Implement support for serde "partially untagged" enums#1521
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See here: https://serde.rs/variant-attrs.html#untagged > Irrespective of the enum representation, serialize and deserialize this variant as untagged, i.e. simply as the variant's data with no record of the variant name. For a simple example: ```rust enum SomeEnum { Tagged(u32) #[serde(untagged)] Untagged(u32) } ``` This would serialize to an optionally tagged integer: ```json {"Tagged": 3} 3 ``` With resulting JSON Schema: ```json { "oneOf": [ {"type": "object", "properties": {"Tagged": {"type": "integer"} }, "required": ["Tagged"] }, {"type": "integer"} ] } ``` --- Somewhat related, but __NOT__ implemented in this PR (as it seems like it will take some deeper refactoring of the "plain enum" handling): ```rust enum SomeEnum { One, Two, #[serde(untagged)] Null } ``` For this enum, serde would emit JSON values `"One"`, `"Two"` and `null`, but utoipa still generates a JSON schema for an enum: ```json { "enum": ["One", "Two", "Null"], "type": "string" } ```
foxfriends
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Feb 14, 2026
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| #[test] | ||
| #[ignore = "not implemented"] | ||
| fn derive_unit_variants_serde_partially_untagged() { |
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I left this ignored test here to bring attention to the unimplemented functionality for plain enums
foxfriends
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Feb 14, 2026
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| #[test] | ||
| fn mixed_enum_serde_partially_untagged_named_fields_proof() { | ||
| #[derive(Serialize)] | ||
| enum Foo { | ||
| One { | ||
| n: i32, | ||
| }, | ||
| #[serde(untagged)] | ||
| Two { | ||
| m: i32, | ||
| }, | ||
| } | ||
|
|
||
| assert_eq!( | ||
| serde_json::to_string(&Foo::One { n: 3 }).unwrap(), | ||
| r#"{"One":{"n":3}}"# | ||
| ); | ||
| assert_eq!( | ||
| serde_json::to_string(&Foo::Two { m: 3 }).unwrap(), | ||
| r#"{"m":3}"# | ||
| ); | ||
| } |
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I also included these "proof" tests, to validate this behaviour of serde, since it is not as widely known as functionality. Can be removed if not wanted, as this doesn't actually test utoipa functionality, only serde functionality.
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See here: https://serde.rs/variant-attrs.html#untagged
For a simple example:
This would serialize to an optionally tagged integer:
{"Tagged": 3} 3With resulting JSON Schema:
{ "oneOf": [ {"type": "object", "properties": {"Tagged": {"type": "integer"} }, "required": ["Tagged"] }, {"type": "integer"} ] }Somewhat related, but NOT implemented in this PR (as it seems like it will take some deeper refactoring of the "plain enum" handling):
For this enum, serde would emit JSON values
"One","Two"andnull, but utoipa still generates a JSON schema for an enum:{ "enum": ["One", "Two", "Null"], "type": "string" }