Implement a deprecation mechanism for aliases of existing options

[Gedochao]

Compiler version

5a2cff6

Description

We seem to be missing a mechanism for deprecating an alias of a command line option within the compiler.
We currently can deprecate an existing flag altogether, but we can't deprecate a particular alias of it.
This is particularly useful when private (-Y*) options get stabilised and promoted to -X*/root/other.

Existing aliases in need of deprecation

• -Yjava-tasty (currently -Xjava-tasty)
• -Yearly-tasty-output (currently -Xearly-tasty-output)
• -YallowOutlineFromTasty (currently -Xallow-outline-from-tasty)
• -usejavacp (will be -Yusejavacp after Make -usejavacp a private setting as -Yusejavacp #24304 gets merged)

Extra context

• Make -usejavacp a private setting as -Yusejavacp #24304 (comment)
• Lift Scala Settings from experimental to stabilized #20199

[som-snytt]

-Y doesn't require deprecation, according to Draconian law. Scala 2 has a mechanism to make a setting cascade to another. With that, the -usejavacp remains a regular deprecated option but sets -Yusejavacp. (Probably the look-up supports also having the alias on the -Y?)

[Gedochao]

-Y doesn't require deprecation, according to Draconian law.

While this generally is true (-Y options don't have any guarantees whatsoever), we do have some -Y options which were widely tested and adopted in builds, where a potential sudden removal would not be wise.

I guess it's a simplicity thing - we'd like it to become simple to deprecate an alias.

[som-snytt]

The description omits the most egregious aliases in need of deprecation:

• Xprint
• Xfatal-warnings
• Xshow-phases

[som-snytt]

The PR proposes a simple syntax -opt* to mean deprecated alias. That is from * for footnotes, or from linguistics where it means not an attested form. Also from sports when a record has a caveat.