x/pkgsite: show the stdlib/module version when a symbol was added

[cespare]

It would be helpful if pkg.go.dev could show the version where a symbol was added.

This is something that is shown for the stdlib by the godoc HTTP server but not a feature that gddo has.

For instance, consider https://golang.org/pkg/log/#Writer:

The equivalent pkg.go.dev page is https://pkg.go.dev/log#Writer:

To begin with, perhaps this could be shown only for the stdlib (to reach feature parity with godoc). It seems that godoc displays the first Go version where a symbol (type, function, method) existed if it was added after 1.0.

As a further improvement, it would be great to show this information for third-party modules as well. Here's one possible heuristic:

• If the symbol existed in the earliest version of that module (i.e., the first release with the same major version), don't show anything.
• Otherwise, show the first version where the symbol exists.

So if a module had releases

• 1.0.0 with symbols {A}
• 2.0.0 with symbols {A, B}
• 2.1.0 with symbols {A, B, C}
• 2.1.1 with symbols {A, B, C}

then the 1.0.0 and 2.0.0 doc pages wouldn't show any versions, whereas the 2.1.0 and 2.1.1 pages would show that C was added in 2.1.0.

Apologies if this feature request already exists somewhere. I tried a few github searches and couldn't turn up anything.

[julieqiu]

Thanks for the suggestion, @cespare! I appreciate the added context. This seems like a duplicate of #37102, so closing as a duplicate of that issue.