Broken line debug info generated for `for` loops

[vadimcn]

extern crate test;
fn main() {
    let mut sum = 0;
    for i in range(0, 10i) { // `s<CR>` lands on this line twice
        sum += i;
    }
    test::black_box(sum);
}

Compile, with -g, step line-by-line in gdb.
You'll notice that in the beginning of each loop iteration it takes two s<CR> to advance to the next line. The culprit seems to be a couple of IR instructions in the loop (which apparently store the value into i), that do not have debug line metadata attached to them.

[alexcrichton]

cc @michaelwoerister

[michaelwoerister]

I've been working on stepping improvements here: #19737
I closed the PR because I wanted to extend it more but it turned out to cause an incredible amount of churn once I started to handle loads and stores correctly, so this is on hold for now. But I have a version that is a clear improvement over the current situation and I hope to get it merged before the alpha.

[michaelwoerister]

#20642 should improve the situation here.

[michaelwoerister]

Update: Since #20642, this only happens when initially entering the loop. In subsequent iterations it works as expected. I'll take a closer look when I find the time.

[steveklabnik]

Triage: I think this might be fixed?

Updated code:

#![feature(test)]

extern crate test;
fn main() {
    let mut sum = 0;
    for i in 0..10 { // `s<CR>` lands on this line twice
        sum += i;
    }
    test::black_box(sum);
}

gdb session:

$ gdb hello 
GNU gdb (Debian 7.7.1+dfsg-5) 7.7.1
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License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from hello...done.
warning: Missing auto-load scripts referenced in section .debug_gdb_scripts
of file /home/steve/tmp/hello
Use `info auto-load python-scripts [REGEXP]' to list them.
(gdb) break _ZN4main20h5165c54df1b2632dfaaE
Breakpoint 1 at 0x4834: file hello.rs, line 5.
(gdb) s
The program is not being run.
(gdb) run
Starting program: /home/steve/tmp/hello 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, hello::main () at hello.rs:5
5       let mut sum = 0;
(gdb) s
6       for i in 0..10 { // `s<CR>` lands on this line twice
(gdb) s
hello::iter::I.IntoIterator::into_iter (self=...) at ../src/libcore/iter.rs:2638
2638    ../src/libcore/iter.rs: No such file or directory.
(gdb) s
hello::iter::ops::Range<A>.Iterator::next (self=0x7fffffffddb0)
    at ../src/libcore/iter.rs:4470
4470    in ../src/libcore/iter.rs
(gdb) s
hello::cmp::impls::i32.PartialOrd::lt (self=0x7fffffffddb0, other=0x7fffffffddb4)
    at ../src/libcore/cmp.rs:470
470 ../src/libcore/cmp.rs: No such file or directory.
(gdb) s
hello::iter::ops::Range<A>.Iterator::next (self=0x7fffffffddb0)
    at ../src/libcore/iter.rs:4471
4471    ../src/libcore/iter.rs: No such file or directory.

It looks like just once for me. Since I'm not aware of any work done in this area, I'm not going to give it a close, though.