Fixes #10609: Adds lint to detect construction of unit struct using default

[Icxolu]

Using default to construct a unit struct increases code complexity and adds a function call. This can be avoided by simply removing the call to default and simply construct by name.

changelog: [default_constructed_unit_structs]: detects construction of unit structs using default

fixes #10609

[rustbot]

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[Manishearth]

Some of the tests fail.

Also, this suggestion from the tests seems incorrect:

     fn vals(_: Self) -> Self {
-        Self::default()
+        Self::()
     }

[Icxolu]

I updated the span to fix the broken suggestion and updated the uitest output to reflect this as well as the updated diagnostics message (forget about that in the last commit)

[Manishearth]

Tests still fail, since a bunch of the other tests use this syntax. You should allow this lint in those tests, or update the test code to not have this problem (if possible)

[Manishearth]

@bors r+

Thanks!

[bors]

📌 Commit 48ae5a0 has been approved by Manishearth

It is now in the queue for this repository.

[bors]

⌛ Testing commit 48ae5a0 with merge f9c1d15...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing f9c1d15 to master...