bugfix: tweak upgrade to ignore aliases of old things

[mitchellwrosen]

Overview

This PR fixes an issue that caused upgrade to behave unintuitively.

The algorithm essentially works as follows (before and after this PR):

• When upgrading library dependency old to new, render out to a Unison file all dependents of all terms in lib.old (e.g. lib.old.thingy#oldthingy), but with the old segment swapped out for new, then parse and typecheck that.

  For example, consider the namespace:

  lib.old.thingy#oldthingy = ...
  lib.new.thingy#newthingy = ...
  myfoo = #oldthingy + 100
  

  On upgrade old new, we render, parse, and typecheck a Unison file like

  myfoo = lib.new.thingy + 100
  

The issue was to do with aliases. Prior to this PR, additional aliases of things in lib.old.* would essentially prevent upgrading as follows.

Consider the namespace:

lib.old.thingy#oldthingy = ...
lib.new.thingy#newthingy = ...
mythingy#oldthingy = ...
myfoo = #oldthingy + 100

The old implementation would render, parse, and typecheck a Unison file like

myfoo = mythingy + 100

as the "old" reference #oldthingy still had a set of names { lib.new.thingy, mythingy } from which we make the pretty-print environment.

This behavior is especially bad considering dependencies' dependencies are actually in scope for developers (something we neglected to think about when implementing the algorithm the first time around).

While in the example above, the user's project code itself has a name for #oldthingy, it's probably much more common for #oldthingy to be referenced by some dependency that still depends on old (which the user is trying to upgrade to `new):

lib.old.thingy#oldthingy = ...
lib.someOtherDependency.lib.old.thingy#oldthingy = ...

This would also keep #oldthingy alive / prevent an upgrade to #newthingy in the exact same way.

So, the solution implemented here is simply to pretty-print hashes that are directly referenced by the old dependency, i.e. things at the top level of lib.old.* (no matter what's in lib.old.lib.*), as lib.new.* (appropriately suffixified), no matter if there are any other aliases.

[aryairani]

@mitchellwrosen Looks good! Could you update the transcript to view bar also after the upgrade?

[mitchellwrosen]

@aryairani sure, done

[aryairani]

We found another bug where upgrade doesn't work if you haven't loaded any scratch file yet in your ucm session. @mitchellwrosen is working on a fix for that now 🙏