Enforce constraints for unnamed enums (#3884)

### Enforces Constraints for Unnamed Enums

This PR addresses the issue where, on the server side, unnamed enums
were incorrectly treated as infallible during deserialization, allowing
any string value to be converted without validation. The solution
introduces a `ConstraintViolation` and `TryFrom` implementation for
unnamed enums, ensuring that deserialized values conform to the enum
variants defined in the Smithy model.

The following is an example of an unnamed enum:

```smithy
@enum([
    { value: "MONDAY" },
    { value: "TUESDAY" }
])
string UnnamedDayOfWeek
```

On the server side the following type is generated for the Smithy shape:

```rust
pub struct UnnamedDayOfWeek(String);

impl ::std::convert::TryFrom<::std::string::String> for UnnamedDayOfWeek {
    type Error = crate::model::unnamed_day_of_week::ConstraintViolation;

    fn try_from(
        s: ::std::string::String,
    ) -> ::std::result::Result<Self, <Self as ::std::convert::TryFrom<::std::string::String>>::Error>
    {
        match s.as_str() {
            "MONDAY" | "TUESDAY" => Ok(Self(s)),
            _ => Err(crate::model::unnamed_day_of_week::ConstraintViolation(s)),
        }
    }
}
```

This change prevents invalid values from being deserialized into unnamed
enums and raises appropriate constraint violations when necessary.

There is one difference between the Rust code generated for
`TryFrom<String>` for named enums versus unnamed enums. The
implementation for unnamed enums passes the ownership of the `String`
parameter to the generated structure, and the implementation for
`TryFrom<&str>` delegates to `TryFrom<String>`.

```rust
impl ::std::convert::TryFrom<::std::string::String> for UnnamedDayOfWeek {
    type Error = crate::model::unnamed_day_of_week::ConstraintViolation;
    fn try_from(
        s: ::std::string::String,
    ) -> ::std::result::Result<Self, <Self as ::std::convert::TryFrom<::std::string::String>>::Error>
    {
        match s.as_str() {
            "MONDAY" | "TUESDAY" => Ok(Self(s)),
            _ => Err(crate::model::unnamed_day_of_week::ConstraintViolation(s)),
        }
    }
}

impl ::std::convert::TryFrom<&str> for UnnamedDayOfWeek {
    type Error = crate::model::unnamed_day_of_week::ConstraintViolation;
    fn try_from(
        s: &str,
    ) -> ::std::result::Result<Self, <Self as ::std::convert::TryFrom<&str>>::Error> {
        s.to_owned().try_into()
    }
}
```

On the client side, the behaviour is unchanged, and the client does not
validate for backward compatibility reasons. An [existing
test](https://github.com/smithy-lang/smithy-rs/pull/3884/files#diff-021ec60146cfe231105d21a7389f2dffcd546595964fbb3f0684ebf068325e48R82)
has been modified to ensure this.

```rust
#[test]
fn generate_unnamed_enums() {
    let result = "t2.nano"
        .parse::<crate::types::UnnamedEnum>()
        .expect("static value validated to member");
    assert_eq!(result, UnnamedEnum("t2.nano".to_owned()));
    let result = "not-a-valid-variant"
        .parse::<crate::types::UnnamedEnum>()
        .expect("static value validated to member");
    assert_eq!(result, UnnamedEnum("not-a-valid-variant".to_owned()));
}
```

Fixes issue #3880

---------

Co-authored-by: Fahad Zubair <fahadzub@amazon.com>

.changelog/4329788.md

@@ -0,0 +1,18 @@
+---
+applies_to: ["server"]
+authors: ["drganjoo"]
+references: ["smithy-rs#3880"]
+breaking: true
+new_feature: false
+bug_fix: true
+---
+Unnamed enums now validate assigned values and will raise a `ConstraintViolation` if an unknown variant is set.
+
+The following is an example of an unnamed enum:
+```smithy
+@enum([
+    { value: "MONDAY" },
+    { value: "TUESDAY" }
+])
+string UnnamedDayOfWeek
+```

codegen-client/src/main/kotlin/software/amazon/smithy/rust/codegen/client/smithy/generators/ClientEnumGenerator.kt

