src/libsyntax_ext/deriving/encodable.rs

   1 //! The compiler code necessary to implement the `#[derive(Encodable)]`
   2 //! (and `Decodable`, in `decodable.rs`) extension. The idea here is that
   3 //! type-defining items may be tagged with `#[derive(Encodable, Decodable)]`.
   4 //!
   5 //! For example, a type like:
   6 //!
   7 //! ```
   8 //! #[derive(Encodable, Decodable)]
   9 //! struct Node { id: usize }
  10 //! ```
  11 //!
  12 //! would generate two implementations like:
  13 //!
  14 //! ```
  15 //! # struct Node { id: usize }
  16 //! impl<S: Encoder<E>, E> Encodable<S, E> for Node {
  17 //!     fn encode(&self, s: &mut S) -> Result<(), E> {
  18 //!         s.emit_struct("Node", 1, |this| {
  19 //!             this.emit_struct_field("id", 0, |this| {
  20 //!                 Encodable::encode(&self.id, this)
  21 //!                 /* this.emit_usize(self.id) can also be used */
  22 //!             })
  23 //!         })
  24 //!     }
  25 //! }
  26 //!
  27 //! impl<D: Decoder<E>, E> Decodable<D, E> for Node {
  28 //!     fn decode(d: &mut D) -> Result<Node, E> {
  29 //!         d.read_struct("Node", 1, |this| {
  30 //!             match this.read_struct_field("id", 0, |this| Decodable::decode(this)) {
  31 //!                 Ok(id) => Ok(Node { id: id }),
  32 //!                 Err(e) => Err(e),
  33 //!             }
  34 //!         })
  35 //!     }
  36 //! }
  37 //! ```
  38 //!
  39 //! Other interesting scenarios are when the item has type parameters or
  40 //! references other non-built-in types. A type definition like:
  41 //!
  42 //! ```
  43 //! # #[derive(Encodable, Decodable)] struct Span;
  44 //! #[derive(Encodable, Decodable)]
  45 //! struct Spanned<T> { node: T, span: Span }
  46 //! ```
  47 //!
  48 //! would yield functions like:
  49 //!
  50 //! ```
  51 //! # #[derive(Encodable, Decodable)] struct Span;
  52 //! # struct Spanned<T> { node: T, span: Span }
  53 //! impl<
  54 //!     S: Encoder<E>,
  55 //!     E,
  56 //!     T: Encodable<S, E>
  57 //! > Encodable<S, E> for Spanned<T> {
  58 //!     fn encode(&self, s: &mut S) -> Result<(), E> {
  59 //!         s.emit_struct("Spanned", 2, |this| {
  60 //!             this.emit_struct_field("node", 0, |this| self.node.encode(this))
  61 //!                 .unwrap();
  62 //!             this.emit_struct_field("span", 1, |this| self.span.encode(this))
  63 //!         })
  64 //!     }
  65 //! }
  66 //!
