src/librustc_builtin_macros/deriving/encodable.rs

   1 //! The compiler code necessary to implement the `#[derive(RustcEncodable)]`
   2 //! (and `RustcDecodable`, in `decodable.rs`) extension. The idea here is that
   3 //! type-defining items may be tagged with
   4 //! `#[derive(RustcEncodable, RustcDecodable)]`.
   5 //!
   6 //! For example, a type like:
   7 //!
   8 //! ```
   9 //! #[derive(RustcEncodable, RustcDecodable)]
  10 //! struct Node { id: usize }
  11 //! ```
  12 //!
  13 //! would generate two implementations like:
  14 //!
  15 //! ```
  16 //! # struct Node { id: usize }
  17 //! impl<S: Encoder<E>, E> Encodable<S, E> for Node {
  18 //!     fn encode(&self, s: &mut S) -> Result<(), E> {
  19 //!         s.emit_struct("Node", 1, |this| {
  20 //!             this.emit_struct_field("id", 0, |this| {
  21 //!                 Encodable::encode(&self.id, this)
  22 //!                 /* this.emit_usize(self.id) can also be used */
  23 //!             })
  24 //!         })
  25 //!     }
  26 //! }
  27 //!
  28 //! impl<D: Decoder<E>, E> Decodable<D, E> for Node {
  29 //!     fn decode(d: &mut D) -> Result<Node, E> {
  30 //!         d.read_struct("Node", 1, |this| {
  31 //!             match this.read_struct_field("id", 0, |this| Decodable::decode(this)) {
  32 //!                 Ok(id) => Ok(Node { id: id }),
  33 //!                 Err(e) => Err(e),
  34 //!             }
  35 //!         })
  36 //!     }
  37 //! }
  38 //! ```
  39 //!
  40 //! Other interesting scenarios are when the item has type parameters or
  41 //! references other non-built-in types. A type definition like:
  42 //!
  43 //! ```
  44 //! # #[derive(RustcEncodable, RustcDecodable)]
  45 //! # struct Span;
  46 //! #[derive(RustcEncodable, RustcDecodable)]
  47 //! struct Spanned<T> { node: T, span: Span }
  48 //! ```
  49 //!
  50 //! would yield functions like:
  51 //!
  52 //! ```
  53 //! # #[derive(RustcEncodable, RustcDecodable)]
  54 //! # struct Span;
  55 //! # struct Spanned<T> { node: T, span: Span }
  56 //! impl<
  57 //!     S: Encoder<E>,
  58 //!     E,
  59 //!     T: Encodable<S, E>
  60 //! > Encodable<S, E> for Spanned<T> {
  61 //!     fn encode(&self, s: &mut S) -> Result<(), E> {
  62 //!         s.emit_struct("Spanned", 2, |this| {
  63 //!             this.emit_struct_field("node", 0, |this| self.node.encode(this))
  64 //!                 .unwrap();
  65 //!             this.emit_struct_field("span", 1, |this| self.span.encode(this))
  66 //!         })
  67 //!     }
  68 //! }
  69 //!
