Error on enums with explicit discriminant values in SpacetimeType

crates/bindings-macro/src/lib.rs

@@ -132,6 +132,7 @@ mod sym {
     symbol!(update);
     symbol!(default);
     symbol!(event);
+    symbol!(allow_discriminants);
 
     symbol!(u8);
     symbol!(i8);

crates/bindings-macro/src/sats.rs

@@ -63,7 +63,39 @@ pub(crate) fn sats_type_from_derive(
             SatsTypeData::Product(fields.collect())
         }
         syn::Data::Enum(enu) => {
+            // Check if #[sats(allow_discriminants)] is set, which opts out of the
+            // discriminant check. This is used by internal enums that set discriminant
+            // values for non-codegen reasons (e.g. for `as u8` casts).
+            let allow_discriminants = input.attrs.iter().any(|attr| {
+                if !attr.path().is_ident("sats") {
+                    return false;
+                }
+                let mut found = false;
+                // Use parse_nested_meta with proper error handling for keyword paths
+                // like `crate = crate`. We consume unknown meta items gracefully.
+                let _ = attr.parse_nested_meta(|meta| {
+                    if meta.path.is_ident("allow_discriminants") {
+                        found = true;
+                    } else if meta.input.peek(syn::Token![=]) {
+                        // Consume `= value` for other keys (e.g. `crate = crate`).
+                        meta.input.parse::<syn::Token![=]>()?;
+                        meta.input.parse::<proc_macro2::TokenTree>()?;
+                    }
+                    Ok(())
+                });
+                found
+            });
             let variants = enu.variants.iter().map(|var| {
+                if !allow_discriminants {
+                    if let Some((eq, _)) = &var.discriminant {
+                        return Err(syn::Error::new(
+                            eq.span(),
+                            "explicit discriminant values are not supported by SpacetimeDB; \
+                             SATS assigns variant tags by declaration order, so discriminant \
+                             values would be silently ignored",
+                        ));
+                    }
+                }
                 let (member, ty) = variant_data(var)?.unzip();
                 Ok(SatsVariant {
                     ident: &var.ident,
@@ -118,6 +150,8 @@ pub(crate) fn extract_sats_type<'a>(
                     let v = value.parse::<LitStr>()?;
                     name = Some(v);
                 }
+                // Parsed earlier in sats_type_from_derive; just accept and ignore here.
+                sym::allow_discriminants => {}
             });
             Ok(())
         })?;

crates/bindings/tests/ui/enum_discriminants.rs

@@ -0,0 +1,23 @@
+// Enums with explicit discriminant values should produce a compile error,
+// since SATS assigns variant tags by declaration order and ignores discriminants.
+
+#[derive(spacetimedb::SpacetimeType)]
+enum ExplicitValues {
+    A = 1,
+    B = 2,
+}
+
+#[derive(spacetimedb::SpacetimeType)]
+enum MixedValues {
+    X,
+    Y = 5,
+}
+
+// This should compile fine — no explicit discriminants.
+#[derive(spacetimedb::SpacetimeType)]
+enum NoValues {
+    Foo,
+    Bar,
+}
+
+fn main() {}

crates/bindings/tests/ui/enum_discriminants.stderr

@@ -0,0 +1,11 @@
+error: explicit discriminant values are not supported by SpacetimeDB; SATS assigns variant tags by declaration order, so discriminant values would be silently ignored
+ --> tests/ui/enum_discriminants.rs:6:7
+  |
+6 |     A = 1,
+  |       ^
+
+error: explicit discriminant values are not supported by SpacetimeDB; SATS assigns variant tags by declaration order, so discriminant values would be silently ignored
+  --> tests/ui/enum_discriminants.rs:13:7
+   |
+13 |     Y = 5,
+   |       ^

crates/client-api-messages/src/websocket/v2.rs

@@ -84,7 +84,7 @@ pub struct Unsubscribe {
 }
 
 #[derive(Clone, Copy, Default, PartialEq, Eq, SpacetimeType)]
-#[sats(crate = spacetimedb_lib)]
+#[sats(crate = spacetimedb_lib, allow_discriminants)]
 pub enum UnsubscribeFlags {
     #[default]
     Default = 0,

crates/lib/src/db/raw_def/v8.rs

@@ -80,7 +80,7 @@ impl RawSequenceDefV8 {
 ///
 /// Currently only `IndexType::BTree` is allowed.
 #[derive(Debug, Copy, Clone, Eq, PartialEq, PartialOrd, Ord, Display, SpacetimeType)]
-#[sats(crate = crate)]
+#[sats(crate = crate, allow_discriminants)]
 pub enum IndexType {
     /// A BTree index.
     BTree = 0,