Detect duplicate source attributes

impl/src/valid.rs

@@ -1,4 +1,4 @@
-use crate::ast::{Enum, Input, Struct};
+use crate::ast::{Enum, Field, Input, Struct, Variant};
 use syn::{Error, Result};
 
 pub(crate) const CHECKED: &str = "checked in validation";
@@ -14,23 +14,46 @@ impl Input<'_> {
 
 impl Struct<'_> {
     fn validate(&self) -> Result<()> {
-        // nothing for now
+        find_duplicate_source(&self.fields)?;
         Ok(())
     }
 }
 
 impl Enum<'_> {
     fn validate(&self) -> Result<()> {
-        if self.has_display() {
-            for variant in &self.variants {
-                if variant.attrs.display.is_none() {
-                    return Err(Error::new_spanned(
-                        variant.original,
-                        "missing #[error(\"...\")] display attribute",
-                    ));
-                }
+        let has_display = self.has_display();
+        for variant in &self.variants {
+            variant.validate()?;
+            if has_display && variant.attrs.display.is_none() {
+                return Err(Error::new_spanned(
+                    variant.original,
+                    "missing #[error(\"...\")] display attribute",
+                ));
             }
         }
         Ok(())
     }
 }
+
+impl Variant<'_> {
+    fn validate(&self) -> Result<()> {
+        find_duplicate_source(&self.fields)?;
+        Ok(())
+    }
+}
+
+fn find_duplicate_source(fields: &[Field]) -> Result<()> {
+    let mut has_source = false;
+    for field in fields {
+        if let Some(source) = &field.attrs.source {
+            if has_source {
+                return Err(Error::new_spanned(
+                    source.original,
+                    "duplicate #[source] attribute",
+                ));
+            }
+            has_source = true;
+        }
+    }
+    Ok(())
+}