Macro using expr non-terminal can cause incorrect borrowck error

[lilyball]

A macro that uses an expr non-terminal that's used in both arms of an if statement, where the expression given to the macro expression involves borrowing *self as mutable, triggers an incorrect borrowck warning where it claims *self is being borrowed as mutable twice.

Code:

struct Foo {
    i: int
}

macro_rules! foo(
    ($code:expr) => (
        if true {
            $code
        } else {
            $code
        }
    )
)

impl Foo {
    pub fn foo(&mut self) {
        foo!({ self.bar() })
    }

    pub fn bar(&mut self) {
        self.i += 1;
    }
}

fn main() {
    let mut foo = Foo{i: 0};

    foo.foo();
}

Errors:

Untitled.rs:17:15: 17:20 error: cannot borrow `*self` as mutable more than once at a time
Untitled.rs:17         foo!({ self.bar() })
                              ^~~~~
Untitled.rs:5:0: 13:1 note: in expansion of foo!
Untitled.rs:17:8: 18:5 note: expansion site
Untitled.rs:17:15: 17:20 note: second borrow of `*self` as mutable occurs here
Untitled.rs:17         foo!({ self.bar() })
                              ^~~~~
Untitled.rs:5:0: 13:1 note: in expansion of foo!
Untitled.rs:17:8: 18:5 note: expansion site
Untitled.rs:17:15: 17:20 error: cannot borrow `*self` as mutable more than once at a time
Untitled.rs:17         foo!({ self.bar() })
                              ^~~~~
Untitled.rs:5:0: 13:1 note: in expansion of foo!
Untitled.rs:17:8: 18:5 note: expansion site
Untitled.rs:17:15: 17:20 note: second borrow of `*self` as mutable occurs here
Untitled.rs:17         foo!({ self.bar() })
                              ^~~~~
Untitled.rs:5:0: 13:1 note: in expansion of foo!
Untitled.rs:17:8: 18:5 note: expansion site
error: aborting due to 2 previous errors

[lilyball]

Same error happens if I use a block non-terminal instead of an expr one, as in

macro_rules! foo(
    ($code:block) => (
        if true $code else $code
    )
)

[huonw]

There's a few other open issues about this iirc, I can't find them while on my phone though. I think a work-around is assigning the $code to a temporary i.e. let code = $code; /* use code instead of $code */ (although this can't be used in all situations).

[lilyball]

@huonw That won't work for me, though. The real macro that hit this needs $code to see a variable that in one arm is &[u8] and in the other is ~[u8]. The real macro is closer to

macro_rules! foo(
    ($var:ident, $code:expr) => (
        if test($var) {
            let var = cond.raise($var.to_owned());
            $code
        } else {
            $code
        }
    )
)

[lilyball]

Looks like this might be the same as #7971, although that macro has both repetitions of the expression in the same block of code (and, in fact, on the same line).