There is seem [LLVM port for z/OS](https://www.ibm.com/developerworks/community/blogs/5894415f-be62-4bc0-81c5-3956e82276f3/entry/XL_C_C_V2_3_1_for_z_OS_V2_3_web_deliverable_is_available_today_on_March_29_2019?lang=en), and Rust supports s390, so should be possible to support the z/OS system as a full-featured target. See example of the [Swift language porting](https://forums.swift.org/t/z-os-swift-and-encodings/5897) by IBM.
There is seem LLVM port for z/OS, and Rust supports s390, so should be possible to support the z/OS system as a full-featured target.
See example of the Swift language porting by IBM.