We have quite a lot of FIXMEs, mostly in hir-ty, for adding new diagnostics for things (and I'm constantly adding more). The shape of them is like FIXME: Emit an error ... (my own FIXMEs usually use these words, others might use similar). Most are pretty easy to do. The steps to resolve one (in inference) are:

1. Add a variant to InferenceDiagnostic with the necessary information.
2. Call self.push_diagnostic(diag) on the place.
3. In hir/src/diagnostics.rs, add a new struct for the diagnostic and convert the enum variant to it in the functions of that module. Note: this struct must not use hir-ty types, only hir types.
4. Create a new handler in ide-diagnostics/src/handlers/ for the diagnostic (follow existing handlers). A simple handler can take a few lines. Also add some tests.
5. Call the handler from ide-diagnostics/src/lib.rs.

That's it! You can also of course customize the error message to be clearer/clever, and/or add a fix, but that is good enough.