Allow 'null' variants in unions (#3481)

## Motivation and Context
- awslabs/aws-sdk-rust#1095

## Description
Update the JSON parser generator to allow for `null` to be set in
unions. Servers can send unions like this:
```json
{
  "AmazonElasticsearchParameters": null,
  "AmazonOpenSearchParameters": null,
  "AppFlowParameters": null,
  "AthenaParameters": null,
  "AuroraParameters": null,
  "AuroraPostgreSqlParameters": null,
  "AwsIotAnalyticsParameters": null,
  "BigQueryParameters": null,
  "DatabricksParameters": null,
  "Db2Parameters": null,
  "DenodoParameters": null,
  "DocumentDBParameters": null,
  "DremioParameters": null,
  "ExasolParameters": null,
  "GoogleAnalyticsParameters": null,
  "JiraParameters": null,
  "MariaDbParameters": null,
  "MongoAtlasParameters": null,
  "MongoDBParameters": null,
  "MySqlParameters": null,
  "OracleParameters": null,
  "PostgreSqlParameters": null,
  "PrestoParameters": null,
  "RdsParameters": null,
  "RedshiftParameters": null,
  "S3Parameters": {
    "IsUploaded": false,
    "ManifestFileLocation": {
      "Bucket": "deided-bucket.prod.us-east-1",
      "Key": "sales/manifest.json"
    },
    "RoleArn": null
  },
  "SalesforceParameters": null,
  "SapHanaParameters": null,
  "ServiceNowParameters": null,
  "SnowflakeParameters": null,
  "SparkParameters": null,
  "SqlServerParameters": null,
  "StarburstParameters": null,
  "TeradataParameters": null,
  "TrinoParameters": null,
  "TwitterParameters": null
}
```

This caused our parser to fail.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
- [x] New unit test
- [x] Dry run against new [smithy protocol
test](smithy-lang/smithy#2180)

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

build.gradle.kts

@@ -15,8 +15,11 @@ buildscript {
 }
 
 allprojects {
+    val allowLocalDeps: String by project
     repositories {
-        // mavenLocal()
+        if (allowLocalDeps.toBoolean()) {
+         mavenLocal()
+        }
         mavenCentral()
         google()
     }

buildSrc/build.gradle.kts

@@ -12,7 +12,6 @@ plugins {
 repositories {
     mavenCentral()
     google()
-    /* mavenLocal() */
 }
 
 // Load properties manually to avoid hard coding smithy version

codegen-client-test/build.gradle.kts

@@ -114,8 +114,6 @@ val allCodegenTests = listOf(
     ),
     ClientTest("aws.protocoltests.misc#QueryCompatService", "query-compat-test", dependsOn = listOf("aws-json-query-compat.smithy")),
 ).map(ClientTest::toCodegenTest)
-// use this line to run just one test
-// .filter { it.module == "query-compat-test" }
 
 project.registerGenerateSmithyBuildTask(rootProject, pluginName, allCodegenTests)
 project.registerGenerateCargoWorkspaceTask(rootProject, pluginName, allCodegenTests, workingDirUnderBuildDir)

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/parse/JsonParserGenerator.kt

@@ -36,6 +36,7 @@ import software.amazon.smithy.rust.codegen.core.rustlang.withBlockTemplate
 import software.amazon.smithy.rust.codegen.core.smithy.CodegenContext
 import software.amazon.smithy.rust.codegen.core.smithy.CodegenTarget
 import software.amazon.smithy.rust.codegen.core.smithy.RuntimeType
+import software.amazon.smithy.rust.codegen.core.smithy.RuntimeType.Companion.preludeScope
 import software.amazon.smithy.rust.codegen.core.smithy.canUseDefault
 import software.amazon.smithy.rust.codegen.core.smithy.customize.NamedCustomization
 import software.amazon.smithy.rust.codegen.core.smithy.customize.Section
@@ -121,6 +122,7 @@ class JsonParserGenerator(
             "skip_to_end" to smithyJson.resolve("deserialize::token::skip_to_end"),
             "Token" to smithyJson.resolve("deserialize::Token"),
             "or_empty" to orEmptyJson(),
+            *preludeScope,
         )
 
     /**
@@ -560,6 +562,15 @@ class JsonParserGenerator(
                             *codegenScope,
                         ) {
                             objectKeyLoop(hasMembers = shape.members().isNotEmpty()) {
+                                rustTemplate(
+                                    """
+                                    if let #{Some}(#{Ok}(#{Token}::ValueNull { .. })) = tokens.peek() {
+                                        let _ = tokens.next().expect("peek returned a token")?;
+                                        continue;
+                                    }
+                                    """,
+                                    *codegenScope,
+                                )
                                 rustTemplate(
                                     """
                                     let key = key.to_unescaped()?;
@@ -622,6 +633,16 @@ class JsonParserGenerator(
                             *codegenScope,
                         )
                     }
+                    // If we've gotten to the point where the union had a `{ ... }` section, we can't return None
+                    // anymore. If we didn't parse a union at this point, this is an error.
+                    rustTemplate(
+                        """
+                        if variant.is_none() {
+                            return Err(#{Error}::custom("Union did not contain a valid variant."))
+                        }
+                        """,
+                        *codegenScope,
+                    )
                     rust("Ok(variant)")
                 }
             }

codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/parse/JsonParserGeneratorTest.kt

@@ -183,6 +183,26 @@ class JsonParserGeneratorTest {
                 """,
             )
 
+            unitTest(
+                "allow_null_for_variants",
+                """
+                // __type field should be ignored during deserialization
+                let input = br#"{ "top": { "choice": { "blob": null, "boolean": null, "int": 5, "long": null, "__type": "value-should-be-ignored-anyway" } } }"#;
+                let output = ${format(operationGenerator)}(input, test_output::OpOutput::builder()).unwrap().build();
+                use test_model::Choice;
+                assert_eq!(Choice::Int(5), output.top.unwrap().choice);
+                """,
+            )
+
+            unitTest(
+                "all_variants_null",
+                """
+                // __type field should be ignored during deserialization
+                let input = br#"{ "top": { "choice": { "blob": null, "boolean": null, "int": null, "long": null, "__type": "value-should-be-ignored-anyway" } } }"#;
+                let _err = ${format(operationGenerator)}(input, test_output::OpOutput::builder()).expect_err("invalid union");
+                """,
+            )
+
             unitTest(
                 "empty_error",
                 """

gradle.properties

@@ -22,6 +22,7 @@ kotlin.code.style=official
 # codegen
 smithyGradlePluginVersion=0.9.0
 smithyVersion=1.45.0
+allowLocalDeps=false
 
 # kotlin
 kotlinVersion=1.9.20