Downgrade useless_let_if_seq to nursery

[dtolnay]

I feel that this lint has the wrong balance of incorrect suggestions for a default-enabled lint.

The immediate code I faced was something like:

fn main() {
    let mut good = do1();
    if !do2() {
        good = false;
    }
    if good {
        println!("good");
    }
}

fn do1() -> bool { println!("1"); false }
fn do2() -> bool { println!("2"); false }

On this code Clippy calls it unidiomatic and suggests the following diff, which has different behavior in a way that I don't necessarily want.

- let mut good = do1();
- if !do2() {
-     good = false;
- }
+ let good = if !do2() {
+     false
+ } else {
+     do1()
+ };

On exploring issues filed about this lint, I have found that other users have also struggled with inappropriate suggestions (#4124, #3043, #2918, #2176) and suggestions that make the code worse (#3769, #2749). Overall I believe that this lint is still at nursery quality for now and should not be enabled.

---

changelog: Remove useless_let_if_seq from default set of enabled lints

[flip1995]

@bors r+

Thanks!

[bors]

📌 Commit 95399f8 has been approved by flip1995

[bors]

⌛ Testing commit 95399f8 with merge 775e8f4...

[bors]

💥 Test timed out

[dtolnay]

Can't tell what failed; I think this needs to be retried.

[matthiaskrgr]

@bors retry

[bors]

⌛ Testing commit 95399f8 with merge cac9ad0...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: flip1995
Pushing cac9ad0 to master...