No Diagnostic system. Basically use panic-mode & Statement-mode for parsing error.

[mhmdvoid]

Well, We need to start off simple.
Like, building one for grammar error then we can iterate to build more complex for semantic error and similar.
For instance :
var x: int = 10 // Grammar error, variable declaration missing semi-colon ; FixIt -> should properly handle to var x: int = 10;
typealias Foo : bool; // Grammar error, Foo should be followed by = in typealiasDecl. FixIt -> typealias Foo = bool; .
As you can tell the language helps us a lot here, Since it's not a very high level language, not that complex one can predict at least from very simple context what's going on.
For advance , more HL languages It's really hard to predict what's going on inside user mind and what they want to apply semantically.