@@ -79,6 +79,37 @@ data class InfallibleEnumType(
             )
         }
 
+    override fun implFromForStrForUnnamedEnum(context: EnumGeneratorContext): Writable =
+        writable {
+            rustTemplate(
+                """
+                impl<T> #{From}<T> for ${context.enumName} where T: #{AsRef}<str> {
+                    fn from(s: T) -> Self {
+                        ${context.enumName}(s.as_ref().to_owned())
+                    }
+                }
+                """,
+                *preludeScope,
+            )
+        }
+
+    override fun implFromStrForUnnamedEnum(context: EnumGeneratorContext): Writable =
+        writable {
+            // Add an infallible FromStr implementation for uniformity
+            rustTemplate(
+                """
+                impl ::std::str::FromStr for ${context.enumName} {
+                    type Err = ::std::convert::Infallible;
+    
+                    fn from_str(s: &str) -> #{Result}<Self, <Self as ::std::str::FromStr>::Err> {
+                        #{Ok}(${context.enumName}::from(s))
+                    }
+                }
+                """,
+                *preludeScope,
+            )
+        }
+
     override fun additionalEnumImpls(context: EnumGeneratorContext): Writable =
         writable {
             // `try_parse` isn't needed for unnamed enums

codegen-client/src/test/kotlin/software/amazon/smithy/rust/codegen/client/smithy/generators/ClientInstantiatorTest.kt

@@ -69,6 +69,8 @@ internal class ClientInstantiatorTest {
         val shape = model.lookup<StringShape>("com.test#UnnamedEnum")
         val sut = ClientInstantiator(codegenContext)
         val data = Node.parse("t2.nano".dq())
+        // The client SDK should accept unknown variants as valid.
+        val notValidVariant = Node.parse("not-a-valid-variant".dq())
 
         val project = TestWorkspace.testProject(symbolProvider)
         project.moduleFor(shape) {
@@ -77,7 +79,11 @@ internal class ClientInstantiatorTest {
                 withBlock("let result = ", ";") {
                     sut.render(this, shape, data)
                 }
-                rust("""assert_eq!(result, UnnamedEnum("t2.nano".to_owned()));""")
+                rust("""assert_eq!(result, UnnamedEnum("$data".to_owned()));""")
+                withBlock("let result = ", ";") {
+                    sut.render(this, shape, notValidVariant)
+                }
+                rust("""assert_eq!(result, UnnamedEnum("$notValidVariant".to_owned()));""")
             }
         }
         project.compileAndTest()

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

@@ -59,6 +59,12 @@ abstract class EnumType {
     /** Returns a writable that implements `FromStr` for the enum */
     abstract fun implFromStr(context: EnumGeneratorContext): Writable
 
+    /** Returns a writable that implements `From<&str>` and/or `TryFrom<&str>` for the unnamed enum */
+    abstract fun implFromForStrForUnnamedEnum(context: EnumGeneratorContext): Writable
+
+    /** Returns a writable that implements `FromStr` for the unnamed enum */
+    abstract fun implFromStrForUnnamedEnum(context: EnumGeneratorContext): Writable
+
     /** Optionally adds additional documentation to the `enum` docs */
     open fun additionalDocs(context: EnumGeneratorContext): Writable = writable {}
 
@@ -237,32 +243,10 @@ open class EnumGenerator(
                     rust("&self.0")
                 },
         )
-
-        // Add an infallible FromStr implementation for uniformity
-        rustTemplate(
-            """
-            impl ::std::str::FromStr for ${context.enumName} {
-                type Err = ::std::convert::Infallible;
-
-                fn from_str(s: &str) -> #{Result}<Self, <Self as ::std::str::FromStr>::Err> {
-                    #{Ok}(${context.enumName}::from(s))
-                }
-            }
-            """,
-            *preludeScope,
-        )
-
-        rustTemplate(
-            """
-            impl<T> #{From}<T> for ${context.enumName} where T: #{AsRef}<str> {
-                fn from(s: T) -> Self {
-                    ${context.enumName}(s.as_ref().to_owned())
-                }
-            }
-
-            """,
-            *preludeScope,
-        )
+        // impl From<str> for Blah { ... }
+        enumType.implFromForStrForUnnamedEnum(context)(this)
+        // impl FromStr for Blah { ... }
+        enumType.implFromStrForUnnamedEnum(context)(this)
     }
 
     private fun RustWriter.renderEnum() {

codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/smithy/generators/EnumGeneratorTest.kt

