如何在Rust过程宏中创建多个项目？

Question

I am trying to create a macro that, when used like this:

foo!()

expands to two items, a static item and a function. Something like this:

static x: uint = 5;

fn bar() -> uint { x + 1 }

It has been pointed out to me that MacResult 's make_items method supports this, so I just need to create a type that implements it properly. I have done so with this:

struct MacItems {
    items: Vec<::std::gc::Gc<Item>>,
}

impl MacResult for MacItems {
    fn make_def(&self) -> Option<::syntax::ext::base::MacroDef> { None }
    fn make_expr(&self) -> Option<::std::gc::Gc<ast::Expr>> { None }
    fn make_pat(&self) -> Option<::std::gc::Gc<ast::Pat>> { None }
    fn make_stmt(&self) -> Option<::std::gc::Gc<ast::Stmt>> { None }

    fn make_items(&self) -> Option<::syntax::util::small_vector::SmallVector<::std::gc::Gc<Item>>> {
        Some(::syntax::util::small_vector::SmallVector::many(self.items.clone()))
    }
}

I have attempted to use the quote_item! macro, but this macro apparently moves the ExtCtxt , making it not possible to use it twice in a row. Here's my code:

#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
    reg.register_macro("foo", expand);
}

fn expand(cx: &mut ExtCtxt, sp: codemap::Span, _: &[ast::TokenTree]) -> Box<MacResult> {
    let mut v = vec!();

    v.push( quote_item!(cx, static x: uint = 5;).unwrap() );
    v.push( quote_item!(cx, fn bar() -> uint { x + 1 }).unwrap() );

    box MacItems { items: v } as Box<MacResult>
}

And here's the error:

test.rs:37:13: 37:57 error: use of moved value: `cx`
test.rs:37     v.push( quote_item!(cx, fn bar() -> uint { x + 1 }).unwrap() );
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: in expansion of quote_item!
test.rs:37:13: 37:57 note: expansion site
test.rs:36:13: 36:50 note: `cx` moved here because it has type `&mut syntax::ext::base::ExtCtxt<'_>`, which is moved by default (use `ref` to override)
test.rs:36     v.push( quote_item!(cx, static x: uint = 5;).unwrap() );
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: in expansion of quote_item!
test.rs:36:13: 36:50 note: expansion site
error: aborting due to previous error

How can I achieve this? I attempted to use AstBuilder::item_static to create the static item, but I cannot figure out which variant of Ty_ corresponds to uint here. (Also this is just a toy example, my real static declaration is &'static str , so I need to build that item instead).

Am I doing this completely wrong? How can this be achieved?

Answer 1

You need to manually reborrow the cx to a temporary &mut ExtCtxt . Often the compiler can insert the reborrows automatically, but the quote_* macros do not expand to something ameniable to this.

That is,

quote_item!(&mut *cx, ...)