The first is, annoyingly, a relatively common problem and solution when dealing with GC lifetimes in tests. Few interpreters/JITs want to generate extra instructions to null out stack slots or pre clobber registers to ensure something becomes collectible at a specific point. Eager nulling of a variable often gets removed by dead store elimination or even just from being a disconnected SSA node. I've written extra nested scopes or wrappers in Java to deal with this in tests.
If this is needed in tests it needs to be known how it works, it needs to work consistently, and it needs to be documented how it works. It can't be an ad-hoc deep investigation and random fix each time. The comment should then be just // ensure foo is no longer a GC root, see gc_roots.md
Don't know anything about the second one.