Macro is not working very well

[recrack]

zz.rs

macro_rules! twice(
    ($a:expr) => ($a + $a);
)

macro_rules! twice_one(
    ($b:expr) => (1 + twice!($b) + 1);
)

fn main() {
    twice_one!(10);
}

result

#rustc --pretty expanded zz.rs

priv use std::prelude::*;
priv extern mod std;


mod __std_macros {
    #[macro_escape];
    #[doc(hidden)];
    priv use std::prelude::*;
    #[cfg(not(debug))]
    #[macro_escape]
    mod debug_macro {
        priv use std::prelude::*;
    }
}
fn main() { 1 + 10 + 10 + 1; }

zz-1.rs

macro_rules! twice(
    ($a:expr) => ($a + $a);
)

macro_rules! twice_one(
    ($b:expr) => (twice!($b) + 1);
)

fn main() {
    twice_one!(10);
}

result

#rustc --pretty expanded zz-1.rs
priv use std::prelude::*;
priv extern mod std;


mod __std_macros {
    #[macro_escape];
    #[doc(hidden)];
    priv use std::prelude::*;
    #[cfg(not(debug))]
    #[macro_escape]
    mod debug_macro {
        priv use std::prelude::*;
    }
}
fn main() { 10 + 10; }

result is different

[huonw]

Wow, this one appears to be the combination of two separate issues:

1. if a macro invocation is allowed to expand to a item/statement (rather than an expression), it is assumed to do so (Syntax extensions (and macros) that start a line want to be a whole statement #5941)
2. a macro expanding to multiple items is silently truncated at the first one (Macros attempting to expand to multiple items silently only expand to the first one #8012)

#5921 means that, in twice!($b) + 1, the twice!($b) gets treated as a statement/item macro, and then #8012 means that the trailing + 1 gets dropped (and so the behaviour is incorrect, and no error is reported).

This can be fixed by forcing the twice!($b) to be evaluated in an expression context (that is what is happening with the 1 + twice!($b) form), which can be done with some parens:

macro_rules! twice_one(
    ($b:expr) => ( (twice!($b) + 1) );
)

(Closing as a dup of those two issues.)