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\clone.rs:160:9
    |
160 |     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\partial_ord.rs:72:9
   |
72 |     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\partial_eq.rs:19:18
   |
19 |     fn cs_eq(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

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_builtin_macros\src\deriving\cmp\ord.rs:42:19
   |
42 | pub fn cs_cmp(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

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_builtin_macros\src\deriving\generic\mod.rs:917:13
    |
917 |         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:1406:13
     |
1406 |         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:1157:13
     |
1157 |         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:1103:13
     |
1103 |         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:1080:13
     |
1080 |         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:859:13
    |
859 |         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:805:13
    |
805 |         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:467:13
    |
467 |         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:457:13
    |
457 |         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

compiler/rustc_builtin_macros/src/deriving/clone.rs

@@ -157,14 +157,14 @@ fn cs_clone_simple(
 
 fn cs_clone(
     name: &str,
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
 ) -> BlockOrExpr {
     let ctor_path;
     let all_fields;
     let fn_path = cx.std_path(&[sym::clone, sym::Clone, sym::clone]);
-    let subcall = |cx: &mut ExtCtxt<'_>, field: &FieldInfo| {
+    let subcall = |cx: &ExtCtxt<'_>, field: &FieldInfo| {
         let args = thin_vec![field.self_expr.clone()];
         cx.expr_call_global(field.span, fn_path.clone(), args)
     };

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

@@ -39,7 +39,7 @@ pub fn expand_deriving_ord(
     trait_def.expand(cx, mitem, item, push)
 }
 
-pub fn cs_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
+pub fn cs_cmp(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
     let test_id = Ident::new(sym::cmp, span);
     let equal_path = cx.path_global(span, cx.std_path(&[sym::cmp, sym::Ordering, sym::Equal]));
     let cmp_path = cx.std_path(&[sym::cmp, sym::Ord, sym::cmp]);

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

@@ -16,7 +16,7 @@ pub fn expand_deriving_partial_eq(
     push: &mut dyn FnMut(Annotatable),
     is_const: bool,
 ) {
-    fn cs_eq(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
+    fn cs_eq(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
         let base = true;
         let expr = cs_fold(
             true, // use foldl

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

@@ -69,7 +69,7 @@ pub fn expand_deriving_partial_ord(
 }
 
 fn cs_partial_cmp(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     substr: &Substructure<'_>,
     tag_then_data: bool,

compiler/rustc_builtin_macros/src/deriving/decodable.rs

@@ -63,7 +63,7 @@ pub fn expand_deriving_rustc_decodable(
 }
 
 fn decodable_substructure(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
     krate: Symbol,
@@ -186,14 +186,14 @@ fn decodable_substructure(
 /// - `outer_pat_path` is the path to this enum variant/struct
 /// - `getarg` should retrieve the `usize`-th field with name `@str`.
 fn decode_static_fields<F>(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     outer_pat_path: ast::Path,
     fields: &StaticFields,
     mut getarg: F,
 ) -> P<Expr>
 where
-    F: FnMut(&mut ExtCtxt<'_>, Span, Symbol, usize) -> P<Expr>,
+    F: FnMut(&ExtCtxt<'_>, Span, Symbol, usize) -> P<Expr>,
 {
     match fields {
         Unnamed(fields, is_tuple) => {

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

@@ -330,7 +330,7 @@ pub enum SubstructureFields<'a> {
 /// Combine the values of all the fields together. The last argument is
 /// all the fields of all the structures.
 pub type CombineSubstructureFunc<'a> =
-    Box<dyn FnMut(&mut ExtCtxt<'_>, Span, &Substructure<'_>) -> BlockOrExpr + 'a>;
+    Box<dyn FnMut(&ExtCtxt<'_>, Span, &Substructure<'_>) -> BlockOrExpr + 'a>;
 
 pub fn combine_substructure(
     f: CombineSubstructureFunc<'_>,
@@ -454,7 +454,7 @@ fn find_type_parameters(
 impl<'a> TraitDef<'a> {
     pub fn expand(
         self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         mitem: &ast::MetaItem,
         item: &'a Annotatable,
         push: &mut dyn FnMut(Annotatable),
@@ -464,7 +464,7 @@ impl<'a> TraitDef<'a> {
 
     pub fn expand_ext(
         self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         mitem: &ast::MetaItem,
         item: &'a Annotatable,
         push: &mut dyn FnMut(Annotatable),
@@ -802,7 +802,7 @@ impl<'a> TraitDef<'a> {
 
     fn expand_struct_def(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         struct_def: &'a VariantData,
         type_ident: Ident,
         generics: &Generics,
@@ -856,7 +856,7 @@ impl<'a> TraitDef<'a> {
 
     fn expand_enum_def(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         enum_def: &'a EnumDef,
         type_ident: Ident,
         generics: &Generics,
@@ -914,7 +914,7 @@ impl<'a> TraitDef<'a> {
 impl<'a> MethodDef<'a> {
     fn call_substructure_method(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         type_ident: Ident,
         nonselflike_args: &[P<Expr>],
@@ -1077,7 +1077,7 @@ impl<'a> MethodDef<'a> {
     /// ```
     fn expand_struct_method_body<'b>(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'b>,
         struct_def: &'b VariantData,
         type_ident: Ident,
@@ -1100,7 +1100,7 @@ impl<'a> MethodDef<'a> {
 
     fn expand_static_struct_method_body(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         struct_def: &VariantData,
         type_ident: Ident,
@@ -1154,7 +1154,7 @@ impl<'a> MethodDef<'a> {
     /// `Unify`), and possibly a default arm.
     fn expand_enum_method_body<'b>(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'b>,
         enum_def: &'b EnumDef,
         type_ident: Ident,
@@ -1403,7 +1403,7 @@ impl<'a> MethodDef<'a> {
 
     fn expand_static_enum_method_body(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         enum_def: &EnumDef,
         type_ident: Ident,
@@ -1668,13 +1668,13 @@ pub enum CsFold<'a> {
 /// Statics may not be folded over.
 pub fn cs_fold<F>(
     use_foldl: bool,
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substructure: &Substructure<'_>,
     mut f: F,
 ) -> P<Expr>
 where
-    F: FnMut(&mut ExtCtxt<'_>, CsFold<'_>) -> P<Expr>,
+    F: FnMut(&ExtCtxt<'_>, CsFold<'_>) -> P<Expr>,
 {
     match substructure.fields {
         EnumMatching(.., all_fields) | Struct(_, all_fields) => {