Switch to anon-const for wrapping derived impls

Fixes a non_local_definitions warning in the derived code.

    warning: non-local `impl` definition, they should be avoided as they go against expectation
     --> tests/test_derive.rs:5:28
      |
    5 | #[derive(PartialEq, Debug, Serialize, Deserialize)]
      |                            ^^^^^^^^^
      |
      = help: move this `impl` block outside the of the current constant `_IMPL_MINISERIALIZE_FOR_Tag`
      = note: an `impl` definition is non-local if it is nested inside an item and neither the type nor the trait are at the same nesting level as the `impl` block
      = note: one exception to the rule are anon-const (`const _: () = { ... }`) at top-level module and anon-const at the same nesting as the trait or type
      = note: this lint may become deny-by-default in the edition 2024 and higher, see the tracking issue <rust-lang/rust#120363>
      = note: the derive macro `Serialize` may come from an old version of the `mini_internal` crate, try updating your dependency with `cargo update -p mini_internal`
      = note: `#[warn(non_local_definitions)]` on by default
      = note: this warning originates in the derive macro `Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)

derive/src/de.rs

@@ -2,7 +2,7 @@ use crate::{attr, bound};
 use proc_macro2::{Span, TokenStream};
 use quote::quote;
 use syn::{
-    parse_quote, Data, DataEnum, DataStruct, DeriveInput, Error, Fields, FieldsNamed, Ident, Result,
+    parse_quote, Data, DataEnum, DataStruct, DeriveInput, Error, Fields, FieldsNamed, Result,
 };
 
 pub fn derive(input: DeriveInput) -> Result<TokenStream> {
@@ -26,10 +26,6 @@ pub fn derive(input: DeriveInput) -> Result<TokenStream> {
 pub fn derive_struct(input: &DeriveInput, fields: &FieldsNamed) -> Result<TokenStream> {
     let ident = &input.ident;
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
-    let dummy = Ident::new(
-        &format!("_IMPL_MINIDESERIALIZE_FOR_{}", ident),
-        Span::call_site(),
-    );
 
     let fieldname = fields.named.iter().map(|f| &f.ident).collect::<Vec<_>>();
     let fieldty = fields.named.iter().map(|f| &f.ty);
@@ -46,7 +42,7 @@ pub fn derive_struct(input: &DeriveInput, fields: &FieldsNamed) -> Result<TokenS
 
     Ok(quote! {
         #[allow(non_upper_case_globals)]
-        const #dummy: () = {
+        const _: () = {
             #[repr(C)]
             struct __Visitor #impl_generics #where_clause {
                 __out: miniserde::__private::Option<#ident #ty_generics>,
@@ -117,10 +113,6 @@ pub fn derive_enum(input: &DeriveInput, enumeration: &DataEnum) -> Result<TokenS
     }
 
     let ident = &input.ident;
-    let dummy = Ident::new(
-        &format!("_IMPL_MINIDESERIALIZE_FOR_{}", ident),
-        Span::call_site(),
-    );
 
     let var_idents = enumeration
         .variants
@@ -141,7 +133,7 @@ pub fn derive_enum(input: &DeriveInput, enumeration: &DataEnum) -> Result<TokenS
 
     Ok(quote! {
         #[allow(non_upper_case_globals)]
-        const #dummy: () = {
+        const _: () = {
             #[repr(C)]
             struct __Visitor {
                 __out: miniserde::__private::Option<#ident>,

derive/src/ser.rs

@@ -2,7 +2,7 @@ use crate::{attr, bound};
 use proc_macro2::{Span, TokenStream};
 use quote::quote;
 use syn::{
-    parse_quote, Data, DataEnum, DataStruct, DeriveInput, Error, Fields, FieldsNamed, Ident, Result,
+    parse_quote, Data, DataEnum, DataStruct, DeriveInput, Error, Fields, FieldsNamed, Result,
 };
 
 pub fn derive(input: DeriveInput) -> Result<TokenStream> {
@@ -26,10 +26,6 @@ pub fn derive(input: DeriveInput) -> Result<TokenStream> {
 fn derive_struct(input: &DeriveInput, fields: &FieldsNamed) -> Result<TokenStream> {
     let ident = &input.ident;
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
-    let dummy = Ident::new(
-        &format!("_IMPL_MINISERIALIZE_FOR_{}", ident),
-        Span::call_site(),
-    );
 
     let fieldname = &fields.named.iter().map(|f| &f.ident).collect::<Vec<_>>();
     let fieldstr = fields
@@ -46,7 +42,7 @@ fn derive_struct(input: &DeriveInput, fields: &FieldsNamed) -> Result<TokenStrea
 
     Ok(quote! {
         #[allow(non_upper_case_globals)]
-        const #dummy: () = {
+        const _: () = {
             impl #impl_generics miniserde::Serialize for #ident #ty_generics #bounded_where_clause {
                 fn begin(&self) -> miniserde::ser::Fragment {
                     miniserde::ser::Fragment::Map(miniserde::__private::Box::new(__Map {
@@ -89,10 +85,6 @@ fn derive_enum(input: &DeriveInput, enumeration: &DataEnum) -> Result<TokenStrea
     }
 
     let ident = &input.ident;
-    let dummy = Ident::new(
-        &format!("_IMPL_MINISERIALIZE_FOR_{}", ident),
-        Span::call_site(),
-    );
 
     let var_idents = enumeration
         .variants
@@ -113,7 +105,7 @@ fn derive_enum(input: &DeriveInput, enumeration: &DataEnum) -> Result<TokenStrea
 
     Ok(quote! {
         #[allow(non_upper_case_globals)]
-        const #dummy: () = {
+        const _: () = {
             impl miniserde::Serialize for #ident {
                 fn begin(&self) -> miniserde::ser::Fragment {
                     match self {