go/doc: do not nil out FuncBody

[robpike]

The go/doc package has references to the components of the AST that it describes. For some historical reason, though, it sets the FuncBody part of the FuncDecl to nil, which makes it hard to recover the source code associated with a declaration.

It seems that godoc use to do this of the "src" flag was not set, which makes sense. But at some point the action was moved into go/doc itself, and not protected by a flag, which means the source is never available.

As part of the go.12 release, I'd like to make cmd/doc supersede godoc's command line features entirely (can't find the issue number at the moment) and add support for examples (#26715). Both of these are made much more awkward by this property of go/doc.

Finally, as a point of order, it's rude for a package to overwrite a data structure passed to it, especially if the property is not documented.

I'd like to delete the nil-ing out of these fields (the function body is not the only one), or at least provide a mechanism to prevent it.

[robpike]

After talking with @gri, we will try to fix this by adding a new Mode bit called KeepSource or something like that, which will be off by default and therefore backwards-compatible.

[gopherbot]

Change https://golang.org/cl/140958 mentions this issue: go/doc: add new mode bit PreserveAST to control clearing of data in AST

[gopherbot]

Change https://golang.org/cl/140959 mentions this issue: cmd/doc: add a -src flag to show original source