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There are at least three Java runtimes written in Java, Jikes RVM, MaximeVM, and GraalVM.

At least two of which use an unsafe dialect of Java for significant parts of the runtime, which I'm pretty sure you know well (maybe not Graal, but if not it's because it's bootstrapping on top of existing unsafe code).

Easy localizable via grep, and not full of UB and memory corruption issues, which is what the 70% of unsafety issues due to memory corruption on C, C++ and Objective-C relate to.

At some level of the stack some Assembly or compiler intrisics are needed, not at every line of code.

Jikes is the one I'm most familiar with and people working on its runtime absolutely suffered from UB and memory corruption issues... obviously not throughout the whole standard library but that's not the case for other JVMs either. In fact the Jikes people found it nicer to work in Rust than in Java on components like the garbage collector, because it was a better fit for working safely with this kind of code and they didn't have to write in a restricted subset of the language to avoid triggering the GC.

Since when Jikes uses Rust?

Also bootstrapting a language always requiring using its subset for low level layers, apparently not an issue that many parts of C, C++ cannot be implemented only with what ISO provides on the standard.

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