Invalid write with by-copy generic tuple

[brson]

fn fst<T, U>(+pair: (T, U)) -> T {                                                                                                                                             
    let (t, _) = pair;                                                                                                                                                        
    ret t;                                                                                                                                                                    
}                                                                                                                                                                             

fn main() {                                                                                                                                                                   
    fst((10, 20));                                                                                                                                                            
}        

==8450== Thread 3:
==8450== Invalid write of size 4
==8450==    at 0x4C2AF5A: memset (mc_replace_strmem.c:738)
==8450==    by 0x401019: fst::_7abc65a4cbab8e1d (in /home/brian/dev/rust/build/x86_64-unknown-linux-gnu/test/run-pass/test.stage1-x86_64-unknown-linux-gnu)
==8450==    by 0x401154: _rust_main (in /home/brian/dev/rust/build/x86_64-unknown-linux-gnu/test/run-pass/test.stage1-x86_64-unknown-linux-gnu)
==8450==  Address 0x6470700 is 0 bytes after a block of size 21,280 alloc'd
==8450==    at 0x4C28F9F: malloc (vg_replace_malloc.c:236)
==8450==    by 0x50C5AFA: memory_region::malloc(unsigned long, char const*, bool) (memory_region.cpp:104)
==8450==    by 0x50B38F4: new_stk(rust_scheduler*, rust_task*, unsigned long) (rust_task.cpp:193)
==8450==    by 0x50B4752: rust_task::rust_task(rust_scheduler*, rust_task_list*, rust_task*, char const*) (rust_task.cpp:274)
==8450==    by 0x50B1BA8: rust_scheduler::create_task(rust_task*, char const*) (rust_scheduler.cpp:339)
==8450==    by 0x50BA078: rust_kernel::create_task(rust_task*, char const*) (rust_kernel.cpp:157)
==8450==    by 0x50AE267: rust_start (rust.cpp:88)
==8450==    by 0x553230C: (below main) (libc-start.c:226)
==8450==
==8450== Invalid write of size 4
==8450==    at 0x4C2AF4F: memset (mc_replace_strmem.c:738)
==8450==    by 0x401019: fst::_7abc65a4cbab8e1d (in /home/brian/dev/rust/build/x86_64-unknown-linux-gnu/test/run-pass/test.stage1-x86_64-unknown-linux-gnu)
==8450==    by 0x401154: _rust_main (in /home/brian/dev/rust/build/x86_64-unknown-linux-gnu/test/run-pass/test.stage1-x86_64-unknown-linux-gnu)
==8450==  Address 0x6470704 is 4 bytes after a block of size 21,280 alloc'd
==8450==    at 0x4C28F9F: malloc (vg_replace_malloc.c:236)
==8450==    by 0x50C5AFA: memory_region::malloc(unsigned long, char const*, bool) (memory_region.cpp:104)
==8450==    by 0x50B38F4: new_stk(rust_scheduler*, rust_task*, unsigned long) (rust_task.cpp:193)
==8450==    by 0x50B4752: rust_task::rust_task(rust_scheduler*, rust_task_list*, rust_task*, char const*) (rust_task.cpp:274)
==8450==    by 0x50B1BA8: rust_scheduler::create_task(rust_task*, char const*) (rust_scheduler.cpp:339)
==8450==    by 0x50BA078: rust_kernel::create_task(rust_task*, char const*) (rust_kernel.cpp:157)
==8450==    by 0x50AE267: rust_start (rust.cpp:88)
==8450==    by 0x553230C: (below main) (libc-start.c:226)

[brson]

Only happens with optimizations.

[marijnh]

Why on earth did you decide to use a by-copy argument here? (Which doesn't really relate to the bug, but I'm really curious.)

[marijnh]

Here's a version that still segfaults without using any rarely-used features (tuples, by-copy). The problem seems to be assigning a structural type with parameters in it in a last-use position (so that it turns into a move). I'm going to dig a bit further.

fn foo<T: copy, U: copy>(pair: {a: T, b: U}) -> T {
    let x = pair;
    let w = x; // This is where it segfaults
    // log(error, x); // If you add this, x above is not a last use, and the problem goes away
    ret w.a;
}

fn main() {
    foo({a: 10u8, b: 20u8});
}

[marijnh]

This is an instance of #843. Closing this issue an continuing discussion there.