  67 //! impl<
  68 //!     D: Decoder<E>,
  69 //!     E,
  70 //!     T: Decodable<D, E>
  71 //! > Decodable<D, E> for Spanned<T> {
  72 //!     fn decode(d: &mut D) -> Result<Spanned<T>, E> {
  73 //!         d.read_struct("Spanned", 2, |this| {
  74 //!             Ok(Spanned {
  75 //!                 node: this.read_struct_field("node", 0, |this| Decodable::decode(this))
  76 //!                     .unwrap(),
  77 //!                 span: this.read_struct_field("span", 1, |this| Decodable::decode(this))
  78 //!                     .unwrap(),
  79 //!             })
  80 //!         })
  81 //!     }
  82 //! }
  83 //! ```
  84
  85 use crate::deriving::{self, pathvec_std};
  86 use crate::deriving::generic::*;
  87 use crate::deriving::generic::ty::*;
  88
  89 use syntax::ast::{Expr, ExprKind, MetaItem, Mutability};
  90 use syntax::ext::base::{Annotatable, ExtCtxt};
  91 use syntax::ext::build::AstBuilder;
  92 use syntax::ptr::P;
  93 use syntax::symbol::Symbol;
  94 use syntax_pos::Span;
  95
  96 pub fn expand_deriving_rustc_encodable(cx: &mut ExtCtxt<'_>,
  97                                        span: Span,
  98                                        mitem: &MetaItem,
  99                                        item: &Annotatable,
 100                                        push: &mut dyn FnMut(Annotatable)) {
 101     let krate = "rustc_serialize";
 102     let typaram = &*deriving::hygienic_type_parameter(item, "__S");
 103
 104     let trait_def = TraitDef {
 105         span,
 106         attributes: Vec::new(),
 107         path: Path::new_(vec![krate, "Encodable"], None, vec![], PathKind::Global),
 108         additional_bounds: Vec::new(),
 109         generics: LifetimeBounds::empty(),
 110         is_unsafe: false,
 111         supports_unions: false,
 112         methods: vec![
 113             MethodDef {
 114                 name: "encode",
 115                 generics: LifetimeBounds {
 116                     lifetimes: Vec::new(),
 117                     bounds: vec![
 118                         (typaram,
 119                          vec![Path::new_(vec![krate, "Encoder"], None, vec![], PathKind::Global)])
 120                     ],
 121                 },
 122                 explicit_self: borrowed_explicit_self(),
 123                 args: vec![(Ptr(Box::new(Literal(Path::new_local(typaram))),
 124                            Borrowed(None, Mutability::Mutable)), "s")],
 125                 ret_ty: Literal(Path::new_(
 126                     pathvec_std!(cx, result::Result),
 127                     None,
 128                     vec![Box::new(Tuple(Vec::new())), Box::new(Literal(Path::new_(
 129                         vec![typaram, "Error"], None, vec![], PathKind::Local
 130                     )))],
 131                     PathKind::Std
 132                 )),
 133                 attributes: Vec::new(),
 134                 is_unsafe: false,
 135                 unify_fieldless_variants: false,
 136                 combine_substructure: combine_substructure(Box::new(|a, b, c| {
 137                     encodable_substructure(a, b, c, krate)
 138                 })),
 139             }
 140         ],
 141         associated_types: Vec::new(),
 142     };
 143
 144     trait_def.expand(cx, mitem, item, push)
 145 }
 146
 147 fn encodable_substructure(cx: &mut ExtCtxt<'_>,
 148                           trait_span: Span,
 149                           substr: &Substructure<'_>,
 150                           krate: &'static str)
 151                           -> P<Expr> {
 152     let encoder = substr.nonself_args[0].clone();
 153     // throw an underscore in front to suppress unused variable warnings
 154     let blkarg = cx.ident_of("_e");
 155     let blkencoder = cx.expr_ident(trait_span, blkarg);
 156     let fn_path = cx.expr_path(cx.path_global(trait_span,
 157                                               vec![cx.ident_of(krate),
 158                                                    cx.ident_of("Encodable"),
 159                                                    cx.ident_of("encode")]));
 160
 161     return match *substr.fields {
 162         Struct(_, ref fields) => {
 163             let emit_struct_field = cx.ident_of("emit_struct_field");
 164             let mut stmts = Vec::new();
 165             for (i, &FieldInfo { name, ref self_, span, .. }) in fields.iter().enumerate() {
 166                 let name = match name {
 167                     Some(id) => id.name,
 168                     None => Symbol::intern(&format!("_field{}", i)),
 169                 };
 170                 let self_ref = cx.expr_addr_of(span, self_.clone());
 171                 let enc = cx.expr_call(span, fn_path.clone(), vec![self_ref, blkencoder.clone()]);
 172                 let lambda = cx.lambda1(span, enc, blkarg);
 173                 let call = cx.expr_method_call(span,
 174                                                blkencoder.clone(),
 175                                                emit_struct_field,
 176                                                vec![cx.expr_str(span, name),
 177                                                     cx.expr_usize(span, i),
 178                                                     lambda]);
 179
 180                 // last call doesn't need a try!
