Invert build.rs cfgs in serde_derive

serde_derive/build.rs

@@ -13,14 +13,14 @@ fn main() {
 
     // Underscore const names stabilized in Rust 1.37:
     // https://blog.rust-lang.org/2019/08/15/Rust-1.37.0.html#using-unnamed-const-items-for-macros
-    if minor >= 37 {
+    if minor < 37 {
-        println!("cargo:rustc-cfg=underscore_consts");
+        println!("cargo:rustc-cfg=no_underscore_consts");
     }
 
     // The ptr::addr_of! macro stabilized in Rust 1.51:
     // https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html#stabilized-apis
-    if minor >= 51 {
+    if minor < 51 {
-        println!("cargo:rustc-cfg=ptr_addr_of");
+        println!("cargo:rustc-cfg=no_ptr_addr_of");
     }
 }
 

serde_derive/src/dummy.rs

@@ -12,10 +12,10 @@ pub fn wrap_in_const(
 ) -> TokenStream {
     let try_replacement = try::replacement();
 
-    let dummy_const = if cfg!(underscore_consts) {
+    let dummy_const = if cfg!(no_underscore_consts) {
-        format_ident!("_")
-    } else {
         format_ident!("_IMPL_{}_FOR_{}", trait_, unraw(ty))
+    } else {
+        format_ident!("_")
     };
 
     let use_serde = match serde_path {

serde_derive/src/pretend.rs

@@ -97,7 +97,7 @@ fn pretend_fields_used_struct_packed(cont: &Container, fields: &[Field]) -> Toke
 
     let members = fields.iter().map(|field| &field.member).collect::<Vec<_>>();
 
-    #[cfg(ptr_addr_of)]
+    #[cfg(not(no_ptr_addr_of))]
     {
         quote! {
             match _serde::__private::None::<&#type_ident #ty_generics> {
@@ -111,7 +111,7 @@ fn pretend_fields_used_struct_packed(cont: &Container, fields: &[Field]) -> Toke
         }
     }
 
-    #[cfg(not(ptr_addr_of))]
+    #[cfg(no_ptr_addr_of)]
     {
         let placeholders = (0usize..).map(|i| format_ident!("__v{}", i));