If you're modifying memory immediately before freeing it (i.e. after the last time you read it), don't you have to be extra super careful to do so in a way that the compiler won't optimize the operation into nothingness? (I don't program in compiled languages very much, so I don't know the details about this.)
The compiler can, and will, make copies of data behind the scenes. And not erase said copies.
What we really need is a keyword / modifier that says that when X passes out of scope no state related to X may be leaked. Ideally, that can be applied to a function / block as well as a variable.
(Or rather, not necessarily no state. Read "as little state as possible", preferably with modifiers that panic unless the compiler can ensure specific things.)
The C standard works at an abstraction level that makes it unsuitable for security applications, I would advocate for a new language here. It needs serious PL research with informarion flow reasoning, what we need is just a new kind of language much more machine aware (yes, more low-level) than C.
On the other hand, something as simple as a keyword marking a variable as "as secure as possible given hardware constraints (read: should wipe any temporary copies and the variable itself after it goes out of scope, should attempt to prevent it from being written to non-volatile storage, that sort of thing)" (sort of like how inline works), with compilers required to bail if the constraint cannot be done to the level specified, would be a massive step in the right direction.
