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> Interaction with partial, in-progress programs. People wanted to be able to comment out `mod` declarations to temporarily exclude files from the build.

Except the current system actually has the opposite effect -- working with partial programs is actually worse from the standpoint of using rust analyzer. Newbies like myself start writing a new source file and expect that completion and other features will work right away.

> Interaction with version control, e.g. `git stash`. Similar to the above, people want to exclude untracked files from the build when their corresponding `mod` declarations are stashed.

They can just stash the file as well.

This strikes me as making the whole system harder for new people to understand just to smooth over edge cases for power users that definitely have the ability to come up with workarounds.

Note that rust-analyzer could choose to load un-`mod`-ed files with no change to the language.

At a minimum, this could be an off-by-default configuration. Ideally, it would suppress errors caused by these extra files, and downgrade errors caused by missing `mod` declarations to warnings, with a quick-fix to add the declaration.

> They can just stash the file as well.

This is an option, but it's not clear-cut whether it's a better one. See my other comment: https://news.ycombinator.com/item?id=23890842

> This strikes me as making the whole system harder for new people to understand just to smooth over edge cases for power users that definitely have the ability to come up with workarounds.

This certainly was not the goal. The goal of the 2018 module system changes in general was to make the module system easier to learn than it was before; this aspect was simply retained as-is because we couldn't figure out how to make it better without making existing aspects worse.

>This strikes me as making the whole system harder for new people to understand just to smooth over edge cases for power users that definitely have the ability to come up with workarounds.

This is exactly what it is, and due to inertia it won't ever change. At this point, there is no other choice but to either accept it or find peace in smth else.

There's still some interest in fixing it in a future edition, if we can find the right design that won't create a substantial amount of churn or pitfalls.

The 2018 module system was already a substantial redesign, and because of that, we chose to carefully minimize churn to existing programs (which involved two iterations on the design before the 2018 edition released); the vast majority of programs designed for Rust 2015 will Just Work with the 2018 module system.

A future edition could very carefully allow for filesystem-based modules, and focus primarily on that without having to deal with other module system changes at the same time. I can imagine some possibilities that might achieve that, if someone has the bandwidth and diligence to work on it and test it.

> which involved two iterations on the design before the 2018 edition released

Just to be clear, I have the highest respect fo the Rust core team, I've followed some discussions and you guys are going above and beyond carefully weighing all the pros and cons, often to an exhaustive degree.

I feel the main strength of Rust is also its biggest weakness: alway striving for the perfect, platonic ideal solution, and going the extra mile to get there, which then might discard a much simpler but slightly less powerful solution. Often thats a good thing, not sure it was warranted for the module system. I remember there were different opinions in the core team, Aaron being a proponent of the simple version I think.

I don't understand the desire to exclude files using mod. If you are including code from that file via use, then your build will still be broken. And if you're not, then the files would be excluded from the build anyway.

Think about the `git stash` use case. The goal is to go from a messy, WIP state that may or may not compile, back to a more stable, probably-working state represented by a commit.

That messy state may include a new, partially-implemented module file. By default `git stash` will leave it alone, because it's untracked. But `git stash` will also remove its corresponding `mod` declaration, and any uses of it, all at once.

Now say you want to build and run. If that WIP module file gets picked up automatically, it will probably break the build! It may simply be incomplete, or it may depend on some of those stashed changes, or whatever else. Even if it happens to typecheck, now you're assuming that it's all dead code- but it may have some public API (including trait impls!), and still be included in the build!

In fact, we already have a taste of this kind of problem, ever since Cargo started running build.rs scripts automatically, even without a `package.build` key in `Cargo.toml`. If you add a build script with some build-dependencies, run `git stash`, and then build, the build script will fail to compile!

Perhaps you could argue that this is a deficiency in `git stash` that Rust shouldn't have to be concerned with. But if `git stash` removed untracked files as well, that would simply break other use cases instead! For example, I often have a few extra files laying around, which I don't ever intend to commit or .gitignore- sample inputs or outputs, short-term todo lists, or other scratch-pad-like data. Ideally `git stash` would leave those alone- but it's hard for git to tell those apart from the problematic cases above.

If you run git stash with the -u option (`git stash -u`), then it will also stash untracked files. IMO leaving uncommitted/unignored files around is asking for trouble. It's far too easy to accidentally delete or commit them. Why not have a prefix that you use for these files, and use a wildcard gitignore?

I'm perfectly aware of `git stash -u` ("by default") as well as `git add -N`, etc, but as I also very intentionally pointed out, that has its own downsides.

That is, my own personal workflow poses essentially zero risk of deleting or committing those files, so "quit using the file system the way I've always used it" is no less of a stumbling block for me than "maintain `mod` declarations" is for you.

We simply haven't found an approach that doesn't step on someone's toes, so the one that Rust was already using from the 2015 edition stuck.

Hmm... would you object to `use` automatically linking in modules without a `mod` declaration? It seems that that would work in the situations with regard to stashing (a git stash that stashed `mod` statements would also stash `use` statements).

FWIW, if you have a new file in WIP state and have changes related to it that you want to stash it might be better to `git add` it so that it gets stashed/unstashed together with the other related changes.

> And if you're not, then the files would be excluded from the build anyway.

They aren't though. Even if the module isn't `use`d, it's still compiled.

(And it has to be. If nothing else, there could be a trait impl buried in the module, which gets shared without `use`.)

It doesn't have to be. It could default to be compiled if used and not being compiled unless use'd. And in the rare case that a trait impl or similar in a module without importing from that module, then you could use `mod`.

> then the files would be excluded from the build anyway

Why would this be the case? Under this model, are we not allowed to have public symbols anywhere in a library but at the root? In other words, why should I have to "use" something from a module from elsewhere in this crate to get it to build?

I'm already hopelessly confused by your system. The current model is simple, correct, and efficient. No wonder people hate it so much - it's human nature to hate the unfamiliar!

Yes, I know — I’m criticizing the article, not the module system. None of the reasons you mention are demonstrated in the example given, which would be much more straightforward if the module system just followed the filesystem automatically, as the author rightly points out beginners tend to expect it to. Asking people to have an “open mind” and then just showing them the complicated way to do a thing they expect to be simple isn’t very motivating.