Proposal: add context import path upgrade to go tool fix

[SamWhited]

It would be convenient if go tool fix had a rule (possibly off by default?) added to upgrade imports of the context package to the new version in the standard library.

Here is a CL that I'm using to upgrade my projects: https://go-review.googlesource.com/#/c/28872/

This makes the new rewrites list read as follows:

Available rewrites are:

context
        Change imports of golang.org/x/net/context to context

gotypes
        Change imports of golang.org/x/tools/go/{exact,types} to go/{constant,types}

netipv6zone
        Adapt element key to IPAddr, UDPAddr or TCPAddr composite literals.

        https://codereview.appspot.com/6849045/

printerconfig
        Add element keys to Config composite literals.

This isn't strictly the same as some of the other rewrites in the tool because the old context package is still perfectly usable, so maybe it would be best to leave this fix off by default and allow it to be manually specified.

[gopherbot]

CL https://golang.org/cl/28872 mentions this issue.

[quentinmit]

You likely want to remain with the old package for quite a while to support building with Go <1.7. I think perhaps we'd want to do this a year or more from now.

[SamWhited]

Agreed, having it off by default seems sensible, but going ahead and putting it in the tool seems like it would be nice for those who explicitly want to upgrade.

[SamWhited]

I've updated the CL to require -force context for the fix to be run (it looks like force wasn't otherwise used). Would this be an acceptible way to do it? The behavior could then be changed after the 1.8 release cycle or whenever the Go team decides it's more appropriate to be default-on.

[adg]

cc @bradfitz

[bradfitz]

We still ship and use go tool fix? Flashback!

[bradfitz]

But yeah, SGTM. Opt-in sounds good.