  70 //! impl<
  71 //!     D: Decoder<E>,
  72 //!     E,
  73 //!     T: Decodable<D, E>
  74 //! > Decodable<D, E> for Spanned<T> {
  75 //!     fn decode(d: &mut D) -> Result<Spanned<T>, E> {
  76 //!         d.read_struct("Spanned", 2, |this| {
  77 //!             Ok(Spanned {
  78 //!                 node: this.read_struct_field("node", 0, |this| Decodable::decode(this))
  79 //!                     .unwrap(),
  80 //!                 span: this.read_struct_field("span", 1, |this| Decodable::decode(this))
  81 //!                     .unwrap(),
  82 //!             })
  83 //!         })
  84 //!     }
  85 //! }
  86 //! ```
  87
  88 use crate::deriving::generic::ty::*;
  89 use crate::deriving::generic::*;
  90 use crate::deriving::pathvec_std;
  91
  92 use rustc_ast::ast::{Expr, ExprKind, MetaItem, Mutability};
  93 use rustc_ast::ptr::P;
  94 use rustc_expand::base::{Annotatable, ExtCtxt};
  95 use rustc_span::symbol::Symbol;
  96 use rustc_span::Span;
  97
  98 pub fn expand_deriving_rustc_encodable(
  99     cx: &mut ExtCtxt<'_>,
 100     span: Span,
 101     mitem: &MetaItem,
 102     item: &Annotatable,
 103     push: &mut dyn FnMut(Annotatable),
 104 ) {
 105     let krate = "rustc_serialize";
 106     let typaram = "__S";
 107
 108     let trait_def = TraitDef {
 109         span,
 110         attributes: Vec::new(),
 111         path: Path::new_(vec![krate, "Encodable"], None, vec![], PathKind::Global),
 112         additional_bounds: Vec::new(),
 113         generics: LifetimeBounds::empty(),
 114         is_unsafe: false,
 115         supports_unions: false,
 116         methods: vec![MethodDef {
 117             name: "encode",
 118             generics: LifetimeBounds {
 119                 lifetimes: Vec::new(),
 120                 bounds: vec![(
 121                     typaram,
 122                     vec![Path::new_(vec![krate, "Encoder"], None, vec![], PathKind::Global)],
 123                 )],
 124             },
 125             explicit_self: borrowed_explicit_self(),
 126             args: vec![(
 127                 Ptr(Box::new(Literal(Path::new_local(typaram))), Borrowed(None, Mutability::Mut)),
 128                 "s",
 129             )],
 130             ret_ty: Literal(Path::new_(
 131                 pathvec_std!(cx, result::Result),
 132                 None,
 133                 vec![
 134                     Box::new(Tuple(Vec::new())),
 135                     Box::new(Literal(Path::new_(
 136                         vec![typaram, "Error"],
 137                         None,
 138                         vec![],
 139                         PathKind::Local,
 140                     ))),
 141                 ],
 142                 PathKind::Std,
 143             )),
 144             attributes: Vec::new(),
 145             is_unsafe: false,
 146             unify_fieldless_variants: false,
 147             combine_substructure: combine_substructure(Box::new(|a, b, c| {
 148                 encodable_substructure(a, b, c, krate)
 149             })),
 150         }],
 151         associated_types: Vec::new(),
 152     };
 153
 154     trait_def.expand(cx, mitem, item, push)
 155 }
 156
 157 fn encodable_substructure(
 158     cx: &mut ExtCtxt<'_>,
 159     trait_span: Span,
 160     substr: &Substructure<'_>,
 161     krate: &'static str,
 162 ) -> P<Expr> {
 163     let encoder = substr.nonself_args[0].clone();
 164     // throw an underscore in front to suppress unused variable warnings
 165     let blkarg = cx.ident_of("_e", trait_span);
 166     let blkencoder = cx.expr_ident(trait_span, blkarg);
 167     let fn_path = cx.expr_path(cx.path_global(
 168         trait_span,
 169         vec![
 170             cx.ident_of(krate, trait_span),
 171             cx.ident_of("Encodable", trait_span),
 172             cx.ident_of("encode", trait_span),
 173         ],
 174     ));
 175
 176     match *substr.fields {
 177         Struct(_, ref fields) => {
 178             let emit_struct_field = cx.ident_of("emit_struct_field", trait_span);
 179             let mut stmts = Vec::new();
 180             for (i, &FieldInfo { name, ref self_, span, .. }) in fields.iter().enumerate() {
 181                 let name = match name {
 182                     Some(id) => id.name,
 183                     None => Symbol::intern(&format!("_field{}", i)),
 184                 };
 185                 let self_ref = cx.expr_addr_of(span, self_.clone());
 186                 let enc = cx.expr_call(span, fn_path.clone(), vec![self_ref, blkencoder.clone()]);
 187                 let lambda = cx.lambda1(span, enc, blkarg);
 188                 let call = cx.expr_method_call(
 189                     span,
 190                     blkencoder.clone(),
 191                     emit_struct_field,
 192                     vec![cx.expr_str(span, name), cx.expr_usize(span, i), lambda],
 193                 );
 194
 195                 // last call doesn't need a try!
