This prefix disables name mangling in LLVM, but other codegen backends don't support this. We also don't guarantee that it will remain working. As discussed in https://rust-lang.zulipchat.com/#narrow/stream/238009-t-compiler.2Fmeetings/topic/.5Bweekly.5D.202024-09-19/near/471499857 we may choose to add a #[link_name("foo", verbatim)] flag or similar to get the same effect and then deny all usage of \u{1} over an edition boundary.