Building a DSL based on Swift

I'm building a small DSL for some configuration files and I'm exploring the idea of making it a subset of Swift. I've already toyed around with @resultBuilders and things are looking great. However, I'd also like to limit some of features/libraries that Swift normally has.

In other words, code written in this DSL would be valid Swift code, but not the other way around.

Also, this DSL is fully declarative, it will never be compiled, I just need the syntax + autocompletion/limited intellisense, so I'm skipping altogether the compilation part and focusing on lexical analysis / parsing and semantic analysis.

For the lexer I can use libSyntax and limit the tokens to my smaller, supported subset. However, I'm not sure what I should use for semantic analysis. I'm looking into SourceKit right now.

I guess my question would be: is this something achievable within a reasonable amount of effor or should I invest my time in trying to write my own simple lexer/semantic analyser.

It would help if you show us a quick sketch of what you are after.
Is it possible to describe what you want in, say, JSON format? If it is you may just express the thing with structs/enum so it is almost like JSON but has the benefits of compiler checking and autocomplete. There'll be some noise for commas and dots but on the bright side it is available immediately without any effort.