-Zunpretty=expanded does not preserve hygiene

[klutzy]

rustc internally converts for val in iter { ... } to:

match &mut iter {
    i =>
    loop  { match i.next() { None => break , Some(val) => { ... } } }
}

where i is created by let local_ident = token::gensym_ident("i");, so i does not clash with other local variables.

However, pprust just prints it as plain i. So the following code:

let mut i: int = 0;
for _ in iter {
    i += 1;
}

is prettified by rustc as:

let mut i: int = 0;
match &mut iter {
    i =>
    loop  { match i.next() { None => break , Some(_) => { i += 1; } } }
}

which is not correct anymore.

[jbclements]

The problem here is generally that rustc --pretty doesn't respect hygiene. You can see this by expanding this program

#![feature(macro_rules)]

macro_rules! dont_capture {($v:expr) => ({let _x = 13; $v})}

fn main() -> (){
    let _x = 2;
    let y = dont_capture!(_x);
    println!("2 = {}",y);
}

using rustc --pretty expanded, which shows a program where the inner let should clearly capture the inner varref.

As far as I know, rustc --pretty doesn't claim to preserve semantics. I would argue that this is currently not a bug, or perhaps a change request.

[steveklabnik]

Triage: no changes I'm aware of.

[Mark-Simulacrum]

Loops are no longer expanded as suggested by this issue as far as I can tell. However, the macro example still doesn't work quite correctly in that dont_capture!(_x) is expanded to let _x = 13; _x, not the more-proper let _x_mangled = 13; _x. Updating issue title to be more accurate.

[steveklabnik]

Triage: no change

[jyn514]

Triage: no change. However, there is a new option -Zunpretty=expanded,hygiene, which does distinguish the two (although it's quite verbose):

macro_rules! dont_capture
    /*
    1348#0
    */ {
    ($v : expr) => ({ let _x = 13 ; $v })
}

fn main /* 691#0 */() -> () {
    let _x /* 1350#0 */ = 2;
    let y /* 1352#0 */ = { let _x /* 1350#3 */ = 13; _x /* 1350#0 */ };
...