Merge pull request 120 from jonasbb/flatten-deny-unknown-fields

Author: dtolnay

_src/attr-flatten.md

@@ -5,6 +5,12 @@ The `flatten` attribute inlines keys from a field into the parent struct.
 supported only within structs that have named fields, and the field to which it
 is applied must be a struct or map type.
 
+*Note:* `flatten` is not supported in combination with structs that use
+[`deny_unknown_fields`]. Neither the outer nor inner flattened struct should use
+that attribute.
+
+[`deny_unknown_fields`]: container-attrs.md#deny_unknown_fields
+
 The `flatten` attribute serves the following two common use cases:
 
 ### Factor out frequently grouped keys

_src/container-attrs.md

@@ -30,6 +30,11 @@
   this attribute is not present, by default unknown fields are ignored for
   self-describing formats like JSON.
 
+  *Note:* this attribute is not supported in combination with [`flatten`],
+  neither on the outer struct nor on the flattened field.
+
+  [`flatten`]: field-attrs.md#flatten
+
 - ##### `#[serde(tag = "type")]` {#tag}
 
   Use the internally tagged enum representation for this enum, with the given

_src/field-attrs.md

@@ -38,6 +38,12 @@
   with arbitrary string keys. The [struct flattening](attr-flatten.md) page
   provides some examples.
 
+  *Note:* this attribute is not supported in combination with structs that use
+  [`deny_unknown_fields`]. Neither the outer nor inner flattened struct should
+  use that attribute.
+
+  [`deny_unknown_fields`]: container-attrs.md#deny_unknown_fields
+
 - ##### `#[serde(skip)]` {#skip}
 
   Skip this field: do not serialize or deserialize it.