Support nullable Arrow unions using virtual union arms (#2708)

**Commit by commit!**

This PR implements support for top-level nullable Arrow unions using the
"virtual arm" strategy.

Specifically, all unions derived from Flatbuffers definitions now have
an auto-generated arm at type index `0` of type `DataType::Null`.
The data values in this arm are irrelevant: the deserialization code
will treat the union as a whole as `null` any time the type buffer
indicates a value of `0`.

I've decided to make the auto-generated arm visible in the
auto-generated `DataType` definition because... why wouldn't we? It
makes it all much less magical and acts as a reminder that this thing is
indeed here.

The choice of using type index `0` rather than e.g. `nb_arms` is to make
things A) more stable and B) forwards compatible.
It cannot surprise end users in any way since user-defined types defined
in Flatbuffers would just do the right thing, while user-defined types
in raw Arrow don't even go through our (de)serialization logic but
rather through theirs.

In addition, this PR greatly improves error handling in the
auto-generated code, giving the end-user very specific context and a
context-correct backtrace when something goes wrong.


Fixes #2684

### What

### 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/2708) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/2708)
- [Docs
preview](https://rerun.io/preview/pr%3Acmc%2Frust_virtual_union_arms/docs)
- [Examples
preview](https://rerun.io/preview/pr%3Acmc%2Frust_virtual_union_arms/examples)

crates/re_types/Cargo.toml

@@ -28,6 +28,8 @@ glam = ["dep:glam", "dep:macaw"]
 
 
 [dependencies]
+# Rerun
+re_error.workspace = true
 
 # External
 arrow2 = { workspace = true, features = [
@@ -36,6 +38,7 @@ arrow2 = { workspace = true, features = [
   "compute_concatenate",
 ] }
 anyhow.workspace = true
+backtrace = "0.3"
 bytemuck = { version = "1.11", features = ["derive", "extern_crate_alloc"] }
 document-features = "0.2"
 itertools.workspace = true

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

@@ -29,8 +29,7 @@ table AffixFuzzer1 (
   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);
+  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);
@@ -50,8 +49,7 @@ table AffixFuzzer1 (
   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);
+  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);
@@ -70,8 +68,7 @@ table AffixFuzzer1 (
   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);
+  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);
@@ -90,8 +87,7 @@ table AffixFuzzer1 (
   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);
+  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

@@ -108,13 +108,12 @@ table AffixFuzzer14 (
   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 AffixFuzzer15 (
+  "attr.rust.derive": "PartialEq",
+  order: 1500
+) {
+  single_optional_union: rerun.testing.datatypes.AffixFuzzer3 (nullable, order: 115);
+}
 
 table AffixFuzzer16 (
   "attr.rust.derive": "PartialEq",

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.
-289571cd5bcef516a43ead1678f39054c300890396d13933dc26d9294a07dffe
+ba58a7d123c5fa81f04a58a6325ef65029d93f330d571176101db87596a9c0cf