extract helper functions for validation

components/salsa-entity-macros/src/entity.rs

@@ -45,13 +45,7 @@ impl EntityLike {
     }
 
     fn validate_entity(&self) -> syn::Result<()> {
-        // Require that entities are structs for now.
+        self.require_named_fields("entity")?;
-        if !self.has_named_fields() {
-            return Err(syn::Error::new(
-                self.id_ident().span(),
-                "entities must be structs with named fields",
-            ));
-        }
 
         Ok(())
     }

components/salsa-entity-macros/src/entity_like.rs

@@ -201,6 +201,42 @@ impl EntityLike {
 
         }
     }
+
+    /// Return an error unless this is a struct with named fields.
+    ///
+    /// # Parameters
+    ///
+    /// * `kind`, the attribute name (e.g., `input` or `interned`)
+    pub(crate) fn require_named_fields(&self, kind: &str) -> syn::Result<()> {
+        if !self.has_named_fields() {
+            return Err(syn::Error::new(
+                self.id_ident().span(),
+                "`#[salsa::{kind}]` can only be applied to a struct with named fields",
+            ));
+        }
+
+        Ok(())
+    }
+
+    /// Disallow `#[id]` attributes on the fields of this struct.
+    ///
+    /// If an `#[id]` field is found, return an error.
+    ///
+    /// # Parameters
+    ///
+    /// * `kind`, the attribute name (e.g., `input` or `interned`)
+    pub(crate) fn disallow_id_fields(&self, kind: &str) -> syn::Result<()> {
+        for ef in self.all_entity_fields() {
+            if ef.has_id_attr {
+                return Err(syn::Error::new(
+                    ef.name().span(),
+                    "`#[id]` cannot be used with `#[salsa::{kind}]`",
+                ));
+            }
+        }
+
+        Ok(())
+    }
 }
 
 pub(crate) const FIELD_OPTION_ATTRIBUTES: &[(&str, fn(&syn::Attribute, &mut EntityField))] = &[

components/salsa-entity-macros/src/interned.rs

@@ -41,20 +41,7 @@ impl EntityLike {
     }
 
     fn validate_interned(&self) -> syn::Result<()> {
-        // Disallow `#[value]` attributes on interned things.
+        self.disallow_id_fields("interned")?;
-        //
-        // They don't really make sense -- we intern all the fields of something
-        // to create its id. If multiple queries were to intern the same thing with
-        // distinct values for the value field, what would happen?
-        for ef in self.all_entity_fields() {
-            if ef.has_id_attr {
-                return Err(syn::Error::new(
-                    ef.name().span(),
-                    "`#[id]` not required in interned structs",
-                ));
-            }
-        }
-
         Ok(())
     }