Hygiene broken in multi-statement macros

[durka]

Multi-statement macros (macros that expand to multiple statements) are hard to use because hygiene runs separately on each line, meaning it's very easy to "lose" identifiers.

Here are some examples. The macro is trying to declare a variable in the scope where it was called, but it takes multiple statements to construct.

/*
// no good: $n declared in inner scope
macro_rules! m {
    ($n:ident) => ({
        let a = 1;
        let $n = a;
    })
}
*/

/*
// no good: hygiene throws away "a"
macro_rules! m {
    ($n:ident) => (
        let a = 1;
        let $n = a;
    )
}
*/

// workaround
macro_rules! m {
    ($n:ident) => (
        let $n = {
            let a = 1;
            a
        };
    )
}

fn main() {
    m!(b);
    println!("{}", b);
}

This is a long-standing bug, not a regression. I previously reported this here and here (can likely be closed) but it wasn't very visible. ping @Sgeo @chris-chambers

[Sgeo]

I am experimentally playing with the idea of using hygiene to hide one object, then exposing another object that refers to the hidden object. The macro I have is

macro_rules! take_mut {
    ($mut_ref:expr => $into:pat) => {
        let mut scope = Scope::new();
        let $into = Hole::new(&mut scope, $mut_ref);
    }
}

Trying to use this fails with

<anon>:42:36: 42:41 error: unresolved name `scope` [E0425]
<anon>:42         let $into = Hole::new(&mut scope, $mut_ref);
                                             ^~~~~
<anon>:58:9: 58:46 note: in this expansion of take_mut! (defined in <anon>)
<anon>:42:36: 42:41 help: see the detailed explanation for E0425
<anon>:42:36: 42:41 error: unresolved name `scope` [E0425]
<anon>:42         let $into = Hole::new(&mut scope, $mut_ref);
                                             ^~~~~
<anon>:59:9: 59:46 note: in this expansion of take_mut! (defined in <anon>)
<anon>:42:36: 42:41 help: see the detailed explanation for E0425
error: aborting due to 2 previous errors
playpen: application terminated with error code 101

[durka]

For the record, you can write code that relies on this bug. This code

macro_rules! m {
    ($i:ident) => {
        let mut $i = 2;
        $i = $i + 1;
    }
}

fn no_macro() {
    let mut a = 1;
    let mut a = 2;
    a = a + 1;

    println!("{}", a);
}

fn yes_macro() {
    let mut a = 1;
    m!(a);

    println!("{}", a);
}

produces code that looks the same under pretty-expansion, but no_macro (which represents what the code would look like were this bug fixed) prints 3, while yes_macro prints 2.