In fast-compiling languages, it's viable to have recompile-on-file-save or even continuous recompilation. Modern (and they aren't that modern anymore) IDE's will show red squiggles under code with compilation errors without requiring any action at all.

That's already provided by other tools than the compiler like rust analyzer.

This was meant as an example, not something I'd literally do.

The most common trivial change would be to add a dbg! statement.

Build systems usually don't care what you added. Modification of source means rebuild.

Yes, but the programmer knows what they added, and I was asking why they wanted to build again if all they added was a comment? If this was in a CI context then 10 seconds seems unimportant; compile times of 10 seconds are usually only complained about in the context of programmers who've gotten used to tiny edit-compile-test cycles.

In order to ensure the next rebuild on an actual code change doesn't need to compile this comment-changed file, I guess.