Source defaults from the model instead of implicitly (#2985)

## Motivation and Context
We weren't correctly computing defaults which lead to incorrect behavior
when coupled with nullability.

## Description
Minimal changeset to source defaults from the model. Other changes:
- Unify enum parsing across client and server to always use `from_str`
in protocol tests
- Extract `PrimitiveInstantiator` from `Instantiator` so it can be used
to instantiate defaults

## Testing
- regular codegen tests

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

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

CHANGELOG.next.toml

@@ -150,3 +150,9 @@ message = "Fix regression with redacting sensitive HTTP response bodies."
 references = ["smithy-rs#2926", "smithy-rs#2972"]
 meta = { "breaking" = false, "tada" = false, "bug" = true, "target" = "client" }
 author = "ysaito1001"
+
+[[smithy-rs]]
+message = "Source defaults from the default trait instead of implicitly based on type. This has minimal changes in the generated code."
+references = ["smithy-rs#2985"]
+meta = { "breaking" = false, "tada" = false, "bug" = true, "target" = "client" }
+author = "rcoh"

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

@@ -5,7 +5,6 @@
 
 package software.amazon.smithy.rust.codegen.client.smithy.generators
 
-import software.amazon.smithy.codegen.core.Symbol
 import software.amazon.smithy.model.node.Node
 import software.amazon.smithy.model.node.ObjectNode
 import software.amazon.smithy.model.shapes.MemberShape
@@ -14,18 +13,12 @@ import software.amazon.smithy.model.shapes.StructureShape
 import software.amazon.smithy.rust.codegen.client.smithy.ClientCodegenContext
 import software.amazon.smithy.rust.codegen.client.smithy.generators.client.FluentClientGenerator
 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.writable
 import software.amazon.smithy.rust.codegen.core.smithy.CodegenContext
 import software.amazon.smithy.rust.codegen.core.smithy.generators.BuilderGenerator
 import software.amazon.smithy.rust.codegen.core.smithy.generators.Instantiator
 import software.amazon.smithy.rust.codegen.core.smithy.generators.setterName
 
-private fun enumFromStringFn(enumSymbol: Symbol, data: String): Writable = writable {
-    rust("#T::from($data)", enumSymbol)
-}
-
 class ClientBuilderKindBehavior(val codegenContext: CodegenContext) : Instantiator.BuilderKindBehavior {
     override fun hasFallibleBuilder(shape: StructureShape): Boolean =
         BuilderGenerator.hasFallibleBuilder(shape, codegenContext.symbolProvider)
@@ -40,7 +33,6 @@ class ClientInstantiator(private val codegenContext: ClientCodegenContext) : Ins
     codegenContext.model,
     codegenContext.runtimeConfig,
     ClientBuilderKindBehavior(codegenContext),
-    ::enumFromStringFn,
 ) {
     fun renderFluentCall(
         writer: RustWriter,

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/Writable.kt

@@ -30,6 +30,11 @@ fun Writable.map(f: RustWriter.(Writable) -> Unit): Writable {
     return writable { f(self) }
 }
 
+/** Returns Some(..arg) */
+fun Writable.some(): Writable {
+    return this.map { rust("Some(#T)", it) }
+}
+
 fun Writable.isNotEmpty(): Boolean = !this.isEmpty()
 
 operator fun Writable.plus(other: Writable): Writable {

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

@@ -6,6 +6,7 @@
 package software.amazon.smithy.rust.codegen.core.smithy
 
 import software.amazon.smithy.codegen.core.Symbol
+import software.amazon.smithy.model.node.Node
 import software.amazon.smithy.model.shapes.Shape
 import software.amazon.smithy.rust.codegen.core.rustlang.RustModule
 import software.amazon.smithy.rust.codegen.core.rustlang.RustType
@@ -102,6 +103,11 @@ sealed class Default {
      * This symbol should use the Rust `std::default::Default` when unset
      */
     object RustDefault : Default()
+
+    /**
+     * This symbol has a custom default value different from `Default::default`
+     */
+    data class NonZeroDefault(val value: Node) : Default()
 }
 
