Steve wasn't talking about unsafe. He was talking about being able to mint your own non-enum types with user defined niches.

Rust provides for example NonZeroU8 which is an 8-bit unsigned integer that's never zero, leaving it with 255 possible values and a convenient niche. You cannot make one of these yourself directly, because the mechanism used by Rust itself is a deliberately perma-unstable compiler-only proc macro which says "Hey compiler, I promise I only ever use bit patterns 0x01 through 0xFF inclusive".

Today you can either - hide a NonZero type inside your type and use that to get the niche, or, use an enum itself which automatically knows ever pattern it didn't use is a niche. In the future a hypothetical "Pattern Types" feature would let you make such types yourself as easily as Rust does

Personally I would like to make a Balanced set of types, like BalanacedI8 (the 8-bit integers except the most negative, so -127 to +127 inclusive) because I think lots of people have a use for types like i8 or i32 but don't need their unbalanaced most-negative value and could re-purpose it this way. And you can make such types... indeed I have... but it's only really practical in unstable Rust.