These languages do have very theoretical foundations even though they might not be readily apparent. ML has a rich theory much of which is applicable to both Rust and Swift's type systems.
This is even more applicable to Rust where the system that deals with lifetimes and ownership are based on ideas that evolved out of linear/affine type systems that can track resource consumption.
Funnily enough linear type systems have a direct connection to linear logic, and via the Curry-Howard-Lambek there is another correspondence to symmetric/closed monoidial categories. Even though sometimes it appears like there is no theory behind these languages, Rust has definitely been aggressively importing some cutting edge thinking from programming languages.
This is even more applicable to Rust where the system that deals with lifetimes and ownership are based on ideas that evolved out of linear/affine type systems that can track resource consumption.
Funnily enough linear type systems have a direct connection to linear logic, and via the Curry-Howard-Lambek there is another correspondence to symmetric/closed monoidial categories. Even though sometimes it appears like there is no theory behind these languages, Rust has definitely been aggressively importing some cutting edge thinking from programming languages.