compiler: fix few needless_pass_by_ref_mut clippy lints

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\deriving\mod.rs:120:9
    |
120 |     cx: &mut ExtCtxt<'_>,
    |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
    --> compiler\rustc_builtin_macros\src\deriving\generic\mod.rs:1573:13
     |
1573 |         cx: &mut ExtCtxt<'_>,
     |             ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
    --> compiler\rustc_builtin_macros\src\deriving\generic\mod.rs:1556:13
     |
1556 |         cx: &mut ExtCtxt<'_>,
     |             ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
    --> compiler\rustc_builtin_macros\src\deriving\generic\mod.rs:1463:13
     |
1463 |         cx: &mut ExtCtxt<'_>,
     |             ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
    --> compiler\rustc_builtin_macros\src\deriving\generic\mod.rs:1433:36
     |
1433 |     fn summarise_struct(&self, cx: &mut ExtCtxt<'_>, struct_def: &VariantData) -> StaticFields {
     |                                    ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\deriving\generic\mod.rs:953:13
    |
953 |         cx: &mut ExtCtxt<'_>,
    |             ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\deriving\generic\mod.rs:932:13
    |
932 |         cx: &mut ExtCtxt<'_>,
    |             ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\deriving\generic\mod.rs:580:13
    |
580 |         cx: &mut ExtCtxt<'_>,
    |             ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\deriving\generic\mod.rs:989:13
    |
989 |         cx: &mut ExtCtxt<'_>,
    |             ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\deriving\clone.rs:97:9
   |
97 |     cx: &mut ExtCtxt<'_>,
   |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\deriving\cmp\eq.rs:52:9
   |
52 |     cx: &mut ExtCtxt<'_>,
   |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\deriving\hash.rs:50:9
   |
50 |     cx: &mut ExtCtxt<'_>,
   |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\deriving\encodable.rs:150:9
    |
150 |     cx: &mut ExtCtxt<'_>,
    |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\deriving\default.rs:176:9
    |
176 |     cx: &mut ExtCtxt<'_>,
    |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\deriving\default.rs:106:9
    |
106 |     cx: &mut ExtCtxt<'_>,
    |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\deriving\default.rs:57:9
   |
57 |     cx: &mut ExtCtxt<'_>,
   |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\deriving\default.rs:84:9
   |
84 |     cx: &mut ExtCtxt<'_>,
   |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\deriving\debug.rs:212:9
    |
212 |     cx: &mut ExtCtxt<'_>,
    |         ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\deriving\debug.rs:48:26
   |
48 | fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
   |                          ^^^^^^^^^^^^^^^^ help: consider changing to: `&ExtCtxt<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

compiler/rustc_builtin_macros/src/deriving/clone.rs

@@ -94,7 +94,7 @@ pub fn expand_deriving_clone(
 
 fn cs_clone_simple(
     name: &str,
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
     is_union: bool,

compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs

@@ -49,7 +49,7 @@ pub fn expand_deriving_eq(
 }
 
 fn cs_total_eq_assert(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
 ) -> BlockOrExpr {

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -45,7 +45,7 @@ pub fn expand_deriving_debug(
     trait_def.expand(cx, mitem, item, push)
 }
 
-fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
+fn show_substructure(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
     // We want to make sure we have the ctxt set so that we can use unstable methods
     let span = cx.with_def_site_ctxt(span);
 
