Spurious "value assigned is never read" warnings during macro expansion

[bstrie]

The following program uses a macro (mlet!) to abstract away the declaration of several variables:

use task::spawn;

fn main() {
    macro_rules! mlet(
        ($($var:ident),+ = $val:expr) => (
            let $($var = $val),+;
        );
    );

    let (port, chan) = pipes::stream();
    let chan = pipes::SharedChan(chan);

    mlet!(s1, s2, s3 = chan.clone());
    // The above line expands to the following, which doesn't throw any
    // warnings when it's inserted manually rather than generated by the macro:
    //let s1 = chan.clone(), s2 = chan.clone(), s3 = chan.clone();
    do spawn { s1.send( (|a: int| a*2)(10) ) }
    do spawn { s2.send( (|a: int| a*2)(20) ) }
    do spawn { s3.send( (|a: int, b: int| a+b)(30, 40) ) }

    mlet!(x, y, z = port.recv());
    // The above line expands to the following, which doesn't throw any
    // warnings when it's inserted manually rather than generated by the macro:
    //let x = port.recv(), y = port.recv(), z = port.recv();
    io::println(fmt!("%d + %d + %d = %d", x, y, z, x+y+z));
}

When compiling this code, I receive this output:

macro3.rs:6:19: 6:24 warning: value assigned to `s2` is never read
macro3.rs:6             let $($var = $val),+;
                               ^~~~~
macro3.rs:4:4: 8:6 note: in expansion of mlet!
macro3.rs:13:4: 13:37 note: expansion site
macro3.rs:6:19: 6:24 warning: value assigned to `s3` is never read
macro3.rs:6             let $($var = $val),+;
                               ^~~~~
macro3.rs:4:4: 8:6 note: in expansion of mlet!
macro3.rs:13:4: 13:37 note: expansion site
macro3.rs:6:19: 6:24 warning: value assigned to `y` is never read
macro3.rs:6             let $($var = $val),+;
                               ^~~~~
macro3.rs:4:4: 8:6 note: in expansion of mlet!
macro3.rs:21:4: 21:33 note: expansion site
macro3.rs:6:19: 6:24 warning: value assigned to `z` is never read
macro3.rs:6             let $($var = $val),+;
                               ^~~~~
macro3.rs:4:4: 8:6 note: in expansion of mlet!
macro3.rs:21:4: 21:33 note: expansion site

These warnings only appear when using the macros to generate the code. If you run the macro expansion pass (rustc --pretty expanded) you see that the mlet! lines turn into the following:

let s1 = chan.clone(), s2 = chan.clone(), s3 = chan.clone();
let x = port.recv(), y = port.recv(), z = port.recv();

Manually replacing the macro lines with these expanded lines does not cause the warnings to be printed upon compilation. Fix the compiler so that these spurious warnings do not occur during macro expansion.

[catamorphism]

Seems non-critical for 0.7. Nominating for milestone 5 (production-ready).

[graydon]

accepted for production ready

[huonw]

Multiple let declaration like in the original testcase is deprecated, and the following seems to pass without issue, but it presumably exercises different code paths. I'm leaning towards closing this as out-of-date.

use std::comm;
fn main() {
    macro_rules! mlet(
        ($($var:ident = $val:expr),*) => (
            let ($($var),*) = ($($val),*);
        );
    );

    let (port, chan) = comm::stream();
    let chan = comm::SharedChan::new(chan);

    mlet!(s1 = chan.clone(), s2 = chan.clone(), s3 = chan.clone());
    do spawn { s1.send( (|a: int| a*2)(10) ) }
    do spawn { s2.send( (|a: int| a*2)(20) ) }
    do spawn { s3.send( (|a: int, b: int| a+b)(30, 40) ) }

    mlet!(x = port.recv(), y = port.recv(), z = port.recv());
    println(fmt!("%d + %d + %d = %d", x, y, z, x + y + z));
}

[bstrie]

In the interest of preserving the original intent of the macro, I've done some horrendous things to recreate it given our new declaration rules:

use std::comm;
fn main() {
    macro_rules! mlet(
        ($($var:ident),* = $val:expr) => (
            let ($($var),*) = ($({let $var=0; let _ = $var; $val}),*);
        );
    );

    let (port, chan) = comm::stream();
    let chan = comm::SharedChan::new(chan);

    mlet!(s1, s2, s3 = chan.clone());
    do spawn { s1.send( (|a: int| a*2)(10) ) }
    do spawn { s2.send( (|a: int| a*2)(20) ) }
    do spawn { s3.send( (|a: int, b: int| a+b)(30, 40) ) }

    mlet!(x, y, z = port.recv());
    println(fmt!("%d + %d + %d = %d", x, y, z, x + y + z));
}

This does indeed no longer exhibit the bug, so I'm closing it.