Which component should be improved?

Compiler

Describe your feature suggestion in more detail

I checked out that UMT decompiles struct very well, but it can't be compiled back.

Implementation-wise, I think there are two ways (not exclusive to each other) to handle structs. One way is to modify parser: ParseBlock() should detect structs with : token. The other way is to modify lexer: opening/closing braces should be lexed as OpenBlock/CloseBlock or OpenStruct/CloseStruct(to be added) based on the context and then be parsed separately from blocks.

I'm not familiar with GameMaker nor with Compiler stuff so I might be wrong.