That's actually the point. Many of these additions can be phrased as unifying existing features and allowing them to be used in previously unusable ways and contexts. There's basically no real increase in user-perceived complexity. The Rust editions system is a key enabler of this, and C++ has nothing comparable.
Rust editions don't cover all use cases that one can think of regarding language evolution, and requires full access to source code.
What do you mean? Editions don't require full access to source code. Rust in general relies heavily on having access to source code, but that has nothing to do with how editions work
Could you elaborate more on this? It's not obvious to me right now why (for example) Crate A using the 2024 edition and Crate B using the 2015 edition would require both full access to both crates' source beyond the standard lack of a stable ABI.
See the Rust documentation on what editions are allowed to change, and the advanced migration guide on examples regarding manual code migration.
Not so much what has happened thus far, rather the limitations imposed in what is possible to actually break across editions.
To be fair, Rust tooling does tend toward build-from-source. But this is for completely different reasons than the edition system: if you had a way to build a crate and then feed the binary into builds by future compilers, it would require zero additional work to link it into a crate using a different edition.