Suggest to move/copy use statement from parent into inline module declaration

[Mark-Simulacrum]

Given the below Rust code, we can probably produce a more user friendly error message than today's "failed to resolve. Use of undeclared type or module File".

use std::fs::File;

mod foo {
    fn some_function(f: File) {}
}

It'd be nice to suggest something like "Possibly move use std::fs::File from outer scope into module foo". This will help beginners understand how to deal with the File use in the parent module not being in scope within the child module.

[arielb1]

There is supposed to be a hint. cc @jseyfried.

[jseyfried]

@Mark-Simulacrum I think like you're using an old rustc -- that hasn't been the error since 1.14 or earlier. The error on beta and nightly is:

error[E0412]: cannot find type `File` in this scope
 --> example.rs:4:25
  |
4 |     fn some_function(f: File) {}
  |                         ^^^^ not found in this scope
  |
  = help: possible candidate is found in another module, you can import it into scope:
            `use std::fs::File;`

Once we add use std::fs::File; as suggested, the the top-level import becomes an unused import warning.

cc @petrochenkov

[petrochenkov]

I think @Mark-Simulacrum means that the "you can import it into scope" may be useless if beginner assumes that the parent module scope extends to children modules. "Why does it tell me File is not in scope? I've just written use std::fs::File; and imported it!"

I'm not sure this is important enough to implement, but in principle we can detect the situation when some name isn't present in this scope, but presents in some parent module scope, and enlighten the user on the basics of the module system.

(It looks like the wording "failed to resolve. Use of undeclared type or module X" was never used in this context (at least since 1.0), so I suspect @Mark-Simulacrum copy-pasted it from somewhere else and is not necessarily using an old compiler)

[Mark-Simulacrum]

Yes; I copy-pasted the message from IRC; I assumed that it was a direct quote.

I agree with the sentiment that beginners may not understand why it isn't imported, so providing an error message for that case would probably be a good idea.

[durka]

I always forget to do this when writing inline test modules as well. But in that case you usually(?) want simply use super::*;.