cmd/compile: capture implicit constraints in export data

[findleyr]

With #48424, we allow users to elide interface{} in constraint expressions. We expose this in the type checker as func (*Interface) IsImplicit() bool. It would be nice to capture this in export data, so that we can preserve this implicit bit on import.

CC @mdempsky @griesemer

[findleyr]

I can do this. @griesemer and I discussed and this is probably as straightforward as exporting the embedded structural type, rather than the constraint interface itself, for implicit constraint interfaces. Then when importing, we can detect that the constraint is not an interface and wrap it in an implicit interface.

Prototyped in https://golang.org/cl/357109.

[gopherbot]

Change https://golang.org/cl/357796 mentions this issue: go/types, types2: add the Interface.MarkImplicit method

[findleyr]

Update: the test failures in the prototype highlighted that trying to "cheat" and compute whether an interface is implicit based on the constraint type, is unclean. We've decided to opt for an explicit bit in the export format.

[gopherbot]

Change https://golang.org/cl/358034 mentions this issue: go/internal/gcimporter: add support for the Go 1.18 export data version

[griesemer]

@findleyr is there anything left to do here?

[findleyr]

No, I don't think so. Closing.