Fixes 5167
When ``a.rs`` and ``a/mod.rs`` are both present we would emit an error
message telling the user that the module couldn't be found. However,
this behavior is misleading because we're dealing with an ambiguous
module path, not a "file not found" error.
Is the file ``a.rs`` or is it ``a/mod.rs``? Rustfmt can't decide, and
the user needs to resolve this ambiguity themselves.
Now, the error message displayed to the user is in line with what they
would see if they went to compile their code with these conflicting
module names.
We only want to fall back if two conditions are met:
1) Initial module resolution is performed relative to some nested
directory.
2) Module resolution fails because of a ModError::FileNotFound error.
When these conditions are met we can try to fallback to searching for
the module's file relative to the dir_path instead of the nested
relative directory.
Fixes 5198
As demonstrated by 5198, it's possible that a directory name conflicts
with a rust file name. For example, src/lib/ and src/lib.rs.
If src/lib.rs references an external module like ``mod foo;``, then
module resolution will try to resolve ``foo`` to src/lib/foo.rs or
src/lib/foo/mod.rs. Module resolution would fail with a file not
found error if the ``foo`` module were defined at src/foo.rs.
When encountering these kinds of module resolution issues we now fall
back to the current directory and attempt to resolve the module again.
Given the current example, this means that if we can't find the module
``foo`` at src/lib/foo.rs or src/lib/foo/mod.rs, we'll attempt
to resolve the module to src/foo.rs.
Fixes 5119
When the directory structure was laid out as follows:
```
dir
|---mod_a
| |---sub_mod_1.rs
| |---sub_mod_2.rs
|---mod_a.rs
```
And ``mod_a.rs`` contains the following content:
```rust
mod mod_a {
mod sub_mod_1;
mod sub_mod_2;
}
```
rustfmt previously tried to find ``sub_mod_1.rs`` and ``sub_mod_2.rs``
in ``./mod_a/mod_a/``. This directory does not exist and this caused
rustfmt to fail with the error message:
Error writing files: failed to resolve mod
Now, both ``sub_mod_1.rs`` and ``sub_mod_2.rs`` are resolved correctly
and found at ``mod_a/sub_mod_1.rs`` and ``mod_a/sub_mod_2.rs``.
Although the implementation is slightly different than the original PR,
the general idea is the same. After collecting all modules we want to
exclude formatting those that contain the #![rustfmt::skip] attribute.
