#[non_exhaustive] attribute constraints how outside code behaves in that struct or enum variant, specifically it disallows:

implicit constructors

#[non_exhaustive]
struct Point {
	x: f32,
	y: f32
}
 
// outside the crate, this is not allowed
Point {x: 10.0, y: 20.0};

non-exhaustive pattern matches

enum Point {
	2D,
	3D
}
 
// outside the crate, this is not allowed
match point {
	2D => print!("two-dimensional"),
	3D => print!("three-dimensional"),
	// would compile with `_ => {},`
}

Useful when you suspect you will modify a particular type in the future.

See also

OCP - software should be open for extension and closed for modification — #[non_exhaustive] is OCP applied to Rust types: the type is open for extension (you can add fields/variants in future versions) because callers are closed against assuming exhaustiveness
All Observable Behaviours of an API Will Be Depended On by Someone — without #[non_exhaustive], adding a field or variant is a breaking change because callers depend on the exhaustive list; the attribute is a direct defence against Hyrum’s Law
Principle of Least Astonishment — #[non_exhaustive] makes future additions unsurprising: callers who pattern match are forced to handle the _ case, so new variants never break their code silently

References

J. Gjengset, Rust for Rustaceans: Idiomatic Programming for Experienced Developers. No Starch Press, 2021.