Looking for Syntax of HIR and MIR

I am going to do research on HIR and MIR, but I can't find any direct syntax on these languages, like the datatype or function definition.  
Does  anyone got some documents about the syntax of HIR and MIR?
Thanks a lot.

Since this involves an internal part of the compiler, I think you'll have better answers if you ask on the platform where the compiler maintainers work: Zulip :slightly_smiling_face:

HIR is can roughly be considered an alternate representation of the parsed AST after macro expansion and desugaring of for loops and the ? operator. MIR only represents function bodies in a CFG format. In addition the MIR explicitly uses function calls for overloaded operators and Drop terminators for the implicit dropping of values at the end of their scope.