Val is not stable unless path requested by use site

[bishabosha]

it seems that a val is not stable in TASTy unless a stable path is requested for it:

Minimized code

// before
object Consts {
  val msg = "Hello World" // not stable
}
object TestConsts {
  def foo = Consts.msg
}

// after
object Consts {
  val msg = "Hello World" // now it is stable because of a use site
}
object TestConsts {
  def foo: Consts.msg.type = Consts.msg
}

Output

before:

80:       VALDEF(7) 26 [msg]
83:         TYPEREF 27 [String]
85:           SHAREDtype 35
87:         STRINGconst 28 [Hello World]

after:

80:       VALDEF(8) 26 [msg]
83:         TYPEREF 27 [String]
85:           SHAREDtype 35
87:         STRINGconst 28 [Hello World]
89:         STABLE

Expectation

[smarter]

@anatoliykmetyuk I don't have time to take on more issues currently.

[odersky]

STABLE can be either user-declared, or structural, in which case it is a hint only. Absence of STABLE when it is a hint is no problem, since it will be re-generated when tested. But maybe the declared and computed versions should be different flags.

[odersky]

I had a look. It does not seem possible to fix this, unless we spend a flag, and we have very few left. So, what is the problem here exactly? StableRealizable is an assertion that a value is stable and relializable. It is sometimes an assertion, and at other times a cache, meaning that it simply avoids a recomputation of the property. Can't we leave that as it is?

[smarter]

So, what is the problem here exactly?

One problem I can think of is it breaks reproducible builds: depending on whether two files are compiled together or one after another, the STABLE flag will be present in tasty or not. Quite a lot of work went into achieve deterministic compilation in Scala 2 to avoid this sort of things, in particular because it's important for build systems that rely heavily on caching compiler artifacts: scala/scala-dev#405

[smarter]

Could we explicitly compute if something is StableRealizable at Pickler to emit STABLE even if the cache hasn't been set yet?