Auto merge of rust-lang#9302 - Jarcho:sig_drop_nursery, r=flip1995

Move `significant_drop_in_scrutinee` into `nursey`

The current suggestion of extending the lifetime of every sub-expression is not great and doesn't fix the error given in the lint's example, though it does make the potential deadlock easier to see, but it can also cause it's own issues by delaying the drop of the lock guard.

e.g.
```rust
match x.lock().foo {
    ..
}
// some stuff
let y = x.lock();
```
The suggestion would create a deadlock at the second `x.lock()` call.

This also lints even when a significant drop type isn't created as a temporary. (rust-lang#9072)

I agree `@kpreid` (rust-lang/rust-clippy#8987 (comment)) that this should be back-ported before the lint hits stable.

changelog: Move `significant_drop_in_scrutinee` into `nursey`

Author: bors

clippy_lints/src/lib.register_all.rs

@@ -144,7 +144,6 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(matches::MATCH_STR_CASE_MISMATCH),
     LintId::of(matches::NEEDLESS_MATCH),
     LintId::of(matches::REDUNDANT_PATTERN_MATCHING),
-    LintId::of(matches::SIGNIFICANT_DROP_IN_SCRUTINEE),
     LintId::of(matches::SINGLE_MATCH),
     LintId::of(matches::WILDCARD_IN_OR_PATTERNS),
     LintId::of(mem_replace::MEM_REPLACE_OPTION_WITH_NONE),

clippy_lints/src/lib.register_nursery.rs

@@ -13,6 +13,7 @@ store.register_group(true, "clippy::nursery", Some("clippy_nursery"), vec![
     LintId::of(future_not_send::FUTURE_NOT_SEND),
     LintId::of(index_refutable_slice::INDEX_REFUTABLE_SLICE),
     LintId::of(let_if_seq::USELESS_LET_IF_SEQ),
+    LintId::of(matches::SIGNIFICANT_DROP_IN_SCRUTINEE),
     LintId::of(methods::ITER_WITH_DRAIN),
     LintId::of(missing_const_for_fn::MISSING_CONST_FOR_FN),
     LintId::of(mutable_debug_assertion::DEBUG_ASSERT_WITH_MUT_CALL),

clippy_lints/src/lib.register_suspicious.rs

@@ -22,7 +22,6 @@ store.register_group(true, "clippy::suspicious", Some("clippy_suspicious"), vec!
     LintId::of(loops::EMPTY_LOOP),
     LintId::of(loops::FOR_LOOPS_OVER_FALLIBLES),
     LintId::of(loops::MUT_RANGE_BOUND),
-    LintId::of(matches::SIGNIFICANT_DROP_IN_SCRUTINEE),
     LintId::of(methods::NO_EFFECT_REPLACE),
     LintId::of(methods::SUSPICIOUS_MAP),
     LintId::of(mut_key::MUTABLE_KEY_TYPE),

clippy_lints/src/matches/mod.rs

@@ -835,7 +835,7 @@ declare_clippy_lint! {
     /// ```
     #[clippy::version = "1.60.0"]
     pub SIGNIFICANT_DROP_IN_SCRUTINEE,
-    suspicious,
+    nursery,
     "warns when a temporary of a type with a drop with a significant side-effect might have a surprising lifetime"
 }