Generating (de)serialization code for dense unions in Rust backend (#…

…2626)

**COMMIT BY COMMIT!**

What the title says.

Adding support for sparse unions would be trivial, but nobody ever uses
those so I've refrained from doing it for now.

Fixes #2625 
Requires #2618 
Requires #2619

---

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested [demo.rerun.io](https://demo.rerun.io/pr/2626) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/2626)
- [Docs preview](https://rerun.io/preview/pr%3Acmc%2Farrow_union/docs)
- [Examples
preview](https://rerun.io/preview/pr%3Acmc%2Farrow_union/examples)

crates/re_types/definitions/rerun/testing/archetypes/fuzzy.fbs

@@ -28,6 +28,12 @@ table AffixFuzzer1 (
   fuzz1011: rerun.testing.components.AffixFuzzer11 ("attr.rerun.component_required", order: 1011);
   fuzz1012: rerun.testing.components.AffixFuzzer12 ("attr.rerun.component_required", order: 1012);
   fuzz1013: rerun.testing.components.AffixFuzzer13 ("attr.rerun.component_required", order: 1013);
+  fuzz1014: rerun.testing.components.AffixFuzzer14 ("attr.rerun.component_required", order: 1014);
+  // NOTE: nullable union -- illegal!
+  // fuzz1015: rerun.testing.components.AffixFuzzer15 ("attr.rerun.component_required", order: 1015);
+  fuzz1016: rerun.testing.components.AffixFuzzer16 ("attr.rerun.component_required", order: 1016);
+  fuzz1017: rerun.testing.components.AffixFuzzer17 ("attr.rerun.component_required", order: 1017);
+  fuzz1018: rerun.testing.components.AffixFuzzer18 ("attr.rerun.component_required", order: 1018);
 
   fuzz1101: [rerun.testing.components.AffixFuzzer1] ("attr.rerun.component_required", order: 1101);
   fuzz1102: [rerun.testing.components.AffixFuzzer2] ("attr.rerun.component_required", order: 1102);
@@ -42,6 +48,12 @@ table AffixFuzzer1 (
   fuzz1111: [rerun.testing.components.AffixFuzzer11] ("attr.rerun.component_required", order: 1111);
   fuzz1112: [rerun.testing.components.AffixFuzzer12] ("attr.rerun.component_required", order: 1112);
   fuzz1113: [rerun.testing.components.AffixFuzzer13] ("attr.rerun.component_required", order: 1113);
+  fuzz1114: [rerun.testing.components.AffixFuzzer14] ("attr.rerun.component_required", order: 1114);
+  // NOTE: nullable union -- illegal!
+  // fuzz1115: [rerun.testing.components.AffixFuzzer15] ("attr.rerun.component_required", order: 1115);
+  fuzz1116: [rerun.testing.components.AffixFuzzer16] ("attr.rerun.component_required", order: 1116);
+  fuzz1117: [rerun.testing.components.AffixFuzzer17] ("attr.rerun.component_required", order: 1117);
+  fuzz1118: [rerun.testing.components.AffixFuzzer18] ("attr.rerun.component_required", order: 1118);
 
   fuzz2001: rerun.testing.components.AffixFuzzer1 ("attr.rerun.component_optional", nullable, order: 2001);
   fuzz2002: rerun.testing.components.AffixFuzzer2 ("attr.rerun.component_optional", nullable, order: 2002);
@@ -56,6 +68,12 @@ table AffixFuzzer1 (
   fuzz2011: rerun.testing.components.AffixFuzzer11 ("attr.rerun.component_optional", nullable, order: 2011);
   fuzz2012: rerun.testing.components.AffixFuzzer12 ("attr.rerun.component_optional", nullable, order: 2012);
   fuzz2013: rerun.testing.components.AffixFuzzer13 ("attr.rerun.component_optional", nullable, order: 2013);
+  fuzz2014: rerun.testing.components.AffixFuzzer14 ("attr.rerun.component_optional", nullable, order: 2014);
+  // NOTE: nullable union -- illegal!
+  // fuzz2015: rerun.testing.components.AffixFuzzer15 ("attr.rerun.component_optional", nullable, order: 2015);
+  fuzz2016: rerun.testing.components.AffixFuzzer16 ("attr.rerun.component_optional", nullable, order: 2016);
+  fuzz2017: rerun.testing.components.AffixFuzzer17 ("attr.rerun.component_optional", nullable, order: 2017);
+  fuzz2018: rerun.testing.components.AffixFuzzer18 ("attr.rerun.component_optional", nullable, order: 2018);
 
