cx.expr(span, ExprRet(Some(call)))
                };
                stmts.push(cx.stmt_expr(call));
            }

            let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
            cx.expr_method_call(trait_span,
                                encoder,
                                cx.ident_of("emit_struct"),
                                vec!(
                cx.expr_str(trait_span, token::get_ident(substr.type_ident)),
                cx.expr_uint(trait_span, fields.len()),
                blk
            ))
        }

        EnumMatching(idx, variant, ref fields) => {
            // We're not generating an AST that the borrow checker is expecting,
            // so we need to generate a unique local variable to take the
            // mutable loan out on, otherwise we get conflicts which don't
            // actually exist.
            let me = cx.stmt_let(trait_span, false, blkarg, encoder);
            let encoder = cx.expr_ident(trait_span, blkarg);
            let emit_variant_arg = cx.ident_of("emit_enum_variant_arg");
            let mut stmts = Vec::new();
            let last = fields.len() - 1;
            for (i, &FieldInfo { self_, span, .. }) in fields.iter().enumerate() {
                let enc = cx.expr_method_call(span, self_, encode, vec!(blkencoder));
                let lambda = cx.lambda_expr_1(span, enc, blkarg);
                let call = cx.expr_method_call(span, blkencoder,
                                               emit_variant_arg,
                                               vec!(cx.expr_uint(span, i),
                                                 lambda));
                let call = if i != last {
                    cx.expr_try(span, call)
                } else {
                    cx.expr(span, ExprRet(Some(call)))
                };
                stmts.push(cx.stmt_expr(call));
            }

            // enums with no fields need to return Ok()
            if stmts.len() == 0 {
                let ret_ok = cx.expr(trait_span,
                                     ExprRet(Some(cx.expr_ok(trait_span,
                                                             cx.expr_lit(trait_span, LitNil)))));
                stmts.push(cx.stmt_expr(ret_ok));
            }

            let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
            let name = cx.expr_str(trait_span, token::get_ident(variant.node.name));
            let call = cx.expr_method_call(trait_span, blkencoder,
                                           cx.ident_of("emit_enum_variant"),
                                           vec!(name,
                                             cx.expr_uint(trait_span, idx),
                                             cx.expr_uint(trait_span, fields.len()),
                                             blk));
            let blk = cx.lambda_expr_1(trait_span, call, blkarg);
            let ret = cx.expr_method_call(trait_span,
                                          encoder,
                                          cx.ident_of("emit_enum"),
                                          vec!(
                cx.expr_str(trait_span, token::get_ident(substr.type_ident)),
                blk
            ));
            cx.expr_block(cx.block(trait_span, vec!(me), Some(ret)))
        }

        _ => cx.bug("expected Struct or EnumMatching in deriving(Encodable)")
    };
}