Misleading error message from incomplete model

[kggilmer]

A model with @xmlFlattened attrubutes that is missing the referenced structure emits a "trait deprecated" error.

Source model:

namespace com.test

use aws.protocols#restXml

@restXml
service Example {
    version: "1.0.0",
    operations: [
        Foo
    ]
}

    @http(method: "POST", uri: "/foo")
    operation Foo {
        input: FooRequest,
        output: FooRequest
    }  
    
    @xmlName("CustomFooRequest")
    structure FooRequest {
        @xmlFlattened
        listVal: ListOfString,
        @xmlFlattened
        mapVal: MapOfInteger
    }

Expected error:

Only the error about the fact that `ListOfString` and/or `MapOfInteger` are undefined.  The deprecated trait error seems to be incorrect.

Actual Error:

[ERROR] com.test#FooRequest$listVal: Trait `xmlFlattened` cannot be applied to `com.test#FooRequest$listVal`. This trait may only be applied to shapes that match the following selector: :is(structure, union) > :test(member > :test(collection, map)) | TraitTarget test.smithy:24:30
[ERROR] com.test#FooRequest$listVal: member shape targets an unresolved shape `com.test#ListOfString` | Target test.smithy:25:1

Side note: when missing shapes are added there are no errors.

[srchase]

The deprecated trait error is being emitted for the use of the @restXml trait. It's unrelated to the @xmlFlattened trait use.

The broken references, i.e. the missing shape, do result in some confusing errors. Ideally, no further validation would run after broken references are found, that'd eliminate the TraitTarget error in this instance.