Fix server code generation of @httpPayload-bound constrained shapes (#2584)

The issue is we're not changing the return type of the payload
deserializing function to be the unconstrained type (e.g. the builder in
case of an `@httpPayload`-bound structure shape) when the shape is
constrained.

Yet another example of why code-generating `constraints.smithy` (see
#2101)
is important.

Closes #2583.

## Testing

The added integration test operation fails to compile without this
patch.

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

----

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choice._

Author: david-perez

CHANGELOG.next.toml

@@ -122,3 +122,9 @@ message = "Fix generation of constrained shapes reaching `@sensitive` shapes"
 references = ["smithy-rs#2582", "smithy-rs#2585"]
 meta = { "breaking" = false, "tada" = false, "bug" = true, "target" = "server" }
 author = "david-perez"
+
+[[smithy-rs]]
+message = "Fix server code generation bug affecting constrained shapes bound with `@httpPayload`"
+references = ["smithy-rs#2583", "smithy-rs#2584"]
+meta = { "breaking" = false, "tada" = false, "bug" = true, "target" = "server" }
+author = "david-perez"

codegen-core/common-test-models/constraints.smithy

@@ -12,7 +12,9 @@ service ConstraintsService {
     operations: [
         ConstrainedShapesOperation,
         ConstrainedHttpBoundShapesOperation,
+        ConstrainedHttpPayloadBoundShapeOperation,
         ConstrainedRecursiveShapesOperation,
+
         // `httpQueryParams` and `httpPrefixHeaders` are structurually
         // exclusive, so we need one operation per target shape type
         // combination.
@@ -59,6 +61,13 @@ operation ConstrainedHttpBoundShapesOperation {
     errors: [ValidationException]
 }
 
+@http(uri: "/constrained-http-payload-bound-shape-operation", method: "POST")
+operation ConstrainedHttpPayloadBoundShapeOperation {
+    input: ConstrainedHttpPayloadBoundShapeOperationInputOutput,
+    output: ConstrainedHttpPayloadBoundShapeOperationInputOutput,
+    errors: [ValidationException]
+}
+
 @http(uri: "/constrained-recursive-shapes-operation", method: "POST")
 operation ConstrainedRecursiveShapesOperation {
     input: ConstrainedRecursiveShapesOperationInputOutput,
@@ -311,6 +320,12 @@ structure ConstrainedHttpBoundShapesOperationInputOutput {
     enumStringListQuery: ListOfEnumString,
 }
 
+structure ConstrainedHttpPayloadBoundShapeOperationInputOutput {
+    @required
+    @httpPayload
+    httpPayloadBoundConstrainedShape: ConA
+}
+
 structure QueryParamsTargetingMapOfPatternStringOperationInputOutput {
     @httpQueryParams
     mapOfPatternString: MapOfPatternString

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/generators/http/HttpBindingGenerator.kt

@@ -235,7 +235,7 @@ class HttpBindingGenerator(
                 // The output needs to be Optional when deserializing the payload body or the caller signature
                 // will not match.
                 val outputT = symbolProvider.toSymbol(binding.member).makeOptional()
-                rustBlock("pub fn $fnName(body: &[u8]) -> std::result::Result<#T, #T>", outputT, errorSymbol) {
+                rustBlock("pub(crate) fn $fnName(body: &[u8]) -> std::result::Result<#T, #T>", outputT, errorSymbol) {
                     deserializePayloadBody(
                         binding,
                         errorSymbol,

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

@@ -239,7 +239,11 @@ class HttpBoundProtocolPayloadGenerator(
     ) {
         val ref = if (payloadMetadata.takesOwnership) "" else "&"
         val serializer = protocolFunctions.serializeFn(member, fnNameSuffix = "http_payload") { fnName ->
-            val outputT = if (member.isStreaming(model)) symbolProvider.toSymbol(member) else RuntimeType.ByteSlab.toSymbol()
+            val outputT = if (member.isStreaming(model)) {
+                symbolProvider.toSymbol(member)
+            } else {
+                RuntimeType.ByteSlab.toSymbol()
+            }
             rustBlockTemplate(
                 "pub fn $fnName(payload: $ref#{Member}) -> Result<#{outputT}, #{BuildError}>",
                 "Member" to symbolProvider.toSymbol(member),

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

@@ -154,14 +154,15 @@ class JsonParserGenerator(
 
     override fun payloadParser(member: MemberShape): RuntimeType {
         val shape = model.expectShape(member.target)
+        val returnSymbolToParse = returnSymbolToParse(shape)
         check(shape is UnionShape || shape is StructureShape || shape is DocumentShape) {
-            "payload parser should only be used on structures & unions"
+            "Payload parser should only be used on structure shapes, union shapes, and document shapes."
         }
         return protocolFunctions.deserializeFn(shape, fnNameSuffix = "payload") { fnName ->
             rustBlockTemplate(
-                "pub fn $fnName(input: &[u8]) -> Result<#{Shape}, #{Error}>",
+                "pub(crate) fn $fnName(input: &[u8]) -> Result<#{ReturnType}, #{Error}>",
                 *codegenScope,
-                "Shape" to symbolProvider.toSymbol(shape),
+                "ReturnType" to returnSymbolToParse.symbol,
             ) {
                 val input = if (shape is DocumentShape) {
                     "input"