@@ -209,7 +209,7 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
 /// }
 /// ```
 fn show_fieldless_enum(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     def: &EnumDef,
     substr: &Substructure<'_>,

compiler/rustc_builtin_macros/src/deriving/default.rs

@@ -54,7 +54,7 @@ pub fn expand_deriving_default(
 }
 
 fn default_struct_substructure(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
     summary: &StaticFields,
@@ -81,7 +81,7 @@ fn default_struct_substructure(
 }
 
 fn default_enum_substructure(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     enum_def: &EnumDef,
 ) -> BlockOrExpr {
@@ -103,7 +103,7 @@ fn default_enum_substructure(
 }
 
 fn extract_default_variant<'a>(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     enum_def: &'a EnumDef,
     trait_span: Span,
 ) -> Result<&'a rustc_ast::Variant, ErrorGuaranteed> {
@@ -173,7 +173,7 @@ fn extract_default_variant<'a>(
 }
 
 fn validate_default_attribute(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     default_variant: &rustc_ast::Variant,
 ) -> Result<(), ErrorGuaranteed> {
     let attrs: SmallVec<[_; 1]> =

compiler/rustc_builtin_macros/src/deriving/encodable.rs

@@ -147,7 +147,7 @@ pub fn expand_deriving_rustc_encodable(
 }
 
 fn encodable_substructure(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
     krate: Symbol,

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -577,7 +577,7 @@ impl<'a> TraitDef<'a> {
     /// therefore does not get bound by the derived trait.
     fn create_derived_impl(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         type_ident: Ident,
         generics: &Generics,
         field_tys: Vec<P<ast::Ty>>,
@@ -929,7 +929,7 @@ impl<'a> MethodDef<'a> {
 
     fn get_ret_ty(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         generics: &Generics,
         type_ident: Ident,
@@ -950,7 +950,7 @@ impl<'a> MethodDef<'a> {
     //   `&self`.
     fn extract_arg_details(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         type_ident: Ident,
         generics: &Generics,
@@ -986,7 +986,7 @@ impl<'a> MethodDef<'a> {
 
     fn create_method(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         type_ident: Ident,
         generics: &Generics,
@@ -1430,7 +1430,7 @@ impl<'a> MethodDef<'a> {
 
 // general helper methods.
 impl<'a> TraitDef<'a> {
-    fn summarise_struct(&self, cx: &mut ExtCtxt<'_>, struct_def: &VariantData) -> StaticFields {
+    fn summarise_struct(&self, cx: &ExtCtxt<'_>, struct_def: &VariantData) -> StaticFields {
         let mut named_idents = Vec::new();
         let mut just_spans = Vec::new();
         for field in struct_def.fields() {
@@ -1460,7 +1460,7 @@ impl<'a> TraitDef<'a> {
 
     fn create_struct_patterns(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         struct_path: ast::Path,
         struct_def: &'a VariantData,
         prefixes: &[String],
@@ -1553,7 +1553,7 @@ impl<'a> TraitDef<'a> {
 
     fn create_struct_pattern_fields(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         struct_def: &'a VariantData,
         prefixes: &[String],
     ) -> Vec<FieldInfo> {
@@ -1570,7 +1570,7 @@ impl<'a> TraitDef<'a> {
 
     fn create_struct_field_access_fields(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         selflike_args: &[P<Expr>],
         struct_def: &'a VariantData,
         is_packed: bool,

compiler/rustc_builtin_macros/src/deriving/hash.rs

@@ -46,11 +46,7 @@ pub fn expand_deriving_hash(
     hash_trait_def.expand(cx, mitem, item, push);
 }
 
-fn hash_substructure(
-    cx: &mut ExtCtxt<'_>,
-    trait_span: Span,
-    substr: &Substructure<'_>,
-) -> BlockOrExpr {
+fn hash_substructure(cx: &ExtCtxt<'_>, trait_span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
     let [state_expr] = substr.nonselflike_args else {
         cx.dcx().span_bug(trait_span, "incorrect number of arguments in `derive(Hash)`");
     };

compiler/rustc_builtin_macros/src/deriving/mod.rs

@@ -117,7 +117,7 @@ fn call_unreachable(cx: &ExtCtxt<'_>, span: Span) -> P<ast::Expr> {
 }
 
 fn assert_ty_bounds(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     stmts: &mut ThinVec<ast::Stmt>,
     ty: P<ast::Ty>,
     span: Span,