@@ -494,6 +494,16 @@ class EnumGeneratorTest {
                         // intentional no-op
                     }
 
+                override fun implFromForStrForUnnamedEnum(context: EnumGeneratorContext): Writable =
+                    writable {
+                        // intentional no-op
+                    }
+
+                override fun implFromStrForUnnamedEnum(context: EnumGeneratorContext): Writable =
+                    writable {
+                        // intentional no-op
+                    }
+
                 override fun additionalEnumMembers(context: EnumGeneratorContext): Writable =
                     writable {
                         rust("// additional enum members")

codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/smithy/generators/TestEnumType.kt

@@ -10,6 +10,7 @@ import software.amazon.smithy.rust.codegen.core.rustlang.rust
 import software.amazon.smithy.rust.codegen.core.rustlang.rustTemplate
 import software.amazon.smithy.rust.codegen.core.rustlang.writable
 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.util.dq
 
 object TestEnumType : EnumType() {
@@ -49,4 +50,35 @@ object TestEnumType : EnumType() {
                 """,
             )
         }
+
+    override fun implFromForStrForUnnamedEnum(context: EnumGeneratorContext): Writable =
+        writable {
+            rustTemplate(
+                """
+                impl<T> #{From}<T> for ${context.enumName} where T: #{AsRef}<str> {
+                    fn from(s: T) -> Self {
+                        ${context.enumName}(s.as_ref().to_owned())
+                    }
+                }
+                """,
+                *preludeScope,
+            )
+        }
+
+    override fun implFromStrForUnnamedEnum(context: EnumGeneratorContext): Writable =
+        writable {
+            // Add an infallible FromStr implementation for uniformity
+            rustTemplate(
+                """
+                impl ::std::str::FromStr for ${context.enumName} {
+                    type Err = ::std::convert::Infallible;
+    
+                    fn from_str(s: &str) -> #{Result}<Self, <Self as ::std::str::FromStr>::Err> {
+                        #{Ok}(${context.enumName}::from(s))
+                    }
+                }
+                """,
+                *preludeScope,
+            )
+        }
 }

codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/generators/ServerEnumGenerator.kt

@@ -5,10 +5,9 @@
 package software.amazon.smithy.rust.codegen.server.smithy.generators
 
 import software.amazon.smithy.model.shapes.StringShape
+import software.amazon.smithy.rust.codegen.core.rustlang.RustWriter
 import software.amazon.smithy.rust.codegen.core.rustlang.Writable
 import software.amazon.smithy.rust.codegen.core.rustlang.rust
-import software.amazon.smithy.rust.codegen.core.rustlang.rustBlock
-import software.amazon.smithy.rust.codegen.core.rustlang.rustBlockTemplate
 import software.amazon.smithy.rust.codegen.core.rustlang.rustTemplate
 import software.amazon.smithy.rust.codegen.core.rustlang.writable
 import software.amazon.smithy.rust.codegen.core.smithy.RuntimeType
@@ -39,16 +38,14 @@ open class ConstrainedEnum(
         }
     private val constraintViolationSymbol = constraintViolationSymbolProvider.toSymbol(shape)
     private val constraintViolationName = constraintViolationSymbol.name
-    private val codegenScope =
-        arrayOf(
-            "String" to RuntimeType.String,
-        )
 
-    override fun implFromForStr(context: EnumGeneratorContext): Writable =
-        writable {
-            withInlineModule(constraintViolationSymbol.module(), codegenContext.moduleDocProvider) {
-                rustTemplate(
-                    """
+    private fun generateConstraintViolation(
+        context: EnumGeneratorContext,
+        generateTryFromStrAndString: RustWriter.(EnumGeneratorContext) -> Unit,
+    ) = writable {
+        withInlineModule(constraintViolationSymbol.module(), codegenContext.moduleDocProvider) {
+            rustTemplate(
+                """
                     ##[derive(Debug, PartialEq)]
                     pub struct $constraintViolationName(pub(crate) #{String});
                     
@@ -60,47 +57,86 @@ open class ConstrainedEnum(
 
                     impl #{Error} for $constraintViolationName {}
                     """,
-                    *codegenScope,
-                    "Error" to RuntimeType.StdError,
-                    "Display" to RuntimeType.Display,
-                )
+                *preludeScope,
+                "Error" to RuntimeType.StdError,
+                "Display" to RuntimeType.Display,
+            )
 
