iOS implements TCP/IP in userspace and most operating systems have drivers at least partially implemented in userspace. Many of the secondary processors in a computer run a microkernel with isolated components. Most smartphones have *at least* one L4 implementation in them.
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You talk about microkernels / isolated drivers and memory safe languages as if they're not already been broadly used in the wild. Most code is written in memory safe languages these days. Systems code is increasingly moving to them too. Kernels and drivers are *mostly* not yet.
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Java is a traditional example of a type / memory safe language. It has a very poor quality type system, like C, and it's not good at safety beyond memory safety. It can't do automatic integer overflow checking either, but at least it doesn't lead right to memory corruption bugs.
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Most languages are memory safe. That's not a unique thing about Rust. The unique aspect is that it's a memory safe low level language encoding ownership and lifetime concepts into the type system rather than relying on garbage collection or pervasive automatic reference counting.
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Swift takes the approach of pervasive automatic (atomic) reference counting. Android heavily uses Java for much of the systems code on top of the kernel and drivers. iOS is increasingly using Swift for that. Most OS services above the kernel don't need a low-level language.
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So, Android's web browser is written in Java? That's news to me!
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The browser is an application, not part of the OS code that I was talking about. By the way, Mozilla's interest in Rust (and why they adopted it as a project) was heavily based on their needs writing a web browser, since using C and C++ is a massive security and robustness issue.
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Servo is their reimplementation of a web rendering engine, but they also use a lot of the components in Firefox. I also don't see why you're continuing to misrepresent and spin what I'm saying. I specifically said that Android heavily uses Java for systems programming.
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I explicitly did not say that they solely use Java or that they don't use a lot of C and C++ alongside it. A huge portion of the OS implementation above the kernel and driver layer is done in Java, including most of what actually makes it Android rather than a different OS.
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Java obviously isn't Rust. It's not a good fit for a lot of the lower level portions. They do use it to implement a huge amount of the underlying OS below the application layer though.