История изменений

Видимо, вот это:

https://github.com/rust-lang/rust/issues/53491#issuecomment-414504548

LLVM defines uninitialized data as Poison, which is distinct from «some arbitrary but valid bit pattern.» Branching based on a Poison value or using it to compute an address which is then dereferenced is UB.

Хм, хотя в примере никакого использования `x` нет.

Видимо, вот это:

https://github.com/rust-lang/rust/issues/53491#issuecomment-414504548

LLVM defines uninitialized data as Poison, which is distinct from «some arbitrary but valid bit pattern.» Branching based on a Poison value or using it to compute an address which is then dereferenced is UB.