incompatible_disable_sysroot_from_configuration: libc_top should be set by attribute

[katre]

Flag: --incompatible_disable_sysroot_from_configuration
Available since: 0.20
Will be flipped in: 0.21
Enclosing tracking issue: #6543

The sysroot (also called grte_top and libc_top) was previously set by default in the CROSSTOOL file, using the default_grte_top property. As this is a build label, and creates a new dependency in the build, it needs to be moved into an actual build target.

The cc_toolchain rule now has an attribute called libc_top that is used to set this value.

Migration notes:
To migrate, remove any default_grte_top properties in your CROSSTOOL file, and set the libc_top attribute on the corresponding cc_toolchain target to be the same value. The libc_top needs to be a fully-qualified label, so in many cases the value from CROSSTOOL will need to have :everything added to refer to a specific target.

[katre]

The new incompatible change flag will be available in the 0.20 release, and the default value will be flipped to true in the 0.21 release.

[dslomov]

Migration notes:
To migrate, remove any default_grte_top properties in your CROSSTOOL file, and set the libc_top attribute on the corresponding cc_toolchain target to be the same value. The libc_top needs to be a fully-qualified label, so in many cases the value from CROSSTOOL will need to have :everything added to refer to a specific target.

If people do that, will it work without the flag?

[katre]

Yes, the migration is backwards compatible.