Verify TASTy versions before releases

[bishabosha]

We need to verify before releases that TASTy is set to a correct version.

Here is a very simplistic approach that could be automated to ensure that
we don't accidentally release experimental TASTy in a stable release,
or stable TASTy in a Nightly release:

(mStable refers to the TastyFormat.MinorVersion of the most recent stable Scala release)

if MajorBump:
  raise "are you really sure?"
else if Nightly or (MinorBump and RC or Milestone):
  assert TastyFormat.ExperimentalVersion > 0
  assert TastyFormat.MinorVersion > mStable
else: # release is final
  assert ExperimentalVersion == 0
  if PatchBump:
    assert TastyFormat.MinorVersion == mStable
  else if MinorBump:
    assert TastyFormat.MinorVersion == mStable + 1

with the proposal above, the only clear weak link is that
TastyFormat.MinorVersion should be increased in a nightly release
before releasing the next Minor, but I propose that this is
difficult to miss as it should be clear that
merging a needs-minor-release PR needs a TASTy version bump

A next step would be to automate the weak-link I described above.

A simple way could be automating the creation of an issue whenever
a needs-minor-release PR is merged, otherwise it will probably
be too complex to analyse automatically (
requires analysing TreePickler, TastyFormat, API additions, Code synthesis changes, more)

[sjrd]

M and RC versions should still be experimental. Imagine if a serious blocker is discovered during the RC cycle that requires a TASTy format change to fix it. If you've already used the non-experimental TASTy version in the RC release, you're stuck.

So the first line of the script above should be changed to

if Nightly or Milestone or RC or anything with a '-' in it:

[bishabosha]

M and RC versions should still be experimental.

My suggestion then here is that the experimental should at least be increased within the same minor version, to account for new needs-minor-release PR's that were merged

[bishabosha]

So with the rule of RCs having experimental versions then this is what the future of releases could look like - I have included examples of both a patch release and a minor release.

each row represents the releases made each 6 weeks

Edit: removed diagram - see #13447 (comment) for updated diagram

[bishabosha]

I have updated the diagram above with the potential future of the current state of things - the most tricky part is RCs for patch releases, if we insist on them having experimental TASTy then either we keep the same minor version and 3.1.1-RC1 can't depend on 3.1.0, or we have to do a weird step where we go to the next TASTy minor series and downgrade again, e.g. going from 28.2-1 for 3.1.1-RC1 to 28.1-0 for 3.1.1. What do you think @sjrd @smarter

[smarter]

I think it's fine for 3.1.1-RC1 to use the same tasty version as 3.1.0 (since the tasty format should be exactly the same between these two versions) as long as 3.1.1-RC1 doesn't let you use experimental features like a nightly does, but you still need to move from 28.2-1 to 28.1-0 when branching release-3.1.1 from master.

[bishabosha]

ok here is an updated diagram with past and future - hopefully this makes a coherent model:

        Stable             Beta             Nightly

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
   ┌───────────────┐                   ┌───────────────┐
   │ T: 28.0-0     │                   │ T: 28.1-1     │
   │ S: 3.0.0      │                   │ S: 3.0.1-NI-a │
   └───────────────┘ ┌───────────────┐ └───────────────┘
           │         │ T: 28.0-0     │         │
           │         │ S: 3.0.1-RC1  │         ▼
           │         └───────────────┘ ┌───────────────┐
           │                 │         │ T: 28.1-1     │
           │                 ▼         │ S: 3.0.2-NI-b │
           │         ┌───────────────┐ └───────────────┘
           │         │ T: 28.0-0     │         │
           ▼         │ S: 3.0.1-RC2  │         │
   ┌───────────────┐ └───────────────┘         │
   │ T: 28.0-0     │         │                 │
   │ S: 3.0.1      │         ▼                 │
   └───────────────┘ ┌───────────────┐         │
           │         │ T: 28.0-0     │         │
           │         │ S: 3.0.2-RC1  │         ▼
           │         └───────────────┘ ┌───────────────┐
           │                 │         │ T: 28.2-1     │
           │                 ▼         │ S: 3.1.0-NI-c │
           │         ┌───────────────┐ └───────────────┘
           │         │ T: 28.0-0     │         │
           ▼         │ S: 3.0.2-RC2  │         │
   ┌───────────────┐ └───────────────┘         │
   │ T: 28.0-0     │         │                 │
   │ S: 3.0.2      │         ▼                 │
   └───────────────┘ ┌───────────────┐         │
           │         │ T: 28.2-1     │         │
           │         │ S: 3.1.0-RC1  │         ▼
           │         └───────────────┘ ┌───────────────┐
           │                 │         │ T: 28.2-1     │
           │                 │         │ S: 3.1.1-NI-d │
           │      ┏━━━━━━━━┓ │         └───────────────┘
           │      ┃  past  ┃ │                 │
▣━━━━━━━━━━╋━━━━━━┻━━━━━━━━┻━╋━┳━━━━━━━━┳━━━━━━╋━━━━━━━━━▣
           │                 │ ┃ future ┃      │
           │                 │ ┗━━━━━━━━┛      │
           │                 ▼                 │
           │         ┌───────────────┐         │
           │         │ T: 28.1-2     │         │
           ▼         │ S: 3.1.0-RC2  │         │
   ┌───────────────┐ └───────────────┘         │
   │ T: 28.1-0     │         │                 │
   │ S: 3.1.0      │         ▼                 │
   └───────────────┘ ┌───────────────┐         │
           │         │ T: 28.1-0     │         │
           │         │ S: 3.1.1-RC1  │         ▼
           │         └───────────────┘ ┌───────────────┐
           │                 │         │ T: 28.3-1     │
           ▼                 │         │ S: 3.2.0-NI-e │
   ┌───────────────┐         │         └───────────────┘
   │ T: 28.1-0     │         │                 │
   │ S: 3.1.1      │         ▼                 │
   └───────────────┘ ┌───────────────┐         │
           │         │ T: 28.2-2     │         │
           │         │ S: 3.2.0-RC1  │         ▼
           │         └───────────────┘ ┌───────────────┐
           │                 │         │ T: 28.3-1     │
           │                 ▼         │ S: 3.2.1-NI-f │
           │         ┌───────────────┐ └───────────────┘
           │         │ T: 28.2-2     │
           ▼         │ S: 3.2.0-RC2  │
   ┌───────────────┐ └───────────────┘
   │ T: 28.2-0     │
   │ S: 3.2.0      │
   └───────────────┘