why not derive(Debug) by default

[clouds56]

and use attribute #[nodebug] to specific others?

in the document, it says

Format trait for the :? format. Useful for debugging, all types should implement this.
Generally speaking, you should just derive a Debug implementation.

but sometimes missing it, and I could only manually add it package by package.

[abonander]

At this point it'd be too widespread of a breaking change to auto-derive Debug by default, because basically all crates would have to be updated.

This would work great as a note or warning lint, though. CC @Manishearth about the viability of this as an idea for clippy.

[Manishearth]

It's possible as a lint (if all children derive trait Foo, lint that you should too), but perhaps should be Allow. derive ing everything isn't always necessary.

[steveklabnik]

making a change like this, as @cybergeek94 mentions, would require an RFC. Please pursue one if you're interested in making this, but yeah, there are backwards compatibility challenges here.

[omega-tree]

Now that we have epochs, Please revisit this!