Allow syntax extensions which modify and decorate

This adds a new SyntaxExtension type - Renovator.

A renovator is a syntax extension which can modify *and* decorate the
items it is attached to - the union of `MultiModifier` and
`MultiDecorator`.

This allows things such as syntax extensions which modify existing
methods for types, and implement traits all in one.

src/librustc_plugin/registry.rs

@@ -16,7 +16,7 @@ use rustc::session::Session;
 use rustc::mir::transform::MirMapPass;
 
 use syntax::ext::base::{SyntaxExtension, NamedSyntaxExtension, NormalTT};
-use syntax::ext::base::{IdentTT, MultiModifier, MultiDecorator};
+use syntax::ext::base::{IdentTT, MultiModifier, MultiDecorator, Renovator};
 use syntax::ext::base::{MacroExpanderFn, MacroRulesTT};
 use syntax::codemap::Span;
 use syntax::parse::token;
@@ -112,6 +112,7 @@ impl<'a> Registry<'a> {
             }
             MultiDecorator(ext) => MultiDecorator(ext),
             MultiModifier(ext) => MultiModifier(ext),
+            Renovator(ext) => Renovator(ext),
             MacroRulesTT => {
                 self.sess.err("plugin tried to register a new MacroRulesTT");
                 return;

src/libsyntax/ext/base.rs

@@ -149,6 +149,33 @@ impl<F> MultiItemModifier for F
     }
 }
 
+pub trait ItemRenovator {
+    fn expand(&self,
+              ecx: &mut ExtCtxt,
+              span: Span,
+              meta_item: &ast::MetaItem,
+              item: Annotatable,
+              push: &mut FnMut(Annotatable))
+              -> Annotatable;
+}
+
+impl<F> ItemRenovator for F
+    where F : Fn(&mut ExtCtxt,
+                 Span,
+                 &ast::MetaItem,
+                 Annotatable,
+                 &mut FnMut(Annotatable)) -> Annotatable
+{
+    fn expand(&self,
+              ecx: &mut ExtCtxt,
+              sp: Span,
+              meta_item: &ast::MetaItem,
+              item: Annotatable,
+              push: &mut FnMut(Annotatable)) -> Annotatable {
+        (*self)(ecx, sp, meta_item, item, push)
+    }
+}
+
 /// Represents a thing that maps token trees to Macro Results
 pub trait TTMacroExpander {
     fn expand<'cx>(&self,
@@ -419,6 +446,12 @@ pub enum SyntaxExtension {
     /// in-place. More flexible version than Modifier.
     MultiModifier(Box<MultiItemModifier + 'static>),
 
+    /// A syntax extension that is attached to an item, modifying it in
+    /// place *and* creating new items based upon it.
+    ///
+    /// It can be thought of as the union of `MultiDecorator` and `MultiModifier`.
+    Renovator(Box<ItemRenovator + 'static>),
+
     /// A normal, function-like syntax extension.
     ///
     /// `bytes!` is a `NormalTT`.

src/libsyntax/ext/expand.rs

@@ -725,7 +725,7 @@ fn expand_annotatable(a: Annotatable,
 
     let mut decorator_items = SmallVector::zero();
     let mut new_attrs = Vec::new();
-    expand_decorators(a.clone(), fld, &mut decorator_items, &mut new_attrs);
+    let a = expand_renovators_and_decorators(a, fld, &mut decorator_items, &mut new_attrs);
 
     let mut new_items: SmallVector<Annotatable> = match a {
         Annotatable::Item(it) => match it.node {
@@ -816,47 +816,63 @@ macro_rules! partition {
 partition!(multi_modifiers, MultiModifier);
 
