Auto merge of #5599 - dtolnay:letif, r=flip1995

Downgrade useless_let_if_seq to nursery

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:

```rust
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.

```diff
- 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 (https://github.com/rust-lang/rust-clippy/issues/4124, https://github.com/rust-lang/rust-clippy/issues/3043, https://github.com/rust-lang/rust-clippy/issues/2918, https://github.com/rust-lang/rust-clippy/issues/2176) and suggestions that make the code worse (https://github.com/rust-lang/rust-clippy/issues/3769, https://github.com/rust-lang/rust-clippy/issues/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

clippy_lints/src/let_if_seq.rs

@@ -50,7 +50,7 @@ declare_clippy_lint! {
     /// };
     /// ```
     pub USELESS_LET_IF_SEQ,
-    style,
+    nursery,
     "unidiomatic `let mut` declaration followed by initialization in `if`"
 }
 

clippy_lints/src/lib.rs

@@ -1266,7 +1266,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&large_enum_variant::LARGE_ENUM_VARIANT),
         LintId::of(&len_zero::LEN_WITHOUT_IS_EMPTY),
         LintId::of(&len_zero::LEN_ZERO),
-        LintId::of(&let_if_seq::USELESS_LET_IF_SEQ),
         LintId::of(&let_underscore::LET_UNDERSCORE_LOCK),
         LintId::of(&lifetimes::EXTRA_UNUSED_LIFETIMES),
         LintId::of(&lifetimes::NEEDLESS_LIFETIMES),
@@ -1476,7 +1475,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&inherent_to_string::INHERENT_TO_STRING),
         LintId::of(&len_zero::LEN_WITHOUT_IS_EMPTY),
         LintId::of(&len_zero::LEN_ZERO),
-        LintId::of(&let_if_seq::USELESS_LET_IF_SEQ),
         LintId::of(&literal_representation::INCONSISTENT_DIGIT_GROUPING),
         LintId::of(&loops::EMPTY_LOOP),
         LintId::of(&loops::FOR_KV_MAP),
@@ -1728,6 +1726,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&floating_point_arithmetic::IMPRECISE_FLOPS),
         LintId::of(&floating_point_arithmetic::SUBOPTIMAL_FLOPS),
         LintId::of(&future_not_send::FUTURE_NOT_SEND),
+        LintId::of(&let_if_seq::USELESS_LET_IF_SEQ),
         LintId::of(&missing_const_for_fn::MISSING_CONST_FOR_FN),
         LintId::of(&mutable_debug_assertion::DEBUG_ASSERT_WITH_MUT_CALL),
         LintId::of(&mutex_atomic::MUTEX_INTEGER),

src/lintlist/mod.rs

@@ -2469,7 +2469,7 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
     },
     Lint {
         name: "useless_let_if_seq",
-        group: "style",
+        group: "nursery",
         desc: "unidiomatic `let mut` declaration followed by initialization in `if`",
         deprecation: None,
         module: "let_if_seq",