 /**

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

@@ -10,6 +10,7 @@ import software.amazon.smithy.codegen.core.SymbolProvider
 import software.amazon.smithy.model.Model
 import software.amazon.smithy.model.knowledge.NullableIndex
 import software.amazon.smithy.model.knowledge.NullableIndex.CheckMode
+import software.amazon.smithy.model.node.Node
 import software.amazon.smithy.model.shapes.BigDecimalShape
 import software.amazon.smithy.model.shapes.BigIntegerShape
 import software.amazon.smithy.model.shapes.BlobShape
@@ -37,6 +38,7 @@ import software.amazon.smithy.model.shapes.StringShape
 import software.amazon.smithy.model.shapes.StructureShape
 import software.amazon.smithy.model.shapes.TimestampShape
 import software.amazon.smithy.model.shapes.UnionShape
+import software.amazon.smithy.model.traits.DefaultTrait
 import software.amazon.smithy.model.traits.EnumTrait
 import software.amazon.smithy.model.traits.ErrorTrait
 import software.amazon.smithy.rust.codegen.core.rustlang.Attribute
@@ -48,6 +50,7 @@ import software.amazon.smithy.rust.codegen.core.smithy.traits.RustBoxTrait
 import software.amazon.smithy.rust.codegen.core.util.PANIC
 import software.amazon.smithy.rust.codegen.core.util.hasTrait
 import software.amazon.smithy.rust.codegen.core.util.letIf
+import software.amazon.smithy.rust.codegen.core.util.orNull
 import software.amazon.smithy.rust.codegen.core.util.toPascalCase
 import software.amazon.smithy.rust.codegen.core.util.toSnakeCase
 import kotlin.reflect.KClass
@@ -79,16 +82,18 @@ data class MaybeRenamed(val name: String, val renamedFrom: String?)
 /**
  * Make the return [value] optional if the [member] symbol is as well optional.
  */
-fun SymbolProvider.wrapOptional(member: MemberShape, value: String): String = value.letIf(toSymbol(member).isOptional()) {
-    "Some($value)"
-}
+fun SymbolProvider.wrapOptional(member: MemberShape, value: String): String =
+    value.letIf(toSymbol(member).isOptional()) {
+        "Some($value)"
+    }
 
 /**
  * Make the return [value] optional if the [member] symbol is not optional.
  */
-fun SymbolProvider.toOptional(member: MemberShape, value: String): String = value.letIf(!toSymbol(member).isOptional()) {
-    "Some($value)"
-}
+fun SymbolProvider.toOptional(member: MemberShape, value: String): String =
+    value.letIf(!toSymbol(member).isOptional()) {
+        "Some($value)"
+    }
 
 /**
  * Services can rename their contained shapes. See https://awslabs.github.io/smithy/1.0/spec/core/model.html#service
@@ -111,7 +116,7 @@ fun Shape.contextName(serviceShape: ServiceShape?): String {
  */
 open class SymbolVisitor(
     settings: CoreRustSettings,
-    override val model: Model,
+    final override val model: Model,
     private val serviceShape: ServiceShape?,
     override val config: RustSymbolProviderConfig,
 ) : RustSymbolProvider, ShapeVisitor<Symbol> {
@@ -170,7 +175,7 @@ open class SymbolVisitor(
     }
 
     private fun simpleShape(shape: SimpleShape): Symbol {
-        return symbolBuilder(shape, SimpleShapes.getValue(shape::class)).setDefault(Default.RustDefault).build()
+        return symbolBuilder(shape, SimpleShapes.getValue(shape::class)).build()
     }
 
     override fun booleanShape(shape: BooleanShape): Symbol = simpleShape(shape)
@@ -263,13 +268,20 @@ open class SymbolVisitor(
 
     override fun memberShape(shape: MemberShape): Symbol {
         val target = model.expectShape(shape.target)
+        val defaultValue = shape.getMemberTrait(model, DefaultTrait::class.java).orNull()?.let { trait ->
+            when (val value = trait.toNode()) {
+                Node.from(""), Node.from(0), Node.from(false), Node.arrayNode(), Node.objectNode() -> Default.RustDefault
+                Node.nullNode() -> Default.NoDefault
+                else -> Default.NonZeroDefault(value)
+            }
+        } ?: Default.NoDefault
         // Handle boxing first, so we end up with Option<Box<_>>, not Box<Option<_>>.
         return handleOptionality(
             handleRustBoxing(toSymbol(target), shape),
             shape,
             nullableIndex,
             config.nullabilityCheckMode,
-        )
+        ).toBuilder().setDefault(defaultValue).build()
     }
 
     override fun timestampShape(shape: TimestampShape?): Symbol {
@@ -297,7 +309,12 @@ fun symbolBuilder(shape: Shape?, rustType: RustType): Symbol.Builder =
         // If we ever generate a `thisisabug.rs`, there is a bug in our symbol generation
         .definitionFile("thisisabug.rs")
 