-                if (shape.isReachableFromOperationInput()) {
-                    rustTemplate(
-                        """
+            if (shape.isReachableFromOperationInput()) {
+                rustTemplate(
+                    """
                         impl $constraintViolationName {
                             #{EnumShapeConstraintViolationImplBlock:W}
                         }
                         """,
-                        "EnumShapeConstraintViolationImplBlock" to
-                            validationExceptionConversionGenerator.enumShapeConstraintViolationImplBlock(
-                                context.enumTrait,
-                            ),
-                    )
-                }
+                    "EnumShapeConstraintViolationImplBlock" to
+                        validationExceptionConversionGenerator.enumShapeConstraintViolationImplBlock(
+                            context.enumTrait,
+                        ),
+                )
             }
-            rustBlock("impl #T<&str> for ${context.enumName}", RuntimeType.TryFrom) {
-                rust("type Error = #T;", constraintViolationSymbol)
-                rustBlockTemplate("fn try_from(s: &str) -> #{Result}<Self, <Self as #{TryFrom}<&str>>::Error>", *preludeScope) {
-                    rustBlock("match s") {
-                        context.sortedMembers.forEach { member ->
-                            rust("${member.value.dq()} => Ok(${context.enumName}::${member.derivedName()}),")
+        }
+
+        generateTryFromStrAndString(context)
+    }
+
+    override fun implFromForStr(context: EnumGeneratorContext): Writable =
+        generateConstraintViolation(context) {
+            rustTemplate(
+                """
+                impl #{TryFrom}<&str> for ${context.enumName} {
+                    type Error = #{ConstraintViolation};
+                    fn try_from(s: &str) -> #{Result}<Self, <Self as #{TryFrom}<&str>>::Error> {
+                        match s {
+                            #{MatchArms}
+                            _ => Err(#{ConstraintViolation}(s.to_owned()))
                         }
-                        rust("_ => Err(#T(s.to_owned()))", constraintViolationSymbol)
                     }
                 }
-            }
+                impl #{TryFrom}<#{String}> for ${context.enumName} {
+                    type Error = #{ConstraintViolation};
+                    fn try_from(s: #{String}) -> #{Result}<Self, <Self as #{TryFrom}<#{String}>>::Error> {
+                        s.as_str().try_into()
+                    }
+                }
+                """,
+                *preludeScope,
+                "ConstraintViolation" to constraintViolationSymbol,
+                "MatchArms" to
+                    writable {
+                        context.sortedMembers.forEach { member ->
+                            rust("${member.value.dq()} => Ok(${context.enumName}::${member.derivedName()}),")
+                        }
+                    },
+            )
+        }
+
+    override fun implFromForStrForUnnamedEnum(context: EnumGeneratorContext): Writable =
+        generateConstraintViolation(context) {
             rustTemplate(
                 """
+                impl #{TryFrom}<&str> for ${context.enumName} {
+                    type Error = #{ConstraintViolation};
+                    fn try_from(s: &str) -> #{Result}<Self, <Self as #{TryFrom}<&str>>::Error> {
+                        s.to_owned().try_into()
+                    }
+                }
                 impl #{TryFrom}<#{String}> for ${context.enumName} {
                     type Error = #{ConstraintViolation};
                     fn try_from(s: #{String}) -> #{Result}<Self, <Self as #{TryFrom}<#{String}>>::Error> {
-                        s.as_str().try_into()
+                        match s.as_str() {
+                            #{Values} => Ok(Self(s)),
+                            _ => Err(#{ConstraintViolation}(s))
+                        }
                     }
                 }
                 """,
                 *preludeScope,
                 "ConstraintViolation" to constraintViolationSymbol,
+                "Values" to
+                    writable {
+                        rust(context.sortedMembers.joinToString(" | ") { it.value.dq() })
+                    },
             )
         }
 
@@ -118,6 +154,8 @@ open class ConstrainedEnum(
                 "ConstraintViolation" to constraintViolationSymbol,
             )
         }
+
+    override fun implFromStrForUnnamedEnum(context: EnumGeneratorContext) = implFromStr(context)
 }
 
 class ServerEnumGenerator(