Add unneeded-wildcard-pattern lint

[ghost]

changelog: Add unneeded-wildcard-pattern lint

[flip1995]

Implementation LGTM.

But I'm concerned, that _, ../.., _ and .. isn't quite the same. While the _, .. means, there is at least one more variant in this struct/tuple, .. means, there are 0 or more variants in this struct/tuple:

struct TS(u32, u32);

fn main() {
    let a = TS(1, 2);
    match a {
        TS(_, _, ..) => (),
    };
}

compiles just fine.

If we'd remove a field from the struct, this code would error, while the .. variant, would still compile. I don't think, this could really lead to an error, since the fields aren't used anyway, but I would add the difference between _, .. and .. at least as a note to the lint documentation (not necessarily to the Known Problems section).

[ghost]

I made all the changes suggested apart from adding a note between the difference between .. and .., _. I still thinking about how best to do that.

If anyone wants to fork this branch and finish that off, I'm happy to pull the changes. (Maintainers can, of course, just push to this branch directly.)

[flip1995]

if let (_, .., 2, 3) = t {} doesn't get linted, does it? Can you add a test for it? This could actually lead to refactoring errors, because with the _, .. it means, there is at least one element before the 2.

[ghost]

It absolutely will. This is the purpose of this lint.
What errors could this lead to and how is this different from the case (0, .., _, _) →(0, ..)?

[flip1995]

I can't remember what I had in mind. And I also cannot come up with something. Can you add a test for it though?

[flip1995]

Only a test for the _, .., 0 case is missing.

[bors]

☔ The latest upstream changes (presumably #4511) made this pull request unmergeable. Please resolve the merge conflicts.

[phansch]

@bors r+

[bors]

📌 Commit ca6d36b has been approved by phansch

[bors]

⌛ Testing commit ca6d36b with merge c23b375...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: phansch
Pushing c23b375 to master...