more clippy::style fixes:

get_first
single_char_add_str
unnecessary_mut_passed
manual_map
manual_is_ascii_check

compiler/rustc_ast/src/util/comments.rs

@@ -62,7 +62,7 @@ fn get_horizontal_trim(lines: &[&str], kind: CommentKind) -> Option<String> {
             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 @@ pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'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 @@ pub fn remove(&mut self, key: &K) -> Option<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_monomorphize/src/partitioning.rs

@@ -1041,10 +1041,7 @@ fn list(ns: &[usize]) -> String {
             }
             elem(curr, curr_count);
 
-            let mut s = "[".to_string();
-            s.push_str(&v.join(", "));
-            s.push_str("]");
-            s
+            format!("[{}]", v.join(", "))
         }
     };
 

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 @@ fn try_fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
                 mapped_regions,
                 mapped_types,
                 mapped_consts,
-                &mut self.universes,
+                &self.universes,
                 result,
             ))
         } else {
@@ -224,7 +224,7 @@ fn try_fold_const(&mut self, ct: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Sel
                 mapped_regions,
                 mapped_types,
                 mapped_consts,
-                &mut self.universes,
+                &self.universes,
                 result,
             ))
         } else {