Here is the reasoning behind that change:
Historically, all the declarations generated by compiler plugins
were marked with `SYNTHESIZED` member kind in Kotlin Metadata
by K1 compiler.
In K1 IDE, descriptors were deserialized directly from the Kotlin
Metadata, and they saw all the generated declarations from it.
In the stubs, however, such declarations were not materialized at all.
It caused no troubles, since K1 IDE relied on descriptors to get the
essential resolution information in completion and other subsystems.
So, the resolution of members from jars processed
by compiler plugins (e.g. `kotlinx-serialization-json`) was mostly fine.
In K2 IDE, however, we use stubs to "deserialize" FIR declarations from
them, to later create `KtSymbol`s upon that FIR.
If we see a library file which was processed by compiler plugins, we
build stubs for it based on the Kotlin Metadata, and then use stubs to
create FIR.
But if stubs do not contain information about the generated
declarations,
then the resulting FIR will also not contain such declarations.
In the end, the K2 IDE would also be blind to such declarations, since
there is no other way to retrieve them from the library jar.
By meterializing all the synthethic declarations in stubs (with some
minor exceptions for data classes), we would avoid such problems,
and the resolve of such declarations from compiler jars in
K2 IDE would become possible.
Important note: currently, the Kotlin Metadata format between K1 and K2
frontends is not 100% the same; most notably, in K2, the generated
declarations are marked as regular declarations,
not as `SYNTHESIZED` ones.
Hence, if you compile a jar with the current version of K2 frontend,
then all the generated declarations would have a regular `DECLARATION`
origin, and there would be no such issues as described above.
There are two notes here:
1. K2 IDE still has to support the jars compiled by the K1 compiler,
so it still makes sense to alter the stubs and make the generated
declarations visible.
2. The issue about the different stub formats has been reported
(see KT-64924), and might be resolved in the future.
So it is possible that K2 frontend's metadata will also start
marking the generated declarations as `SYNTHESIZED`.
^KT-64808 Fixed
But still compile stdlib, reflect, kotlin.test and scripting runtimes
with JVM target 1.6 to simplify migration from Kotlin 1.6 to 1.7.
#KT-45165 Fixed
There seems to be no point in configuring the compiler argument per
project. This argument will be deleted soon anyway, when we remove
support for JDK 1.6 & 1.7.
Also remove `disableDeprecatedJvmTargetWarning`. It didn't have any
effect in all modules where it was applied because these modules
reassign `freeCompilerArgs` anyway, with
`-Xsuppress-deprecated-jvm-target-warning` in it.
Roman Golyshev