Is metadata or attributes allowed in Golang?

How are these various validation libraries adding this meta data to structs like:

type Post struct {
    Title    string `valid:"alphanum,required"`
    Message  string `valid:"duck,ascii"`
    AuthorIP string `valid:"ipv4"`
    Date     string `valid:"-"`
}

I'm confused, the property is Title, the type is string. Beyond that how can you just add valid:"alphanum,required" Is this using reflection?

Is this like attributes in other languages?

[Required]
public int Title { get;set; }

Go doesn't have attributes in general sense. The strings in the struct are struct tags:

>A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration. The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored.
>
// A struct corresponding to the TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
microsec uint64 "field 1"
serverIP6 uint64 "field 2"
process string "field 3"
}

You can't add or change them, but you can access them using the reflect package.

Another thing that kinda looks like attributes are "magic comments" like

// +build amd64

or

//go:noinline

These are compiler-specific and IIRC are not a part of the language specification.

Those are tags. From the language spec (towards the end of Struct types):

> A field declaration may be followed by an optional string literal tag,
> which becomes an attribute for all the fields in the corresponding
> field declaration. The tags are made visible through a reflection
> interface and take part in type identity for structs but are otherwise
> ignored.

As per the quote above, you can use them with reflection.

ALso , from the reflect package doc:

> A StructTag is the tag string in a struct field.
>
> By convention, tag strings are a concatenation of optionally
> space-separated key:"value" pairs. Each key is a non-empty string
> consisting of non-control characters other than space (U+0020 ' '),
> quote (U+0022 '"'), and colon (U+003A ':'). Each value is quoted using
> U+0022 '"' characters and Go string literal syntax.

There is an excellent executable example at the link above.

Tags are also used extensively with json. From the json package docs:

> The object's default key string is the struct field name but can be
> specified in the struct field's tag value. The "json" key in the
> struct field's tag value is the key name, followed by an optional
> comma and options.

Loads of additional data on that page.

This earlier question may also be helpful.