"cannot declare a new module at this ___location" should specify good locations

[durka]

The error has two notes full of weasel words (2x "maybe", 1x "possibly") and it should probably say something straightforward like "modules are declared at the crate root, or mod.rs at the root of a source directory".

[igilham]

This is a particularly annoying error message as it fails to tell the user what the problem is, leading to the impression that modules only work when some randomly shifting set of undocumented requirements are met.

It also fails to offer any useful suggestions when refactoring and moving code around.

The prevailing term that comes up when searching for this issue, "from the root", is also misleading as it appears to mean the root file (main.rs, lib.rs), not the root or src directory in the project.

[fmease]

Triage: 9 years later I had to reverse engineer what diagnostic was even talked about here (given the lack of version and reproducer). The setup was:

• mod_file_not_owning.rs:

  mod mod_file_not_owning_aux1;
  
  fn main() {}

• mod_file_not_owning_aux1.rs:

  mod mod_file_not_owning_aux2;

• mod_file_not_owning_aux2.rs: empty

Version 1.13 outputs:

error: cannot declare a new module at this ___location
 --> mod_file_not_owning_aux1.rs:1:5
  |
1 | mod mod_file_not_owning_aux2;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: maybe move this module `mod_file_not_owning_aux1` to its own directory via `mod_file_not_owning_aux1/mod.rs`
 --> mod_file_not_owning_aux1.rs:1:5
  |
1 | mod mod_file_not_owning_aux2;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^
note: ... or maybe `use` the module `mod_file_not_owning_aux2` instead of possibly redeclaring it
 --> mod_file_not_owning_aux1.rs:1:5
  |
1 | mod mod_file_not_owning_aux2;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^

Output with 1.90 (nightly) is:

error[E0583]: file not found for module `mod_file_not_owning_aux2`
 --> mod_file_not_owning_aux1.rs:1:1
  |
1 | mod mod_file_not_owning_aux2;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: to create the module `mod_file_not_owning_aux2`, create file "mod_file_not_owning_aux1/mod_file_not_owning_aux2.rs" or "mod_file_not_owning_aux1/mod_file_not_owning_aux2/mod.rs"
  = note: if there is a `mod mod_file_not_owning_aux2` elsewhere in the crate already, import it with `use crate::...` instead

I'd say this is fixed.