Improve invalid HTTP status code responses failure mode handling

If the model only uses the `@http` trait to define the response code,
Smithy already checks that this is a valid status code, so the
conversion cannot fail, and we can avoid rejecting with
`ResponseRejection::InvalidHttpStatusCode`.

This commit also removes this variant from RPC protocols, where HTTP
binding traits are not allowed, and so this failure mode can never
happen.

codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/protocols/ServerHttpBoundProtocolGenerator.kt

@@ -439,17 +439,18 @@ class ServerHttpBoundProtocolTraitImplGenerator(
         Attribute.AllowUnusedMut.render(this)
         rustTemplate("let mut builder = #{http}::Response::builder();", *codegenScope)
         serverRenderResponseHeaders(operationShape)
-        // Fallback to the default code of `@http`, 200.
+        // Fallback to the default code of `@http`, which should be 200.
         val httpTraitDefaultStatusCode = HttpTrait
             .builder().method("GET").uri(UriPattern.parse("/")) /* Required to build */
             .build()
             .code
+        check(httpTraitDefaultStatusCode == 200)
         val httpTraitStatusCode = operationShape.getTrait<HttpTrait>()?.code ?: httpTraitDefaultStatusCode
         bindings.find { it.location == HttpLocation.RESPONSE_CODE }
             ?.let {
                 serverRenderResponseCodeBinding(it, httpTraitStatusCode)(this)
             }
-            // no binding, use http's
+            // No binding, use `@http`.
             ?: serverRenderHttpResponseCode(httpTraitStatusCode)(this)
 
         operationShape.outputShape(model).findStreamingMember(model)?.let {
@@ -555,46 +556,42 @@ class ServerHttpBoundProtocolTraitImplGenerator(
         )
     }
 
-    private fun serverRenderHttpResponseCode(
-        defaultCode: Int,
-    ): Writable {
-        return writable {
-            rustTemplate(
-                """
-                let status = $defaultCode;
-                let http_status: u16 = status.try_into()
-                    .map_err(|_| #{ResponseRejection}::InvalidHttpStatusCode)?;
-                builder = builder.status(http_status);
-                """.trimIndent(),
-                *codegenScope,
-            )
+    private fun serverRenderHttpResponseCode(defaultCode: Int) = writable {
+        check(defaultCode in 100..999) {
+            """
+           Smithy library lied to us. According to https://smithy.io/2.0/spec/http-bindings.html#http-trait,
+           "The provided value SHOULD be between 100 and 599, and it MUST be between 100 and 999".
+           """.replace("\n", "").trimIndent()
         }
+        rustTemplate(
+            """
+            let http_status: u16 = $defaultCode;
+            builder = builder.status(http_status);
+            """,
+            *codegenScope,
+        )
     }
 
     private fun serverRenderResponseCodeBinding(
         binding: HttpBindingDescriptor,
+        /** This is the status code to fall back on if the member shape bound with `@httpResponseCode` is not
+         * `@required` and the user did not provide a value for it at runtime. **/
         fallbackStatusCode: Int,
     ): Writable {
         check(binding.location == HttpLocation.RESPONSE_CODE)
 
         return writable {
             val memberName = symbolProvider.toMemberName(binding.member)
-            rust("let status = output.$memberName")
-            if (symbolProvider.toSymbol(binding.member).isOptional()) {
-                rustTemplate(
-                    """
-                    .unwrap_or($fallbackStatusCode)
-                    """.trimIndent(),
-                    *codegenScope,
-                )
+            withBlock("let status = output.$memberName", ";") {
+                if (symbolProvider.toSymbol(binding.member).isOptional()) {
+                    rust(".unwrap_or($fallbackStatusCode)")
+                }
             }
             rustTemplate(
                 """
-                ;
-                let http_status: u16 = status.try_into()
-                    .map_err(|_| #{ResponseRejection}::InvalidHttpStatusCode)?;
+                let http_status: u16 = status.try_into().map_err(#{ResponseRejection}::InvalidHttpStatusCode)?;
                 builder = builder.status(http_status);
-                """.trimIndent(),
+                """,
                 *codegenScope,
             )
         }

rust-runtime/aws-smithy-http-server/src/proto/aws_json/rejection.rs

@@ -9,7 +9,6 @@ use crate::rejection::MissingContentTypeReason;
 
 #[derive(Debug, Display)]
 pub enum ResponseRejection {
-    InvalidHttpStatusCode,
     Serialization(crate::Error),
     Http(crate::Error),
 }

rust-runtime/aws-smithy-http-server/src/proto/rest_json_1/rejection.rs

@@ -47,6 +47,8 @@
 //!
 //! Consult `crate::proto::$protocolName::rejection` for rejection types for other protocols.
 
+use std::num::TryFromIntError;
+
 use strum_macros::Display;
 
 use crate::rejection::MissingContentTypeReason;
@@ -58,7 +60,7 @@ pub enum ResponseRejection {
     /// Used when the service implementer provides an integer outside the 100-999 range for a
     /// member targeted by `httpResponseCode`.
     /// See <https://github.com/awslabs/smithy/issues/1116>.
-    InvalidHttpStatusCode,
+    InvalidHttpStatusCode(TryFromIntError),
 
     /// Used when an invalid HTTP header value (a value that cannot be parsed as an
     /// `[http::header::HeaderValue]`) is provided for a shape member bound to an HTTP header with

rust-runtime/aws-smithy-http-server/src/proto/rest_xml/rejection.rs

@@ -7,11 +7,13 @@
 //! [`crate::proto::rest_json_1::rejection::RequestRejection::JsonDeserialize`] is swapped for
 //! [`RequestRejection::XmlDeserialize`].
 
+use std::num::TryFromIntError;
+
 use strum_macros::Display;
 
 #[derive(Debug, Display)]
 pub enum ResponseRejection {
-    InvalidHttpStatusCode,
+    InvalidHttpStatusCode(TryFromIntError),
     Build(crate::Error),
     Serialization(crate::Error),
     Http(crate::Error),