Recently I read Rust, reflection and access rules. Among other things, it describes how a hypothetical Rust reflection system couldn't safely allow access to private fields of things, and especially how it couldn't allow code to set them through reflection. My short paraphrase of the article's discussion is that in Rust, private fields can be in use as part of invariants that allow unsafe operations to be done safely through suitable public APIs. This brought into clarity what had previously been a somewhat odd seeming restriction in Go's reflect package.

Famously (for people who've dabbled in reflect), you can only set exported struct fields. This is covered in both the Value.CanSet() package documentation and The Laws of Reflection (in passing). Since one of the uses of reflection is for going between JSON and structs, encoding/json only works on exported struct fields and you'll find a lot of such fields in lots of code. This requirement can be a bit annoying. Wouldn't it be nice if you didn't have to make your fields public just to serialize them easily?

(You can use encoding/json and still serialize non-exported struct fields, but you have to write some custom methods instead of just marking struct fields the way you could if they were exported.)

Go has this reflect restriction, presumably, for the same reason that reflection in Rust wouldn't be able to modify private fields. Since private fields in a Go struct may be used by functions and methods in the package to properly manage the struct, modifying those fields yourself is unsafe (in the general sense). The reflect package will let you see the fields (and their values) but not change their values. You're allowed to change exported fields because (in theory) arbitrary Go code can already change the value of those fields, and so code in the struct's package can't count on them having any particular value. It can at least sort of count on private fields having approved values (or the zero value, I believe).

(I understand why the reflect documentation doesn't explain the logic of not being able to modify private fields, since package documentation isn't necessarily the right place for a rationale. Also, perhaps it was considered obvious.)