 
-fn expand_decorators(a: Annotatable,
-                     fld: &mut MacroExpander,
-                     decorator_items: &mut SmallVector<Annotatable>,
-                     new_attrs: &mut Vec<ast::Attribute>)
+fn expand_renovators_and_decorators(mut item: Annotatable,
+                                    fld: &mut MacroExpander,
+                                    decorator_items: &mut SmallVector<Annotatable>,
+                                    new_attrs: &mut Vec<ast::Attribute>) -> Annotatable
 {
-    for attr in a.attrs() {
+    let attrs: Vec<_> = item.attrs().iter().cloned().collect();
+
+    for attr in attrs {
         let mname = intern(&attr.name());
+
+        fld.cx.bt_push(ExpnInfo {
+            call_site: attr.span,
+            callee: NameAndSpan {
+                format: MacroAttribute(mname),
+                span: Some(attr.span),
+                // attributes can do whatever they like,
+                // for now.
+                allow_internal_unstable: true,
+            }
+        });
+
+        // we'd ideally decorator_items.push_all(expand_annotatable(ann, fld)),
+        // but that double-mut-borrows fld
+        let mut items: SmallVector<Annotatable> = SmallVector::zero();
+
         match fld.cx.syntax_env.find(mname) {
             Some(rc) => match *rc {
                 MultiDecorator(ref dec) => {
                     attr::mark_used(&attr);
 
-                    fld.cx.bt_push(ExpnInfo {
-                        call_site: attr.span,
-                        callee: NameAndSpan {
-                            format: MacroAttribute(mname),
-                            span: Some(attr.span),
-                            // attributes can do whatever they like,
-                            // for now.
-                            allow_internal_unstable: true,
-                        }
-                    });
-
-                    // we'd ideally decorator_items.push_all(expand_annotatable(ann, fld)),
-                    // but that double-mut-borrows fld
-                    let mut items: SmallVector<Annotatable> = SmallVector::zero();
                     dec.expand(fld.cx,
                                attr.span,
                                &attr.node.value,
-                               &a,
+                               &item,
                                &mut |ann| items.push(ann));
-                    decorator_items.extend(items.into_iter()
-                        .flat_map(|ann| expand_annotatable(ann, fld).into_iter()));
+                },
+                Renovator(ref ren) => {
+                    attr::mark_used(&attr);
 
-                    fld.cx.bt_pop();
-                }
-                _ => new_attrs.push((*attr).clone()),
+                    item = ren.expand(fld.cx,
+                                      attr.span,
+                                      &attr.node.value,
+                                      item,
+                                      &mut |ann| items.push(ann));
+                },
+                _ => new_attrs.push(attr),
             },
-            _ => new_attrs.push((*attr).clone()),
+            _ => new_attrs.push(attr),
         }
+
+        decorator_items.extend(items.into_iter()
+            .flat_map(|ann| expand_annotatable(ann, fld).into_iter()));
+
+        fld.cx.bt_pop();
     }
+
+    item
 }
 
 fn expand_item_multi_modifier(mut it: Annotatable,

src/test/run-pass-fulldeps/auxiliary/macro_crate_test.rs

@@ -39,6 +39,9 @@ pub fn plugin_registrar(reg: &mut Registry) {
         token::intern("duplicate"),
         // FIXME (#22405): Replace `Box::new` with `box` here when/if possible.
         MultiDecorator(Box::new(expand_duplicate)));
+    reg.register_syntax_extension(
+        token::intern("wrap"),
+        Renovator(Box::new(expand_wrap)));
 }
 
 fn expand_make_a_1(cx: &mut ExtCtxt, sp: Span, tts: &[TokenTree])
@@ -138,4 +141,25 @@ fn expand_duplicate(cx: &mut ExtCtxt,
     }
 }
 
+fn expand_wrap(cx: &mut ExtCtxt,
+               sp: Span,
+               meta_item: &MetaItem,
+               mut item: Annotatable,
+               push: &mut FnMut(Annotatable)) -> Annotatable
+{
+
+    match item {
+        Annotatable::Item(item) => {
+            push(Annotatable::Item(item.clone()));
+
+            Annotatable::Item(P(Item {
+                attrs: item.attrs.clone(),
+                ..(*quote_item!(cx, enum Foo2 { Bar2, Baz2 }).unwrap()).clone()
+            }))
+        },
+        _ => item,
+    }
+}
+
 pub fn foo() {}
+pub fn main() { }

src/test/run-pass-fulldeps/macro-crate.rs

@@ -33,6 +33,14 @@ impl Qux for i32 {
 
 impl Qux for u8 {}
 
+#[wrap]
+struct Bar;
+
+impl Bar
+{
+    fn new() -> Self { Bar }
+}
+
 pub fn main() {
     assert_eq!(1, make_a_1!());
     assert_eq!(2, exported_macro!());