reflect: add StructField.IsExported and Method.IsExported

[dsnet]

The StructField.PkgPath field is populated if and only if the field is unexported. However, there is a hit to readability:

for i := 0; i < t.NumField(); i++ {
    f := t.Field(i)
    if f.PkgPath != "" {
        continue
    }
    ...
}

It's not obvious to most readers why the f.PkgPath != "" check exist. For this reason, a vast majority of cases have a comment that says:

    // Skip unexported fields.
    if f.PkgPath != "" {
        continue
    }

This documentation seems unnecessary if we had just added a helper method on StructField like:

// IsExported reports whether the struct field is exported.
func (f StructField) IsExported() bool {
    return f.PkgPath == ""
}

Thus, use of it becomes obvious from a readability perspective:

for i := 0; i < t.NumField(); i++ {
    f := t.Field(i)
    if !f.IsExported() {
        continue
    }
    ...
}

Bikeshedding: I don't have a preference whether this should be called IsExported or simply Exported. The former matches IsValid, IsNil, or IsZero, while the latter matches StructField.Anonymous. It seems that the reflect package is already internally inconsistent about naming of predicate functionality.

[ianlancetaylor]

Minor side note: the field name Anonymous is already wrong, as we now refer to that concept as an embedded field. So although we can't change Anonymous we shouldn't take it as a precedent for anything.

[rsc]

I'm OK with adding IsExported() bool.
I wouldn't want to add a new struct field that could be out of sync with PkgPath.

[rsc]

Based on the discussion above, this seems like a likely accept.

[dsnet]

I should note that we probably want to add an IsExported method to the Method type as well since it has a PkgPath field with the same semantics.

[rsc]

SGTM

[dsnet]

Another note: #22075 makes it such that NumMethod no longer counts unexported methods, which subsequently means that you can't retrieve unexported methods. If this is the case, we should mark Method.PkgPath as deprecated instead of adding a Method.IsExported method.

\cc @mdempsky

[rsc]

That change to NumMethod is likely to be rolled back.

[rsc]

No change in consensus, so accepted.