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The recent change seems to make it possible to end up with one of the block fields being a null pointer. A patch that exposes this behavior is available at https://gist.github.com/2925753
Here's a backtrace:
[host-5-154:build eholk]$ gdb --args x86_64-apple-darwin/stage1/bin/rustc --cfg stage1 -O --target=x86_64-apple-darwin -o x86_64-apple-darwin/stage1/lib/rustc/x86_64-apple-darwin/lib/libcore.dylib /Users/eholk/Documents/projects/mozilla/rust/src/libcore/core.rc
GNU gdb 6.3.50-20050815 (Apple version gdb-1705) (Fri Jul 1 10:50:06 UTC 2011)
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This GDB was configured as "x86_64-apple-darwin"...Reading symbols for shared libraries .. done
Starting program: /Users/eholk/Documents/projects/mozilla/rust/build/x86_64-apple-darwin/stage1/bin/rustc --cfg stage1 -O --target=x86_64-apple-darwin -o x86_64-apple-darwin/stage1/lib/rustc/x86_64-apple-darwin/lib/libcore.dylib /Users/eholk/Documents/projects/mozilla/rust/src/libcore/core.rc
Reading symbols for shared libraries .....+................................................................. done
Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x00000000000000a8
[Switching to process 56322 thread 0x1e03]
0x0000000100cbd9c3 in middle::trans::base::bcx_icx::meth2293::insn_ctxt::_3e984ab9a2e9bf70::_02 ()
#0 0x0000000100cbd9c3 in middle::trans::base::bcx_icx::meth2293::insn_ctxt::_3e984ab9a2e9bf70::_02 ()
#1 0x000000010107a1ae in str2622 ()
#2 0x0000000100cdf5b2 in middle::trans::base::move_val::_dd8bbd759ed35a3d::_02 ()
#3 0x0000000100ce72b1 in middle::trans::base::trans_assign_op::_dcbae8ce5868daa8::_02 ()
#4 0x0000000100d1d1fb in middle::trans::base::trans_expr::unrooted::_e05f72d83f6e492c::_02 ()
#5 0x0000000100ce3e51 in middle::trans::base::trans_expr::_e05f72d83f6e492c::_02 ()
#6 0x0000000100d23b65 in middle::trans::base::trans_stmt::_17322c3f90f22da6::_02 ()
#7 0x0000000100cee351 in middle::trans::base::trans_block::_e3aa2aeff1a48c5c::_02 ()
#8 0x0000000100ceec75 in middle::trans::base::trans_while::_9effb9d55ffc7be5::_02 ()
#9 0x0000000100d1cda6 in middle::trans::base::trans_expr::unrooted::_e05f72d83f6e492c::_02 ()
#10 0x0000000101029804 in __morestack ()
Previous frame inner to this frame (gdb could not unwind past this frame)
This branch should have exactly the code to make this happen on a Mac: https://github.com/eholk/rust/commits/issue-2548
See #2633 for the root cause of this problem. There are at least two possible easy workarounds, though: change move_val not to take an lval_result (it doesn't actually use the bcx field in its lval_result argument), and just take the kind and val as separate arguments; or make move_val's block argument pass-by-copy. I'm implementing one of these. Or if you get impatient, @eholk , you should be able to do the same in your local branch :-)
Thanks for tracking this down!
It was a fun one (though it turned out to not really be about classes at all!)
@eholk If you rebase your branch against incoming, a14df27 should fix the segfault. I'm closing this for now, but reopen if it doesn't work.
That seems to fix the problem, but since the segfault is in rustc, we are unfortunately going to need a snapshot.