 196                 let last = fields.len() - 1;
 197                 let call = if i != last {
 198                     cx.expr_try(span, call)
 199                 } else {
 200                     cx.expr(span, ExprKind::Ret(Some(call)))
 201                 };
 202
 203                 let stmt = cx.stmt_expr(call);
 204                 stmts.push(stmt);
 205             }
 206
 207             // unit structs have no fields and need to return Ok()
 208             let blk = if stmts.is_empty() {
 209                 let ok = cx.expr_ok(trait_span, cx.expr_tuple(trait_span, vec![]));
 210                 cx.lambda1(trait_span, ok, blkarg)
 211             } else {
 212                 cx.lambda_stmts_1(trait_span, stmts, blkarg)
 213             };
 214
 215             cx.expr_method_call(
 216                 trait_span,
 217                 encoder,
 218                 cx.ident_of("emit_struct", trait_span),
 219                 vec![
 220                     cx.expr_str(trait_span, substr.type_ident.name),
 221                     cx.expr_usize(trait_span, fields.len()),
 222                     blk,
 223                 ],
 224             )
 225         }
 226
 227         EnumMatching(idx, _, variant, ref fields) => {
 228             // We're not generating an AST that the borrow checker is expecting,
 229             // so we need to generate a unique local variable to take the
 230             // mutable loan out on, otherwise we get conflicts which don't
 231             // actually exist.
 232             let me = cx.stmt_let(trait_span, false, blkarg, encoder);
 233             let encoder = cx.expr_ident(trait_span, blkarg);
 234             let emit_variant_arg = cx.ident_of("emit_enum_variant_arg", trait_span);
 235             let mut stmts = Vec::new();
 236             if !fields.is_empty() {
 237                 let last = fields.len() - 1;
 238                 for (i, &FieldInfo { ref self_, span, .. }) in fields.iter().enumerate() {
 239                     let self_ref = cx.expr_addr_of(span, self_.clone());
 240                     let enc =
 241                         cx.expr_call(span, fn_path.clone(), vec![self_ref, blkencoder.clone()]);
 242                     let lambda = cx.lambda1(span, enc, blkarg);
 243                     let call = cx.expr_method_call(
 244                         span,
 245                         blkencoder.clone(),
 246                         emit_variant_arg,
 247                         vec![cx.expr_usize(span, i), lambda],
 248                     );
 249                     let call = if i != last {
 250                         cx.expr_try(span, call)
 251                     } else {
 252                         cx.expr(span, ExprKind::Ret(Some(call)))
 253                     };
 254                     stmts.push(cx.stmt_expr(call));
 255                 }
 256             } else {
 257                 let ok = cx.expr_ok(trait_span, cx.expr_tuple(trait_span, vec![]));
 258                 let ret_ok = cx.expr(trait_span, ExprKind::Ret(Some(ok)));
 259                 stmts.push(cx.stmt_expr(ret_ok));
 260             }
 261
 262             let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
 263             let name = cx.expr_str(trait_span, variant.ident.name);
 264             let call = cx.expr_method_call(
 265                 trait_span,
 266                 blkencoder,
 267                 cx.ident_of("emit_enum_variant", trait_span),
 268                 vec![
 269                     name,
 270                     cx.expr_usize(trait_span, idx),
 271                     cx.expr_usize(trait_span, fields.len()),
 272                     blk,
 273                 ],
 274             );
 275             let blk = cx.lambda1(trait_span, call, blkarg);
 276             let ret = cx.expr_method_call(
 277                 trait_span,
 278                 encoder,
 279                 cx.ident_of("emit_enum", trait_span),
 280                 vec![cx.expr_str(trait_span, substr.type_ident.name), blk],
 281             );
 282             cx.expr_block(cx.block(trait_span, vec![me, cx.stmt_expr(ret)]))
 283         }
 284
 285         _ => cx.bug("expected Struct or EnumMatching in derive(Encodable)"),
 286     }
 287 }