Why isn't go embedding files?

I'm trying to embed a text file within my go application, but I can't figure out why it's not working.

I have a file named hello.txt in the same directory as my go program, but when I compile and run the code below it doesn't print anything, when it should print the contents of hello.txt.

package main

import (
	_ "embed"
	"fmt"
)

// go:embed hello.txt
var hello string

func main() {
	fmt.Println(hello)
}

I also tried making hello a []byte and even using a filesystem and listing all files, both show that there's nothing embedded.

You cannot have a space between the // and the go:embed, otherwise the compiler just treats it as a comment.

The embed documentation doesn't make this clear, but it's explained in the documentation for go compile

> The compiler accepts directives in the form of comments. To distinguish them from non-directive comments, directives require no space between the comment opening and the name of the directive. However, since they are comments, tools unaware of the directive convention or of a particular directive can skip over a directive like any other comment.