more clippy fixes: clippy::{iter_cloned_collect, unwarp_or_else_default, option_map_or_none}

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -1317,7 +1317,7 @@ fn compare_number_of_generics<'tcx>(
                         impl_count,
                         kind,
                         pluralize!(impl_count),
-                        suffix.unwrap_or_else(String::new),
+                        suffix.unwrap_or_default(),
                     ),
                 );
             }

compiler/rustc_mir_transform/src/coverage/debug.rs

@@ -292,10 +292,8 @@ impl DebugCounters {
     }
 
     pub fn some_block_label(&self, operand: ExpressionOperandId) -> Option<&String> {
-        self.some_counters.as_ref().map_or(None, |counters| {
-            counters
-                .get(&operand)
-                .map_or(None, |debug_counter| debug_counter.some_block_label.as_ref())
+        self.some_counters.as_ref().and_then(|counters| {
+            counters.get(&operand).and_then(|debug_counter| debug_counter.some_block_label.as_ref())
         })
     }
 

compiler/rustc_transmute/src/layout/tree.rs

@@ -196,7 +196,7 @@ pub(crate) mod rustc {
         fn from(err: LayoutError<'tcx>) -> Self {
             match err {
                 LayoutError::Unknown(..) => Self::Unknown,
-                err @ _ => unimplemented!("{:?}", err),
+                err => unimplemented!("{:?}", err),
             }
         }
     }

src/librustdoc/clean/auto_trait.rs

@@ -141,7 +141,7 @@ where
         let f = auto_trait::AutoTraitFinder::new(tcx);
 
         debug!("get_auto_trait_impls({:?})", ty);
-        let auto_traits: Vec<_> = self.cx.auto_traits.iter().copied().collect();
+        let auto_traits: Vec<_> = self.cx.auto_traits.to_vec();
         let mut auto_traits: Vec<Item> = auto_traits
             .into_iter()
             .filter_map(|trait_def_id| {