You don't want to make the compiler itself interactive. Compilers have a well-known interface: either they spit out an output binary and exit cleanly, or they spit out an error. This is a case where I'd imagine the compiler could just spit out an error: something like "not 100% confident on the correct way to optimize the following lines: [...]" (this is oddly reminiscent of INTERCAL's "please" directives, actually.)
What the compiler would be expecting, would be an "optimization annotation file" to exist alongside each source file, that it would take as batch input. If said file has all the answers it needs, it compiles cleanly; if it doesn't (because the program changed, or because it doesn't exist yet) then it'll error out.
The important bit is that an IDE can read such an error, and transform that into some form of semi-interactive process to create/modify the optimization-annotation file. But to the compiler, everything stays a pipeline, like usual.
Could those be specified in comments in the file? Most development nowdays is done in an IDE that could intelligently "hide" the comments when you're just reading the code, and then when you're looking at optimizing the code you could start annotating things that way.
A similar approach is to use #pragma directives, though many languages do not have an analogue to that.
Java Annotations are probably wrong. The compiler can benefit from more freedom (and finer grained detail), so for Java comments seem like a better choice to me.
What the compiler would be expecting, would be an "optimization annotation file" to exist alongside each source file, that it would take as batch input. If said file has all the answers it needs, it compiles cleanly; if it doesn't (because the program changed, or because it doesn't exist yet) then it'll error out.
The important bit is that an IDE can read such an error, and transform that into some form of semi-interactive process to create/modify the optimization-annotation file. But to the compiler, everything stays a pipeline, like usual.