Go designers distinguish between Go language as defined by Go spec and implementation details.

//go:fix is something understood by a particular implementation of Go. Another implementation could implement Go without implementing support for //go:fix and it would be a fully compliant implementation of Go, the language.

If they made it part of the syntax, that would require other implementations to implement it.

Because these are instructions for users for making tool-assisted changes to their source code, not a behavior that exists at runtime (or even compile time). A new syntax wouldn't make sense for it.

For other things, like `//go:noinline`, this is fair criticism. `//go:fix inline` is quite different in every way.

The //go:xyz comments are an established pattern in the Go tooling.