Reject transparent attr in field attribute position

impl/src/valid.rs

@@ -163,6 +163,12 @@ fn check_field_attrs(fields: &[Field]) -> Result<()> {
             backtrace_field = Some(field);
             has_backtrace = true;
         }
+        if let Some(transparent) = field.attrs.transparent {
+            return Err(Error::new_spanned(
+                transparent,
+                "#[error(transparent)] needs to go outside the enum or struct, not on an individual field",
+            ));
+        }
         has_backtrace |= field.is_backtrace();
     }
     if let (Some(from_field), Some(source_field)) = (from_field, source_field) {

tests/ui/bad-field-attr.rs

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

tests/ui/bad-field-attr.stderr

@@ -0,0 +1,5 @@
+error: #[error(transparent)] needs to go outside the enum or struct, not on an individual field
+ --> $DIR/bad-field-attr.rs:5:18
+  |
+5 | pub struct Error(#[error(transparent)] std::io::Error);
+  |                  ^^^^^^^^^^^^^^^^^^^^^