Reject source attribute not on a field

impl/src/valid.rs

@@ -1,4 +1,5 @@
 use crate::ast::{Enum, Field, Input, Struct, Variant};
+use crate::attr::Attrs;
 use syn::{Error, Result};
 
 pub(crate) const CHECKED: &str = "checked in validation";
@@ -14,6 +15,7 @@ impl Input<'_> {
 
 impl Struct<'_> {
     fn validate(&self) -> Result<()> {
+        check_no_source(&self.attrs)?;
         find_duplicate_source(&self.fields)?;
         Ok(())
     }
@@ -21,6 +23,7 @@ impl Struct<'_> {
 
 impl Enum<'_> {
     fn validate(&self) -> Result<()> {
+        check_no_source(&self.attrs)?;
         let has_display = self.has_display();
         for variant in &self.variants {
             variant.validate()?;
@@ -37,11 +40,22 @@ impl Enum<'_> {
 
 impl Variant<'_> {
     fn validate(&self) -> Result<()> {
+        check_no_source(&self.attrs)?;
         find_duplicate_source(&self.fields)?;
         Ok(())
     }
 }
 
+fn check_no_source(attrs: &Attrs) -> Result<()> {
+    if let Some(source) = &attrs.source {
+        return Err(Error::new_spanned(
+            source.original,
+            "not expected here; the #[source] attribute belongs on a specific field",
+        ));
+    }
+    Ok(())
+}
+
 fn find_duplicate_source(fields: &[Field]) -> Result<()> {
     let mut has_source = false;
     for field in fields {

tests/ui/unexpected-struct-source.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[source]
+pub struct Error;
+
+fn main() {}

tests/ui/unexpected-struct-source.stderr

@@ -0,0 +1,5 @@
+error: not expected here; the #[source] attribute belongs on a specific field
+ --> $DIR/unexpected-struct-source.rs:4:1
+  |
+4 | #[source]
+  | ^^^^^^^^^