I couldn't quite remember either, and brson told me that https://github.com/rust-lang/rust/issues/11710 was probably it. Can't seem to find where it was actually finally disabled though.
"This is a known valgrind false positive and Rust already has a lot of suppressions for it. It's reported upstream on the LLVM bug tracker but is not really an LLVM issue, just a limitation of valgrind. LLVM is allowed to generate undefined reads if it can not change the program behavior."
[...]
"Just for the record, the undefined read doesn't change the behaviour because it's just a check whether free should not be called, i.e. either it jumps over the free call right away, or it performs the second check."
