I should probably preface this comment by mentioning that I don't think there is anything new in it for either of us. Nor do I think we actually disagree on any of the facts. My earlier comment, and this one, was really just a response predicated on what I think the colloquial meaning of "memory safety" is, and to whether a practical language can be "truly memory safe"…which of course depends on what you see a programming language as being.

Memory safety is, as you have already mentioned, not black and white: I wouldn't even put it on an axis, because that suggests the scale is one-dimensional, and I don't even think it is practical to discuss it in that context. I prefer to categorize languages (for a definition of "language") in a couple of rough groups where most of them hang out.

In the first group is C and C++ as you're typically used to it, where pretty much every operation can do something unsafe and there's really no safe subset of the language, much less safety by default.

The second group is the "safe by default" languages like Rust or Python or Java, were you can write functional programs in the entirely safe subset (which is usually the default). This is where things get more complicated, though, because what the unsafe bits look like differ. Some give you language-level constructs to do unsafe things, such as Rust (with unsafe) and Java (with sun.misc.Unsafe or whatever). I think CPython technically also falls here because of some weird implementation choices where you can corrupt memory, but it's really more of being in the other category where you can do unsafe things via FFI and external interfaces. That's kind of where most Lua implementations live, or nodejs stuff.

Then you have the things which (usually intentionally) do not give you any of these things. That's JavaScript or WebAssembly in a browser. The final stop in this line is where you start placing significant limits to what the language itself can do, such as eBPF running in the kernel, or domain-specific parsers like WUFFS.

I've been pretty sloppy with what I call a "language" here, because you can always take a programming language and slap memory safety on it: though not trivial, you can sandbox it, pick some subsets of it, put in hardware, etc. (FWIW CHERI doesn't actually make C/C++ completely memory safe, it just helps.) And going the other way is pretty easy, you just add features to let programs mess with the execution environment.

I get that the comment that you're replying to is trying to well acktually you and I agree with the rest of your response, but the takeaway I have here is "you [the commenter you were responding to originally are coming in with a definition of memory safety, yes in this context Rust does have these escape hatches and this is what they do, but in vernacular it is safe because this is how we typically evaluate languages for this sort of thing". Which, again, is like 90% of what you wrote already, I just think that it is probably worth bringing up that there is a pretty common environment for a popular language that actually takes things a step further than this, with whatever tradeoffs that entails. Not really a disagreement, just a "hey I think this is worth mentioning".