Adding warning diagnostics for invalid or otherwise unsavory combinations of driver options, and modifying what the driver does when presented with those options, can be accomplished, I think, by modifying lib/Driver/Driver.cpp's validateArgs function, or by modifying the Driver::buildCompilation method, similar to how that method checks for invalid combinations of the -incremental option.

There's a lot of prior art in that file that new contributors could look at, so I think this can be marked as a "starter bug".

Note, however, that anyone who wishes to pick up this task should first check that swiftc still exhibits this behavior. I presume this can be done by running swiftc -emit-sil -embed-bitcode foo.swift, or something.

Ah, actually, I believe this is already done:

Someone should double-check the issue doesn't reproduce, or check whether there's a test, and close this issue.

Confirmed that this does warn now:

% ./bin/swiftc -emit-sil -embed-bitcode /tmp/foo.swift
<unknown>:0: warning: ignoring -embed-bitcode since no object file is being generated

But this did not have the same behavior with `-embed-bitcode-marker`. I've submitted a PR adding that here: #18304

Now the behavior is:

% ./bin/swiftc -emit-sil -embed-bitcode-marker /tmp/foo.swift
<unknown>:0: warning: ignoring -embed-bitcode-marker since no object file is being generated