Changes to StagedTuple

[odersky]

I noted the following problems when trying to make StageTuple optional:

• there seems to be a build problem. If Tuple.stageIt is turned on, we fail with the following two error messages in dotty-bootstrapped/compile:

[error] -- Error: /Users/odersky/workspace/dotty/library/src-scala3/scala/Tuple.scala:315:17 
[error] 315 |    if (stageIt) stagedHead
[error]     |                 ^^^^^^^^^^
[error]     |Could not find class scala.StagedTuple$ in classpath. The most common reason for that is that you apply macros in the compilation run that defines them
[error]     | This location is in code that was inlined at /Users/odersky/workspace/dotty/library/src-scala3/scala/Tuple.scala:479
[error] -- Error: /Users/odersky/workspace/dotty/library/src-scala3/scala/Tuple.scala:346:17 
[error] 346 |    if (stageIt) stagedTail
[error]     |                 ^^^^^^^^^^
[error]     |Could not find class scala.StagedTuple$ in classpath. The most common reason for that is that you apply macros in the compilation run that defines them
[error]     | This location is in code that was inlined at /Users/odersky/workspace/dotty/library/src-scala3/scala/Tuple.scala:479

I am curious why the other 6 occurrences of splices do not give errors?

Other issues with StagedTuple.

• It's for internal use only, so should definitely not be in scala package.
• Inconsistent naming: stagedCons vs headStaged.

[nicolasstucki]

This is the source of the issue is https://github.com/lampepfl/dotty/pull/5392/files#diff-901fed2e7fd7e86c4b92a9f7198b0cc7R797.
At https://github.com/lampepfl/dotty/pull/5392/files#diff-dae2a6166db3f0fad29edec3ff83f555R479 we have a top level splice that gets inlined when trying to compute the rhs of the inlined method.

StagedTuple tuples should be defined in a separate project and the lib should depend on it.

[nicolasstucki]

It works if we use the previous version of the code

inline def unapply[H, T <: Tuple](x: H *: T) =
    (NonEmptyTuple.dynamicHead(x), NonEmptyTuple.dynamicTail(x))

Which is also what gets inlined in the non-staged version and would be by the staged.

[nicolasstucki]

Fixed in #5822 and then StagedTuple were removed from the library in #6311