suggest declaring modules when file found but module not defined

[bend-n]

suggests declaring modules when a module is found but not defined, i.e

├── main.rs: `use thing::thang;`
└── thing.rs: `struct thang`

or

├── main.rs: `use thing::thang;`
└── thing
    └── mod.rs: `struct thang`

which currently is just

error[E0432]: unresolved import `yeah`
 --> src/main.rs:1:1
  |
1 | use thing::thang;
  |     ^^^^^ use of unresolved module or unlinked crate `thing`
  |

but now would have this nice help:

= help: you may have forgotten to declare the module `thing`. use `mod thing` in this file to declare this module.

[bend-n]

@rustbot label A-diagnostics

[rustbot]

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[petrochenkov]

Could you also add a new test exercising the various naming schemes (like …/x/y/mod.rs and others) for the module file?

[bend-n]

Could you also add a new test exercising the various naming schemes (like …/x/y/mod.rs and others) for the module file?

im not sure how to make a test with a normal directory structure? everything seems to use #[path = auxiliary/]...

[petrochenkov]

Well, tests/ui/modules_and_files_visibility/mod_file_disambig.rs caught some file to report, the new test can probably do something similar to that case.

[bend-n]

@rustbot ready

[petrochenkov]

I think with this file you don't need //@ ignore-auxiliary annotations?
(Or vice versa, with //@ ignore-auxiliary annotations you don't need this file)

[petrochenkov]

This wasn't actually addressed for tests/ui/modules/module_suggestion_when_module_not_found/submodule/compiletest-ignore-dir, only for tests/ui/modules/module_suggestion_when_module_not_found/success/compiletest-ignore-dir.

[bend-n]

@rustbot ready

[petrochenkov]

r=me with #142730 (comment) addressed.
@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.