Remove language fragmentation `-Y` flags. Only `-Xsource` allowed henceforth.

[adriaanm]

I've counted close to 20 flags that change language semantics in various ways. 2^20 variants of the language is unacceptable complexity both for users and maintainers. As of 2.13, -Y flags will no longer be allowed to change language semantics. We will retain -Xsource as a migration/preview mechanism. We will clearly list which features can be turned on by specifying a future source version, and as soon as the first milestone of that version is released, the feature will be turned on by default.

A feature submitted to our codebase is only accepted if we expect it to become officially supported. -Xsource is meant to allow previewing and migrating to the new version that offers this feature by default. If the feature is not of high enough quality to be turned on along with the other -Xsource:N features, it is not ready for submission to scala/scala.

The official Scala compiler is not a vehicle for research/experimentation. There are plenty of alternatives for that. Having many rarely exercised code paths lead to bitrot and undertested code. They also set the wrong expectations for users.

The mechanism is also used by the presentation compiler to simulate older versions of the compiler while running the latest version to provide the IDE experience. We will retain this functionality where appropriate.

[Jasper-M]

Does that mean popular features like -Ypartial-unification will become part of flagless Scala 2.13, or that you'll need Typelevel Scala to use them?

[xeno-by]

What about -Yrangepos?

[adriaanm]

Partial unification, assuming it has proven itself (I’d say it has), will be on by default. Ditto for rangepos.

[jvican]

I really like this.

> $ scalac -Y    
Usage: scalac <options> <source files>

-- Notes on option parsing --
Boolean settings are always false unless set.
Where multiple values are accepted, they should be comma-separated.
  example: -Xplugin:option1,option2
<phases> means one or a comma-separated list of:
  (partial) phase names, phase ids, phase id ranges, or the string "all".
  example: -Xprint:all prints all phases.
  example: -Xprint:expl,24-26 prints phases explicitouter, closelim, dce, jvm.
  example: -Xprint:-4 prints only the phases up to typer.

Possible private options include:
  -Ybreak-cycles                           Attempt to break cycles encountered during typing
  -Ybrowse:<phases>                        Browse the abstract syntax tree after <phases>
  -Ycheck:<phases>                         Check the tree at the end of <phases>
  -Ycompact-trees                          Use compact tree printer when displaying trees.
  -Ydebug                                  Increase the quantity of debugging output.
  -Ydelambdafy:<strategy>                  Strategy used for translating lambdas into JVM code. Choices: (inline,method), default: method.
  -YdisableFlatCpCaching                   Do not cache flat classpath representation of classpath elements from jars across compiler instances.
  -Ydump-classes <dir>                     Dump the generated bytecode to .class files (useful for reflective compilation that utilizes in-memory classloaders).
  -Ygen-asmp <dir>                         Generate a parallel output directory of .asmp files (ie ASM Textifier output).
  -Yinfer-argument-types                   Infer types for arguments of overridden methods.
  -Ylog:<phases>                           Log operations during <phases>
  -Ylog-classpath                          Output information about what classpath is being applied.
  -Ymacro-debug-lite                       Trace essential macro-related activities.
  -Ymacro-debug-verbose                    Trace all macro-related activities: compilation, generation of synthetics, classloading, expansion, exceptions.
  -Ymacro-expand:<policy>                  Control expansion of macros, useful for scaladoc and presentation compiler. Choices: (normal,none,discard), default: normal.
  -Yno-adapted-args                        Do not adapt an argument list (either by inserting () or creating a tuple) to match the receiver.
  -Yno-completion                          Disable tab-completion in the REPL.
  -Yno-generic-signatures                  Suppress generation of generic signatures for Java.
  -Yno-imports                             Compile without importing scala.*, java.lang.*, or Predef.
  -Yno-predef                              Compile without importing Predef.
  -Yopt-inline-heuristics:<strategy>       Set the heuristics for inlining decisions. Choices: (at-inline-annotated,everything,default), default: default.
  -Yopt-log-inline <package/Class.method>  Print a summary of inliner activity; `_` to print all, prefix match to select.
  -Yopt-trace <package/Class.method>       Trace the optimizer progress for methods; `_` to print all, prefix match to select.
  -Yoverride-objects                       Allow member objects to be overridden.
  -Yoverride-vars                          Allow vars to be overridden.
  -Ypartial-unification                    Enable partial unification in type constructor inference
  -Ypatmat-exhaust-depth <n>               off
  -Ypresentation-any-thread                Allow use of the presentation compiler from any thread
  -Ypresentation-delay <n>                 Wait number of ms after typing before starting typechecking
  -Ypresentation-log <file>                Log presentation compiler events into file
  -Ypresentation-replay <file>             Replay presentation compiler events from file
  -Ypresentation-strict                    Do not report type errors in sources with syntax errors.
  -Ypresentation-verbose                   Print information about presentation compiler tasks.
  -Yprofile-destination <file>             where to send profiling output - specify a file, default is to the console.
  -Yprofile-enabled                        Enable profiling.
  -Yprofile-external-tool[:phases]         Enable profiling for a phase using an external tool hook. Generally only useful for a single phase <phases> (default: typer)
  -Yprofile-run-gc[:phases]                Run a GC between phases - this allows heap size to be accurate at the expense of more time. Specify a list of phases, or * <phases> (default: _)
  -Yrangepos                               Use range positions for syntax trees.
  -Yrecursion <n>                          Set recursion depth used when locking symbols.
  -Yreify-copypaste                        Dump the reified trees in copypasteable representation.
  -Yrepl-class-based                       Use classes to wrap REPL snippets instead of objects
  -Yrepl-outdir <path>                     Write repl-generated classfiles to given output directory (use "" to generate a temporary dir)
  -Yrepl-sync                              Do not use asynchronous code for repl startup
  -Yresolve-term-conflict:<strategy>       Resolve term conflicts. Choices: (package,object,error), default: error.
  -Yshow:<phases>                          (Requires -Xshow-class or -Xshow-object) Show after <phases>
  -Yshow-member-pos <output style>         Show start and end positions of members
  -Yshow-symkinds                          Print abbreviated symbol kinds next to symbol names.
  -Yshow-symowners                         Print owner identifiers next to symbol names.
  -Yshow-syms                              Print the AST symbol hierarchy after each phase.
  -Yshow-trees                             (Requires -Xprint:) Print detailed ASTs in formatted form.
  -Yshow-trees-compact                     (Requires -Xprint:) Print detailed ASTs in compact form.
  -Yshow-trees-stringified                 (Requires -Xprint:) Print stringifications along with detailed ASTs.
  -Yskip:<phases>                          Skip <phases>
  -Ystatistics:<_,phase,-phase>            Print compiler statistics for specific phases: `_' for all, `-Ystatistics:help' to list choices.
  -Ystop-after:<phases>                    Stop after <phases>
  -Ystop-before:<phases>                   Stop before <phases>
  -Yvirtpatmat                             Enable pattern matcher virtualization
  -Ywarn-adapted-args                      Warn if an argument list is modified to match the receiver.
  -Ywarn-dead-code                         Warn when dead code is identified.
  -Ywarn-extra-implicit                    Warn when more than one implicit parameter section is defined.
  -Ywarn-inaccessible                      Warn about inaccessible types in method signatures.
  -Ywarn-infer-any                         Warn when a type argument is inferred to be `Any`.
  -Ywarn-nullary-override                  Warn when non-nullary `def f()' overrides nullary `def f'.
  -Ywarn-nullary-unit                      Warn when nullary methods return Unit.
  -Ywarn-numeric-widen                     Warn when numerics are widened.
  -Ywarn-unused:<_,warning,-warning>       Enable or disable specific `unused' warnings: `_' for all, `-Ywarn-unused:help' to list choices.
  -Ywarn-unused-import                     Warn when imports are unused.
  -Ywarn-value-discard                     Warn when non-Unit expression results are unused.

