12. A computer system, comprising a memory and at least one processor, the memory storing software executable by the at least one processor, the software comprising: a code editor component that receives input and is operable in an edit mode to allow text editing of the input, the input comprising source code, the source code comprising instructions for mapping of values from one or more fields of a data source to one or more fields of a data destination; a compiler component that automatically executes the source code by compiling the source code into object code and sending the object code to an interface engine, causing the interface engine to receive one or more structured messages from the data source containing values for the mapping, and to extract and collect from the structured messages, for each instruction of the source code from which the object code was derived, values of the one or more fields of the data source referenced by that instruction; a display component that receives the collected field values from the interface engine and during the text editing of the input, for each instruction in the source code, displays an annotation comprising the value of any fields referenced by that instruction as collected for that instruction.
12. A computer system, comprising a memory and at least one processor, the memory storing software executable by the at least one processor, the software comprising: a code editor component that receives input and is operable in an edit mode to allow text editing of the input, the input comprising source code, the source code comprising instructions for mapping of values from one or more fields of a data source to one or more fields of a data destination; a compiler component that automatically executes the source code by compiling the source code into object code and sending the object code to an interface engine, causing the interface engine to receive one or more structured messages from the data source containing values for the mapping, and to extract and collect from the structured messages, for each instruction of the source code from which the object code was derived, values of the one or more fields of the data source referenced by that instruction; a display component that receives the collected field values from the interface engine and during the text editing of the input, for each instruction in the source code, displays an annotation comprising the value of any fields referenced by that instruction as collected for that instruction. 13. The computer system of claim 12 , wherein the compiler component automatically executes the source code in response to a user moving a cursor to a different line.