Memory corruption when matching on an Option of a vector slice

[catamorphism]

use std::io;

fn main() {
// This is ok
    match &[(~5,~7)] {
        ps => {
           let (ref y, _) = ps[0];
           io::println(fmt!("1. y = %d", **y));
           assert!(**y == 5);
        }
    }

// This is not entirely ok
    match Some(&[(~5,)]) {
        Some(ps) => {
           let (ref y,) = ps[0];
           io::println(fmt!("2. y = %d", **y));
           if **y != 5 { io::println("sadness"); }
        }
        None => ()
    }

// This is not ok
    match Some(&[(~5,~7)]) {
        Some(ps) => {
           let (ref y, ref z) = ps[0];
           io::println(fmt!("3. y = %d z = %d", **y, **z));
           assert!(**y == 5);
        }
        None => ()
    }
}

The output:

1. y = 5
2. y = 5
sadness
3. y = 2 z = 3458764513820540928
task <unnamed> failed at 'assertion failed: **y == 5', /Users/tjc/rust/src/test/run-pass/spawn-env-2.rs:28

In the second match, the first print statement prints out the right answer, but reading **y for the second time yields garbage (perhaps y is wrongly getting moved out of here even though it's a ref binding?). The third match just prints out garbage.

[bblum]

there must have been some heroic test-case minimization behind the scenes of this issue

[catamorphism]

@bblum Indeed, at first I thought the problem was process spawning... it's actually the arguments to process spawning that go wrong, even before any process gets spawned!

[catamorphism]

I thought this might be related to #5917, but it's not; the test still fails when compiling from a branch that includes the fix for #5917.

[emberian]

Modernized testcase:

fn main() {
// This is ok
    match &[(~5,~7)] {
        ps => {
           let (ref y, _) = ps[0];
           println!("1. y = {}", **y);
           assert!(**y == 5);
        }
    }

// This is not entirely ok
    match Some(&[(~5,)]) {
        Some(ps) => {
           let (ref y,) = ps[0];
           println!("2. y = {}", **y);
           if **y != 5 { println("sadness"); }
        }
        None => ()
    }

// This is not ok
    match Some(&[(~5,~7)]) {
        Some(ps) => {
           let (ref y, ref z) = ps[0];
           println!("3. y = {} z = {}", **y, **z);
           assert!(**y == 5);
        }
        None => ()
    }
}

New output:

1. y = 5
2. y = 140097404013712
sadness
3. y = 0 z = 140097404013744
task '<main>' failed at 'assertion failed: **y == 5', foo.rs:26

[alexcrichton]

This appears to have been fixed, flagging as needstest.