Remove TODO about required `httpHeader`-bound collection shapes (#1397)

We do support required `httpHeader`-bound collection shapes, as
evidenced by the test operation this commit adds, which is generated by
the server, that does not use `Option` for `required` member shapes.

This is because even though the deserialization always returns an
optional collection, we return `None` if no headers are found (as
opposed to an empty collection), so the call that sets the member field
on the operation structure does nothing; when `build()` gets called, a
`BuildError` is raised because the member is required.

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

@@ -14,7 +14,8 @@ use smithy.test#httpResponseTests
 service MiscService {
     operations: [
         TypeComplexityOperation,
-        InnerRequiredShapeOperation,
+        RequiredInnerShapeOperation,
+        RequiredHeaderCollectionOperation,
         ResponseCodeRequiredOperation,
         ResponseCodeHttpFallbackOperation,
         ResponseCodeDefaultOperation,
@@ -54,12 +55,12 @@ map MapA {
 /// This operation tests that (de)serializing required values from a nested
 /// shape works correctly.
 @http(uri: "/innerRequiredShapeOperation", method: "POST")
-operation InnerRequiredShapeOperation {
-    input: InnerRequiredShapeOperationInputOutput,
-    output: InnerRequiredShapeOperationInputOutput,
+operation RequiredInnerShapeOperation {
+    input: RequiredInnerShapeOperationInputOutput,
+    output: RequiredInnerShapeOperationInputOutput,
 }
 
-structure InnerRequiredShapeOperationInputOutput {
+structure RequiredInnerShapeOperationInputOutput {
     inner: InnerShape
 }
 
@@ -230,3 +231,27 @@ structure ResponseCodeRequiredOutput {
     }
 ])
 operation AcceptHeaderStarService {}
+
+@http(uri: "/required-header-collection-operation", method: "GET")
+operation RequiredHeaderCollectionOperation {
+    input: RequiredHeaderCollectionOperationInputOutput,
+    output: RequiredHeaderCollectionOperationInputOutput,
+}
+
+structure RequiredHeaderCollectionOperationInputOutput {
+    @required
+    @httpHeader("X-Required-List")
+    requiredHeaderList: HeaderList,
+
+    @required
+    @httpHeader("X-Required-Set")
+    requiredHeaderSet: HeaderSet,
+}
+
+list HeaderList {
+    member: String
+}
+
+set HeaderSet {
+    member: String
+}

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

@@ -365,7 +365,6 @@ class HttpBindingGenerator(
                 rust("let $parsedValue = $parsedValue?;")
             }
         }
-        // TODO(https://github.com/awslabs/smithy-rs/issues/837): this doesn't support non-optional vectors
         when (rustType) {
             is RustType.Vec ->
                 rust(