The struct parser does not like token trees, or maybe something weirder

[sorear]

I'm writing an FFI binding with a number of structs that have a fixed part and a variable part. I tried to factor it using a macro which takes the variable part as a sequence of token trees (to allow zero or more fields, possibly with #[cfg] modifiers), but I'm getting a strange parse error; golfed to:

macro_rules! bug1 {
    ($x:tt) => { struct _Bug2 { $x : i32, } }
}

bug1!(_abc);

fn main() {
}

(playpen: http://is.gd/i4195j)

If I change :tt to :ident the short example works, but that doesn't work for my real code which uses a repeated token tree.

Is this supposed to work with :tt ?

[huonw]

Dupe of #5846. Thanks for filing!