auto merge of #7142 : alexcrichton/rust/deriving-zero, r=pcwalton

This allows mass-initialization of large structs without having to specify all the fields.

I'm a bit hesitant, but I wanted to get this out there. I don't really like using the `Zero` trait, because it doesn't really make sense for a type like `HashMap` to use `Zero` as the 'blank allocation' trait. In theory there'd be a new trait, but then that's adding cruft to the language which may not necessarily need to be there.

I do think that this can be useful, but I only implemented `Zero` on the basic types where I thought it made sense, so it may not be all that usable yet. (opinions?)

src/libsyntax/ext/deriving/mod.rs

@@ -31,6 +31,7 @@ pub mod encodable;
 pub mod decodable;
 pub mod rand;
 pub mod to_str;
+pub mod zero;
 
 #[path="cmp/eq.rs"]
 pub mod eq;
@@ -98,6 +99,7 @@ pub fn expand_meta_deriving(cx: @ExtCtxt,
                             "Rand" => expand!(rand::expand_deriving_rand),
 
                             "ToStr" => expand!(to_str::expand_deriving_to_str),
+                            "Zero" => expand!(zero::expand_deriving_zero),
 
                             ref tname => {
                                 cx.span_err(titem.span, fmt!("unknown \

src/libsyntax/ext/deriving/zero.rs

@@ -0,0 +1,96 @@
+// 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.
+
+use core::prelude::*;
+
+use ast::{meta_item, item, expr};
+use codemap::span;
+use ext::base::ExtCtxt;
+use ext::build::AstBuilder;
+use ext::deriving::generic::*;
+
+use core::vec;
+
+pub fn expand_deriving_zero(cx: @ExtCtxt,
+                            span: span,
+                            mitem: @meta_item,
+                            in_items: ~[@item])
+    -> ~[@item] {
+    let trait_def = TraitDef {
+        path: Path::new(~["std", "num", "Zero"]),
+        additional_bounds: ~[],
+        generics: LifetimeBounds::empty(),
+        methods: ~[
+            MethodDef {
+                name: "zero",
+                generics: LifetimeBounds::empty(),
+                explicit_self: None,
+                args: ~[],
+                ret_ty: Self,
+                const_nonmatching: false,
+                combine_substructure: zero_substructure
+            },
+            MethodDef {
+                name: "is_zero",
+                generics: LifetimeBounds::empty(),
+                explicit_self: borrowed_explicit_self(),
+                args: ~[],
+                ret_ty: Literal(Path::new(~["bool"])),
+                const_nonmatching: false,
+                combine_substructure: |cx, span, substr| {
+                    cs_and(|cx, span, _, _| cx.span_bug(span,
+                                                        "Non-matching enum \
+                                                         variant in \
+                                                         deriving(Zero)"),
+                           cx, span, substr)
+                }
+            }
+        ]
+    };
+    trait_def.expand(cx, span, mitem, in_items)
+}
+
+fn zero_substructure(cx: @ExtCtxt, span: span, substr: &Substructure) -> @expr {
+    let zero_ident = ~[
+        cx.ident_of("std"),
+        cx.ident_of("num"),
+        cx.ident_of("Zero"),
+        cx.ident_of("zero")
+    ];
+    let zero_call = || {
+        cx.expr_call_global(span, copy zero_ident, ~[])
+    };
+
+    return match *substr.fields {
+        StaticStruct(_, ref summary) => {
+            match *summary {
+                Left(count) => {
+                    if count == 0 {
+                        cx.expr_ident(span, substr.type_ident)
+                    } else {
+                        let exprs = vec::from_fn(count, |_| zero_call());
+                        cx.expr_call_ident(span, substr.type_ident, exprs)
+                    }
+                }
+                Right(ref fields) => {
+                    let zero_fields = do fields.map |ident| {
+                        cx.field_imm(span, *ident, zero_call())
+                    };
+                    cx.expr_struct_ident(span, substr.type_ident, zero_fields)
+                }
+            }
+        }
+        StaticEnum(*) => {
+            cx.span_fatal(span, "`Zero` cannot be derived for enums, \
+                                 only structs")
+        }
+        _ => cx.bug("Non-static method in `deriving(Zero)`")
+    };
+}