   fuzz2101: [rerun.testing.components.AffixFuzzer1] ("attr.rerun.component_optional", nullable, order: 2101);
   fuzz2102: [rerun.testing.components.AffixFuzzer2] ("attr.rerun.component_optional", nullable, order: 2102);
@@ -70,5 +88,10 @@ table AffixFuzzer1 (
   fuzz2111: [rerun.testing.components.AffixFuzzer11] ("attr.rerun.component_optional", nullable, order: 2111);
   fuzz2112: [rerun.testing.components.AffixFuzzer12] ("attr.rerun.component_optional", nullable, order: 2112);
   fuzz2113: [rerun.testing.components.AffixFuzzer13] ("attr.rerun.component_optional", nullable, order: 2113);
+  fuzz2114: [rerun.testing.components.AffixFuzzer14] ("attr.rerun.component_optional", nullable, order: 2114);
+  // NOTE: nullable union -- illegal!
+  // fuzz2115: [rerun.testing.components.AffixFuzzer15] ("attr.rerun.component_optional", nullable, order: 2115);
+  fuzz2116: [rerun.testing.components.AffixFuzzer16] ("attr.rerun.component_optional", nullable, order: 2116);
+  fuzz2117: [rerun.testing.components.AffixFuzzer17] ("attr.rerun.component_optional", nullable, order: 2117);
+  fuzz2118: [rerun.testing.components.AffixFuzzer18] ("attr.rerun.component_optional", nullable, order: 2118);
 }
-

crates/re_types/definitions/rerun/testing/components/fuzzy.fbs

@@ -101,11 +101,47 @@ table AffixFuzzer13 (
   many_strings_optional: [string] (nullable, order: 113);
 }
 
+table AffixFuzzer14 (
+  "attr.rust.derive": "PartialEq",
+  order: 1400
+) {
+  single_required_union: rerun.testing.datatypes.AffixFuzzer3 (order: 114);
+}
+
+// NOTE: Illegal: union don't have top-level bitmaps! Commenting out will/should break the build.
+// table AffixFuzzer15 (
+//   "attr.rust.derive": "PartialEq",
+//   order: 1500
+// ) {
+//   single_optional_union: rerun.testing.datatypes.AffixFuzzer3 (nullable, order: 115);
+// }
+
+table AffixFuzzer16 (
+  "attr.rust.derive": "PartialEq",
+  order: 1600
+) {
+  many_required_unions: [rerun.testing.datatypes.AffixFuzzer3] (order: 116);
+}
+
+table AffixFuzzer17 (
+  "attr.rust.derive": "PartialEq",
+  order: 1700
+) {
+  many_optional_unions: [rerun.testing.datatypes.AffixFuzzer3] (nullable, order: 117);
+}
+
+table AffixFuzzer18 (
+  "attr.rust.derive": "PartialEq",
+  order: 1800
+) {
+  many_optional_unions: [rerun.testing.datatypes.AffixFuzzer4] (nullable, order: 118);
+}
+
 // TODO(cmc): the ugly bug we need to take care of at some point
 // table AffixFuzzer14 (
 //   "attr.rust.derive": "PartialEq",
 //   order: 1400
 // ) {
 //
-//   many_transparent_optionals: rerun.testing.datatypes.AffixFuzzer2 (order: 107);
+//   many_transparent_optionals: rerun.testing.datatypes.AffixFuzzer2 (nullable, order: 107);
 // }

crates/re_types/definitions/rerun/testing/datatypes/fuzzy.fbs

@@ -19,7 +19,7 @@ table VeryDeeplyFlattenedScalar (transparent, order: 001) {
 }
 
 table SurprisinglyShallowScalar (transparent, order: 001) {
-  value: FlattenedScalar (required, order: 100);
+  value: FlattenedScalar (order: 100);
 }
 
 table AffixFuzzer1 (
@@ -36,6 +36,10 @@ table AffixFuzzer1 (
   almost_flattened_scalar: SurprisinglyShallowScalar (order: 108, transparent);
 }
 
+table __AffixFuzzer1Vec (transparent, order: 0) {
+  inner: [AffixFuzzer1] (order: 0);
+}
+
 table AffixFuzzer2 (
   "attr.arrow.transparent",
   "attr.rust.derive": "PartialEq",
@@ -44,3 +48,29 @@ table AffixFuzzer2 (
 ) {
   single_float_optional: float (nullable, order: 101);
 }
+
+union AffixFuzzer3 (
+  "attr.rust.derive": "PartialEq",
+  order: 300
+) {
+  degrees: FlattenedScalar (transparent, order: 100),
+  radians: FlattenedScalar (transparent, nullable, order: 101),
+  craziness: __AffixFuzzer1Vec (transparent, order: 102),
+}
+
+table __AffixFuzzer3 (transparent, order: 0) {
+  inner: AffixFuzzer3 (order: 0);
+}
+
+table __AffixFuzzer3Vec (transparent, order: 0) {
+  inner: [AffixFuzzer3] (order: 0);
+}
+
+union AffixFuzzer4 (
+  "attr.rust.derive": "PartialEq",
+  order: 300
+) {
+  single_required: __AffixFuzzer3 (transparent, order: 100),
+  many_required: __AffixFuzzer3Vec (transparent, order: 101),
+  many_optional: __AffixFuzzer3Vec (transparent, order: 102, nullable),
+}

crates/re_types/source_hash.txt

@@ -1,4 +1,4 @@
 # This is a sha256 hash for all direct and indirect dependencies of this crate's build script.
 # It can be safely removed at anytime to force the build script to run again.
 # Check out build.rs to see how it's computed.
-b0b0d5d6d0f4131e52331303a2c2dfdfa11e03936e97242ef897c2b158979273
+51e4e1d21a853de2f4d3b55d7d34d75cb3d6f0f7c01b701e9750256c759e9827