All runtime-allocated memory in a Rust program begins its life as uninitialized. In this state the value of the memory is an indeterminate pile of bits that may or may not even reflect a valid state for the type that is supposed to inhabit that location of memory. Attempting to interpret this memory as a value of any type will cause Undefined Behavior. Do Not Do This.
Rust provides mechanisms to work with uninitialized memory in checked (safe) and unchecked (unsafe) ways.