update

lang/attribute/account/src/lib.rs

@@ -116,17 +116,12 @@ pub fn account(
     };
 
     let disc_fn = {
-        if cfg!(feature = "deprecated-layout") {
-            quote! {
-                fn discriminator() -> [u8; 8] {
-                    #discriminator
-                }
-            }
-        } else {
-            quote! {
-                fn discriminator() -> [u8; 4] {
-                    #discriminator
-                }
+        let len: proc_macro2::TokenStream = anchor_common::header::discriminator_len_str()
+            .parse()
+            .unwrap();
+        quote! {
+            fn discriminator() -> [u8; #len] {
+                #discriminator
             }
         }
     };

lang/attribute/event/src/lib.rs

@@ -23,17 +23,12 @@ pub fn event(
     };
 
     let discriminator_trait_impl = {
-        if cfg!(feature = "deprecated_layout") {
-            quote! {
-                fn discriminator() -> [u8; 8] {
-                    #discriminator
-                }
-            }
-        } else {
-            quote! {
-                fn discriminator() -> [u8; 4] {
-                    #discriminator
-                }
+        let len: proc_macro2::TokenStream = anchor_common::header::discriminator_len_str()
+            .parse()
+            .unwrap();
+        quote! {
+            fn discriminator() -> [u8; #len] {
+                #discriminator
             }
         }
     };

lang/common/src/header.rs

@@ -38,15 +38,26 @@ pub fn read_data_mut(account_data: &mut [u8]) -> &mut [u8] {
 }
 
 pub fn write_discriminator(account_data: &mut [u8], discriminator: &[u8]) {
-    #[cfg(feature = "deprecated_layout")]
+    #[cfg(feature = "deprecated-layout")]
     {
         let mut cursor = Cursor::new(account_dst);
         cursor.write_all(discriminator).unwrap();
     }
-    #[cfg(not(feature = "deprecated_layout"))]
+    #[cfg(not(feature = "deprecated-layout"))]
     {
         let dst: &mut [u8] = &mut account_data[2..];
         let mut cursor = Cursor::new(dst);
         cursor.write_all(discriminator).unwrap();
     }
 }
+
+// Bit of a hack. We return the length as a string and then parse it into
+// a token stream in the macro code generation, so that we can isolate all
+// the feature flagging to this one module, here.
+pub fn discriminator_len_str() -> &'static str {
+    if cfg!(feature = "deprecated-layout") {
+        "8"
+    } else {
+        "4"
+    }
+}