Additional debug settings:
  -Ydoc-debug                              Trace all scaladoc activity.
  -Yide-debug                              Generate, validate and output trees using the interactive compiler.
  -Yissue-debug                            Print stack traces when a context issues an error.
  -Ypatmat-debug                           Trace pattern matching translation.
  -Ypos-debug                              Trace position validation.
  -Ypresentation-debug                     Enable debugging output for the presentation compiler.
  -Yquasiquote-debug                       Trace quasiquote-related activities.
  -Yreify-debug                            Trace reification.
  -Ytyper-debug                            Trace all type assignments.

Deprecated settings:
  -Ymacro-no-expand                        Don't expand macros. Might be useful for scaladoc and presentation compiler, but will crash anything which uses macros and gets past typer.
                                             deprecated: Use -Ymacro-expand:none

I guess those -Y flags related to debugging will stay or renamed? And perhaps things like -Yprofile-enabled should be moved to -X?

[adriaanm]

I only intend to change/remove -Y flags that affect language semantics. Debugging flags can stay since they don't fragment the language.

[jvican]

  -Yno-imports                             Compile without importing scala.*, java.lang.*, or Predef.
  -Yno-predef                              Compile without importing Predef.

These affect language semantics, though I guess they will stay too.

[adriaanm]

Yes, exceptions will be needed, just not 19 ;-) I hope the predef/imports options will be consolidated -- there's a PR for that.

[adriaanm]

-Y should be reserved for compiler development. No guarantees. The help text should not mention them, and they should never be used in a build (so that we can drop/change them without fear of breaking the community build and by extension the eco-system).

-X should be for the more adventurous user, and we should do our best to keep them stable acros compiler versoins. I expect -Ximports:... to replace the -Y flags that will disappear.

[som-snytt]

I guess no more override-objects. The repl options could be moved as part of the refactor. There's GenericRunnerSettings for -i, where the repl fragmentation flags could live.

[smarter]

-Y should be reserved for compiler development. No guarantees.

Sounds good to me, you may want to figure out what should happen to the billion flags recommended in https://tpolecat.github.io/2017/04/25/scalac-flags.html

[smarter]

I suggest replacing -Ywarn-xxx by -Wxxx and -Wno-xxx like gcc/clang

[som-snytt]

Flags to increase verbose output like -Ytyper-debug could become -Vtyper.