 181                 let last = fields.len() - 1;
 182                 let call = if i != last {
 183                     cx.expr_try(span, call)
 184                 } else {
 185                     cx.expr(span, ExprKind::Ret(Some(call)))
 186                 };
 187
 188                 let stmt = cx.stmt_expr(call);
 189                 stmts.push(stmt);
 190             }
 191
 192             // unit structs have no fields and need to return Ok()
 193             let blk = if stmts.is_empty() {
 194                 let ok = cx.expr_ok(trait_span, cx.expr_tuple(trait_span, vec![]));
 195                 cx.lambda1(trait_span, ok, blkarg)
 196             } else {
 197                 cx.lambda_stmts_1(trait_span, stmts, blkarg)
 198             };
 199
 200             cx.expr_method_call(trait_span,
 201                                 encoder,
 202                                 cx.ident_of("emit_struct"),
 203                                 vec![cx.expr_str(trait_span, substr.type_ident.name),
 204                                      cx.expr_usize(trait_span, fields.len()),
 205                                      blk])
 206         }
 207
 208         EnumMatching(idx, _, variant, ref fields) => {
 209             // We're not generating an AST that the borrow checker is expecting,
 210             // so we need to generate a unique local variable to take the
 211             // mutable loan out on, otherwise we get conflicts which don't
 212             // actually exist.
 213             let me = cx.stmt_let(trait_span, false, blkarg, encoder);
 214             let encoder = cx.expr_ident(trait_span, blkarg);
 215             let emit_variant_arg = cx.ident_of("emit_enum_variant_arg");
 216             let mut stmts = Vec::new();
 217             if !fields.is_empty() {
 218                 let last = fields.len() - 1;
 219                 for (i, &FieldInfo { ref self_, span, .. }) in fields.iter().enumerate() {
 220                     let self_ref = cx.expr_addr_of(span, self_.clone());
 221                     let enc =
 222                         cx.expr_call(span, fn_path.clone(), vec![self_ref, blkencoder.clone()]);
 223                     let lambda = cx.lambda1(span, enc, blkarg);
 224                     let call = cx.expr_method_call(span,
 225                                                    blkencoder.clone(),
 226                                                    emit_variant_arg,
 227                                                    vec![cx.expr_usize(span, i), lambda]);
 228                     let call = if i != last {
 229                         cx.expr_try(span, call)
 230                     } else {
 231                         cx.expr(span, ExprKind::Ret(Some(call)))
 232                     };
 233                     stmts.push(cx.stmt_expr(call));
 234                 }
 235             } else {
 236                 let ok = cx.expr_ok(trait_span, cx.expr_tuple(trait_span, vec![]));
 237                 let ret_ok = cx.expr(trait_span, ExprKind::Ret(Some(ok)));
 238                 stmts.push(cx.stmt_expr(ret_ok));
 239             }
 240
 241             let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
 242             let name = cx.expr_str(trait_span, variant.node.ident.name);
 243             let call = cx.expr_method_call(trait_span,
 244                                            blkencoder,
 245                                            cx.ident_of("emit_enum_variant"),
 246                                            vec![name,
 247                                                 cx.expr_usize(trait_span, idx),
 248                                                 cx.expr_usize(trait_span, fields.len()),
 249                                                 blk]);
 250             let blk = cx.lambda1(trait_span, call, blkarg);
 251             let ret = cx.expr_method_call(trait_span,
 252                                           encoder,
 253                                           cx.ident_of("emit_enum"),
 254                                           vec![cx.expr_str(trait_span ,substr.type_ident.name),
 255                                                blk]);
 256             cx.expr_block(cx.block(trait_span, vec![me, cx.stmt_expr(ret)]))
 257         }
 258
 259         _ => cx.bug("expected Struct or EnumMatching in derive(Encodable)"),
 260     };
 261 }