cmd/vet: add check for Context.WithValue(_, string, _)

[bradfitz]

Using a context.Context Value key of type string is a terrible idea and walks into the minefield of a global namespace. We should've outlawed it from day 1.

All context value keys should be made from user-defined types.

Also disallow int, bool, etc. But string is what I usually see.

[mdlayher]

I'd be happy to give this a try.

[bradfitz]

Thanks.

[mdlayher]

Submitted an initial CL here: https://go-review.googlesource.com/c/30084/.

I suspect there's probably more I can do, but I would appreciate some review before I pursue this issue further.

[robpike]

This is not a correctness issue and so should not be fixed in vet. It's a better candidate for lint.

[gopherbot]

CL https://golang.org/cl/30084 mentions this issue.

[dsnet]

Merged as golang/lint@c6242af