couple of clippy::style changes

compiler/rustc_ast/src/util/comments.rs

@@ -62,7 +62,7 @@ pub fn beautify_doc_string(data: Symbol, kind: CommentKind) -> Symbol {
             CommentKind::Block => {
                 // Whatever happens, we skip the first line.
                 let mut i = lines
-                    .get(0)
+                    .first()
                     .map(|l| if l.trim_start().starts_with('*') { 0 } else { 1 })
                     .unwrap_or(0);
                 let mut j = lines.len();

compiler/rustc_ast_lowering/src/expr.rs

@@ -286,7 +286,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ExprKind::OffsetOf(container, fields) => hir::ExprKind::OffsetOf(
                     self.lower_ty(
                         container,
-                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::OffsetOf),
+                        &ImplTraitContext::Disallowed(ImplTraitPosition::OffsetOf),
                     ),
                     self.arena.alloc_from_iter(fields.iter().map(|&ident| self.lower_ident(ident))),
                 ),

compiler/rustc_ast_pretty/src/pprust/state/expr.rs

@@ -697,15 +697,15 @@ pub fn reconstruct_format_args_template_string(pieces: &[FormatArgsPiece]) -> St
                 write!(template, "{n}").unwrap();
                 if p.format_options != Default::default() || p.format_trait != FormatTrait::Display
                 {
-                    template.push_str(":");
+                    template.push(':');
                 }
                 if let Some(fill) = p.format_options.fill {
                     template.push(fill);
                 }
                 match p.format_options.alignment {
-                    Some(FormatAlignment::Left) => template.push_str("<"),
-                    Some(FormatAlignment::Right) => template.push_str(">"),
-                    Some(FormatAlignment::Center) => template.push_str("^"),
+                    Some(FormatAlignment::Left) => template.push('<'),
+                    Some(FormatAlignment::Right) => template.push('>'),
+                    Some(FormatAlignment::Center) => template.push('^'),
                     None => {}
                 }
                 match p.format_options.sign {

compiler/rustc_data_structures/src/sso/map.rs

@@ -268,11 +268,7 @@ impl<K: Eq + Hash, V> SsoHashMap<K, V> {
     pub fn remove_entry(&mut self, key: &K) -> Option<(K, V)> {
         match self {
             SsoHashMap::Array(array) => {
-                if let Some(index) = array.iter().position(|(k, _v)| k == key) {
-                    Some(array.swap_remove(index))
-                } else {
-                    None
-                }
+                array.iter().position(|(k, _v)| k == key).map(|index| array.swap_remove(index))
             }
             SsoHashMap::Map(map) => map.remove_entry(key),
         }

compiler/rustc_hir/src/hir.rs

@@ -3013,8 +3013,7 @@ pub struct FieldDef<'hir> {
 impl FieldDef<'_> {
     // Still necessary in couple of places
     pub fn is_positional(&self) -> bool {
-        let first = self.ident.as_str().as_bytes()[0];
-        (b'0'..=b'9').contains(&first)
+        self.ident.as_str().as_bytes()[0].is_ascii_digit()
     }
 }
 

compiler/rustc_hir_typeck/src/demand.rs

@@ -621,7 +621,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // 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 @@ impl<'tcx> MiniGraph<'tcx> {
                 }
             }
         } 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 @@ where
         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

@@ -1041,10 +1041,7 @@ fn debug_dump<'a, 'tcx: 'a>(tcx: TyCtxt<'tcx>, label: &str, cgus: &[CodegenUnit<
             }
             elem(curr, curr_count);
 
-            let mut s = "[".to_string();
-            s.push_str(&v.join(", "));
-            s.push_str("]");
-            s
+            format!("[{}]", v.join(", "))
         }
     };
 
@@ -1229,12 +1226,13 @@ 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 +1285,7 @@ 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_symbol_mangling/src/typeid/typeid_itanium_cxx_abi.rs

@@ -400,7 +400,7 @@ fn encode_ty_name(tcx: TyCtxt<'_>, def_id: DefId) -> String {
         let _ = write!(s, "{}", name.len());
 
         // Prepend a '_' if name starts with a digit or '_'
-        if let Some(first) = name.as_bytes().get(0) {
+        if let Some(first) = name.as_bytes().first() {
             if first.is_ascii_digit() || *first == b'_' {
                 s.push('_');
             }

compiler/rustc_trait_selection/src/solve/normalize.rs

@@ -194,7 +194,7 @@ impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for NormalizationFolder<'_, 'tcx> {
                 mapped_regions,
                 mapped_types,
                 mapped_consts,
-                &mut self.universes,
+                &self.universes,
                 result,
             ))
         } else {
@@ -224,7 +224,7 @@ impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for NormalizationFolder<'_, 'tcx> {
                 mapped_regions,
                 mapped_types,
                 mapped_consts,
-                &mut self.universes,
+                &self.universes,
                 result,
             ))
         } else {

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:?}")
             }
         },