-fun handleOptionality(symbol: Symbol, member: MemberShape, nullableIndex: NullableIndex, nullabilityCheckMode: CheckMode): Symbol =
+fun handleOptionality(
+    symbol: Symbol,
+    member: MemberShape,
+    nullableIndex: NullableIndex,
+    nullabilityCheckMode: CheckMode,
+): Symbol =
     symbol.letIf(nullableIndex.isMemberNullable(member, nullabilityCheckMode)) { symbol.makeOptional() }
 
 /**

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

@@ -32,7 +32,6 @@ import software.amazon.smithy.rust.codegen.core.rustlang.stripOuter
 import software.amazon.smithy.rust.codegen.core.rustlang.withBlock
 import software.amazon.smithy.rust.codegen.core.rustlang.writable
 import software.amazon.smithy.rust.codegen.core.smithy.CodegenContext
-import software.amazon.smithy.rust.codegen.core.smithy.Default
 import software.amazon.smithy.rust.codegen.core.smithy.RuntimeConfig
 import software.amazon.smithy.rust.codegen.core.smithy.RuntimeType
 import software.amazon.smithy.rust.codegen.core.smithy.RuntimeType.Companion.preludeScope
@@ -41,7 +40,6 @@ import software.amazon.smithy.rust.codegen.core.smithy.canUseDefault
 import software.amazon.smithy.rust.codegen.core.smithy.customize.NamedCustomization
 import software.amazon.smithy.rust.codegen.core.smithy.customize.Section
 import software.amazon.smithy.rust.codegen.core.smithy.customize.writeCustomizations
-import software.amazon.smithy.rust.codegen.core.smithy.defaultValue
 import software.amazon.smithy.rust.codegen.core.smithy.expectRustMetadata
 import software.amazon.smithy.rust.codegen.core.smithy.isOptional
 import software.amazon.smithy.rust.codegen.core.smithy.makeOptional
@@ -385,15 +383,22 @@ class BuilderGenerator(
             members.forEach { member ->
                 val memberName = symbolProvider.toMemberName(member)
                 val memberSymbol = symbolProvider.toSymbol(member)
-                val default = memberSymbol.defaultValue()
                 withBlock("$memberName: self.$memberName", ",") {
-                    // Write the modifier
-                    when {
-                        !memberSymbol.isOptional() && default == Default.RustDefault -> rust(".unwrap_or_default()")
-                        !memberSymbol.isOptional() -> withBlock(
-                            ".ok_or_else(||",
-                            ")?",
-                        ) { missingRequiredField(memberName) }
+                    val generator = DefaultValueGenerator(runtimeConfig, symbolProvider, model)
+                    val default = generator.defaultValue(member)
+                    if (!memberSymbol.isOptional()) {
+                        if (default != null) {
+                            if (default.isRustDefault) {
+                                rust(".unwrap_or_default()")
+                            } else {
+                                rust(".unwrap_or_else(#T)", default.expr)
+                            }
+                        } else {
+                            withBlock(
+                                ".ok_or_else(||",
+                                ")?",
+                            ) { missingRequiredField(memberName) }
+                        }
                     }
                 }
             }

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

@@ -0,0 +1,40 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.rust.codegen.core.smithy.generators
+
+import software.amazon.smithy.model.Model
+import software.amazon.smithy.model.shapes.MemberShape
+import software.amazon.smithy.model.shapes.SimpleShape
+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.writable
+import software.amazon.smithy.rust.codegen.core.smithy.Default
+import software.amazon.smithy.rust.codegen.core.smithy.RuntimeConfig
+import software.amazon.smithy.rust.codegen.core.smithy.RustSymbolProvider
+import software.amazon.smithy.rust.codegen.core.smithy.defaultValue
+
+class DefaultValueGenerator(
+    runtimeConfig: RuntimeConfig,
+    private val symbolProvider: RustSymbolProvider,
+    private val model: Model,
+) {
+    private val instantiator = PrimitiveInstantiator(runtimeConfig, symbolProvider)
+
+    data class DefaultValue(val isRustDefault: Boolean, val expr: Writable)
+
+    /** Returns the default value as set by the defaultValue trait */
+    fun defaultValue(member: MemberShape): DefaultValue? {
+        val target = model.expectShape(member.target)
+        return when (val default = symbolProvider.toSymbol(member).defaultValue()) {
+            is Default.NoDefault -> null
+            is Default.RustDefault -> DefaultValue(isRustDefault = true, writable("Default::default"))
+            is Default.NonZeroDefault -> {
+                val instantiation = instantiator.instantiate(target as SimpleShape, default.value)
+                DefaultValue(isRustDefault = false, writable { rust("||#T", instantiation) })
+            }
+        }
+    }
+}

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

@@ -232,13 +232,28 @@ open class EnumGenerator(
             },
         )
 
+        // 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,
         )