[stewoconnor]

(Unison Developer here), yes we've done two such migrations this year. It has typically meant that we have an "are you ready to upgrade" message when you start up and the migration took less than a minute on my pretty large codebase. It's not a big deal

[msla]

It's a solved problem technically, certainly, but assuming all of the relevant source code will always be available ignores some social and legal issues.

[stewoconnor]

The relevant sourcecode is not available, we don't store any source code. All the relevant dependences MUST be availble in AST form though.

I don't know what the social and legal issues might possibly be, though I might be missing somehting, what do you have in mind there?

[msla]

One issue I was thinking of was companies that distribute code in binary form only, not ASTs or anything which could be used to steal their thoughts. The other issue was in reverse, however: A binary-only package is unmaintained and lists as dependencies hashes that no longer exist because they're the hashed versions of ASTs that, for one reason or another, the compiler won't generate, even if the source still exists. Versioning and archiving would help this case, at least.

[jupp0r]

It's a common approach in language design to require this though. Rust has made very similar design choices due to its (current) lack of stable ABIs. IIRC some hash of the compiler version is included in built libraries and prevents accidental linking. You need to rebuild the world for every toolchain upgrade.