fix some clippy::style findings

comparison_to_empty
iter_nth_zero
for_kv_map
manual_next_back
redundant_pattern

compiler/rustc_hir_typeck/src/demand.rs

@@ -621,7 +621,7 @@ fn annotate_expected_due_to_let_ty(
                     // is in a different line, so we point at both.
                     err.span_label(secondary_span, "expected due to the type of this binding");
                     err.span_label(primary_span, format!("expected due to this{post_message}"));
-                } else if post_message == "" {
+                } else if post_message.is_empty() {
                     // We are pointing at either the assignment lhs or the binding def pattern.
                     err.span_label(primary_span, "expected due to the type of this binding");
                 } else {

compiler/rustc_infer/src/infer/region_constraints/leak_check.rs

@@ -425,9 +425,11 @@ fn iterate_region_constraints(
                 }
             }
         } else {
-            for (constraint, _origin) in &region_constraints.data().constraints {
-                each_constraint(constraint)
-            }
+            region_constraints
+                .data()
+                .constraints
+                .keys()
+                .for_each(|constraint| each_constraint(constraint));
         }
     }
 

compiler/rustc_lexer/src/unescape.rs

@@ -372,7 +372,7 @@ fn skip_ascii_whitespace<F>(chars: &mut Chars<'_>, start: usize, callback: &mut
         callback(start..end, EscapeError::MultipleSkippedLinesWarning);
     }
     let tail = &tail[first_non_space..];
-    if let Some(c) = tail.chars().nth(0) {
+    if let Some(c) = tail.chars().next() {
         if c.is_whitespace() {
             // For error reporting, we would like the span to contain the character that was not
             // skipped. The +1 is necessary to account for the leading \ that started the escape.

compiler/rustc_monomorphize/src/partitioning.rs

@@ -1229,12 +1229,13 @@ fn dump_mono_items_stats<'tcx>(
     // Gather instantiated mono items grouped by def_id
     let mut items_per_def_id: FxHashMap<_, Vec<_>> = Default::default();
     for cgu in codegen_units {
-        for (&mono_item, _) in cgu.items() {
+        cgu.items()
+            .keys()
             // Avoid variable-sized compiler-generated shims
-            if mono_item.is_user_defined() {
+            .filter(|mono_item| mono_item.is_user_defined())
+            .for_each(|mono_item| {
                 items_per_def_id.entry(mono_item.def_id()).or_default().push(mono_item);
-            }
-        }
+            });
     }
 
     #[derive(serde::Serialize)]
@@ -1287,7 +1288,7 @@ fn codegened_and_inlined_items(tcx: TyCtxt<'_>, (): ()) -> &DefIdSet {
     let mut result = items.clone();
 
     for cgu in cgus {
-        for (item, _) in cgu.items() {
+        for item in cgu.items().keys() {
             if let MonoItem::Fn(ref instance) = item {
                 let did = instance.def_id();
                 if !visited.insert(did) {

compiler/rustc_parse/src/lexer/unescape_error_reporting.rs

@@ -27,7 +27,7 @@ pub(crate) fn emit_unescape_error(
         lit, span_with_quotes, mode, range, error
     );
     let last_char = || {
-        let c = lit[range.clone()].chars().rev().next().unwrap();
+        let c = lit[range.clone()].chars().next_back().unwrap();
         let span = span.with_lo(span.hi() - BytePos(c.len_utf8() as u32));
         (c, span)
     };

compiler/rustc_ty_utils/src/implied_bounds.rs

@@ -51,7 +51,7 @@ fn assumed_wf_types<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> &'tcx [(Ty<'
             // assumed_wf_types should include those of `Opaque<T>`, `Opaque<T>` itself
             // and `&'static T`.
             DefKind::OpaqueTy => bug!("unimplemented implied bounds for nested opaque types"),
-            def_kind @ _ => {
+            def_kind => {
                 bug!("unimplemented implied bounds for opaque types with parent {def_kind:?}")
             }
         },