Warn when encountering a serde_as attribute on a field with no values

jonasbb · Jan 5, 2021 · c911f26 · c911f26
1 parent 95fc97b
commit c911f26
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diff --git a/serde_with_macros/src/lib.rs b/serde_with_macros/src/lib.rs
@@ -499,6 +499,7 @@ fn serde_as_add_attr_to_field(
     let serde_as_options = SerdeAsOptions::from_field(field)?;
     let serde_with_options = SerdeWithOptions::from_field(field)?;
 
+    // TODO allow multiple attributes
     // Find index of serde_as attribute
     let serde_as_idx = field.attrs.iter().enumerate().find_map(|(idx, attr)| {
         if attr.path.is_ident("serde_as") {
@@ -515,6 +516,10 @@ fn serde_as_add_attr_to_field(
     // serde_as Attribute
     field.attrs.remove(serde_as_idx);
 
+    if !serde_as_options.has_any_set() {
+        return Err(DarlingError::custom("An empty `serde_as` attribute on a field has no effect. You are missing an `as`, `serialize_as`, or `deserialize_as` parameter."));
+    }
+
     // Check if there are any conflicting attributes
     if serde_as_options.has_any_set() && serde_with_options.has_any_set() {
         return Err(DarlingError::custom("Cannot combine `serde_as` with serde's `with`, `deserialize_with`, or `serialize_with`."));

diff --git a/serde_with_macros/tests/compile-fail/serde_as.stderr b/serde_with_macros/tests/compile-fail/serde_as.stderr
@@ -93,3 +93,15 @@ error: Unable to parse attribute: expected literal
    |
 67 |     #[serde_as(serialize_as = "123", deserialize_as = DisplayFromStr)]
    |                                                       ^^^^^^^^^^^^^^
+
+error: An empty `serde_as` attribute on a field has no effect. You are missing an `as`, `serialize_as`, or `deserialize_as` parameter.
+  --> $DIR/serde_as.rs:70:5
+   |
+70 |     no_entries: u32,
+   |     ^^^^^^^^^^
+
+error: An empty `serde_as` attribute on a field has no effect. You are missing an `as`, `serialize_as`, or `deserialize_as` parameter.
+  --> $DIR/serde_as.rs:72:5
+   |
+72 |     no_entries_brackets: u32,
+   |     ^^^^^^^^^^^^^^^^^^^