Merge documentation of same inline modules (#4008)

When two inline modules with the same name are generated from different
parts of the codebase, their documentation should be merged. However,
other metadata must match exactly, as it is an error for one part of the
codebase to define an inline module with pub visibility while another
defines it with pub(crate) visibility.

This PR enables documentation merging while maintaining strict
validation of other metadata fields.

Currently, the following sample model fails to generate code because
both `SomeList` and `member` are generated in the same inline module but
with different doc comments:

```smithy
@documentation("Outer constraint has some documentation")
@Length(max: 3)
list SomeList {
    @Length(max: 8000)
    member: String
}
```

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

Author: drganjoo

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

@@ -352,25 +352,74 @@ class InnerModule(private val moduleDocProvider: ModuleDocProvider, debugMode: B
         inlineModuleList: MutableList<InlineModuleWithWriter>,
         lookForModule: RustModule.LeafModule,
     ): RustWriter {
-        val inlineModuleAndWriter =
-            inlineModuleList.firstOrNull {
-                it.inlineModule.name == lookForModule.name
-            }
-        return if (inlineModuleAndWriter == null) {
+        val index = inlineModuleList.indexOfFirst { it.inlineModule.name == lookForModule.name }
+
+        return if (index == -1) {
             val inlineWriter = createNewInlineModule()
             inlineModuleList.add(InlineModuleWithWriter(lookForModule, inlineWriter))
             inlineWriter
         } else {
-            check(inlineModuleAndWriter.inlineModule == lookForModule) {
-                """The two inline modules have the same name but different attributes on them:
-                1) ${inlineModuleAndWriter.inlineModule}
-                2) $lookForModule"""
+            val existing = inlineModuleList[index]
+
+            // Verify everything except documentation matches.
+            val existingModule = existing.inlineModule
+            check(
+                existingModule.name == lookForModule.name &&
+                    existingModule.rustMetadata == lookForModule.rustMetadata &&
+                    existingModule.parent == lookForModule.parent &&
+                    existingModule.tests == lookForModule.tests,
+            ) {
+                """
+                An inline module with the same name `${lookForModule.name}` was earlier created with different attributes:
+                1) Metadata:
+                    `${existingModule.rustMetadata}`
+                    `${lookForModule.rustMetadata}`
+                2) Parent:
+                    `${existingModule.parent}`
+                    `${lookForModule.parent}`
+                3) Tests:
+                    `${existingModule.tests}`
+                    `${lookForModule.tests}`
+                4) DocumentationOverride:
+                    `${existingModule.documentationOverride}`
+                    `${lookForModule.documentationOverride}`
+                """
             }
 
-            inlineModuleAndWriter.writer
+            // Merge documentation.
+            val mergedDoc =
+                mergeDocumentation(existingModule.documentationOverride, lookForModule.documentationOverride)
+
+            // Replace the element in the list with merged documentation.
+            inlineModuleList[index] =
+                InlineModuleWithWriter(
+                    existingModule.copy(documentationOverride = mergedDoc),
+                    existing.writer,
+                )
+
+            existing.writer
         }
     }
 
+    // @VisibleForTesting
+    internal fun mergeDocumentation(
+        existingModule: String?,
+        lookForModule: String?,
+    ): String? {
+        val mergedDoc =
+            when {
+                existingModule == null && lookForModule == null -> null
+                existingModule == null -> lookForModule
+                lookForModule == null -> existingModule
+                else ->
+                    listOf(
+                        existingModule.trim(),
+                        lookForModule.trim(),
+                    ).joinToString("\n")
+            }
+        return mergedDoc?.trim()
+    }
+
     private fun writeDocs(innerModule: RustModule.LeafModule) {
         docWriters[innerModule]?.forEach {
             it()

codegen-server/src/test/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ConstraintsMemberShapeTest.kt

@@ -14,7 +14,10 @@ import software.amazon.smithy.model.SourceLocation
 import software.amazon.smithy.model.shapes.ShapeId
 import software.amazon.smithy.model.traits.RequiredTrait
 import software.amazon.smithy.model.traits.Trait
+import software.amazon.smithy.rust.codegen.core.rustlang.RustModule
 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.smithy.ModuleDocProvider
 import software.amazon.smithy.rust.codegen.core.smithy.transformers.OperationNormalizer
 import software.amazon.smithy.rust.codegen.core.testutil.IntegrationTestParams
 import software.amazon.smithy.rust.codegen.core.testutil.asSmithyModel
@@ -87,6 +90,7 @@ class ConstraintsMemberShapeTest {
             @pattern("^[g-m]+${'$'}")
             constrainedPatternString : PatternString
 
+            constrainedList : ConstrainedList
             plainStringList : PlainStringList
             patternStringList : PatternStringList
             patternStringListOverride : PatternStringListOverride
@@ -118,6 +122,11 @@ class ConstraintsMemberShapeTest {
             @range(min: 10, max:100)
             long : RangedInteger
         }
+        @length(max: 3)
+        list ConstrainedList {
+            @length(max: 8000)
+            member: String
+        }
         list PlainStringList {
             member: String
         }
@@ -285,7 +294,12 @@ class ConstraintsMemberShapeTest {
 
     @Test
     fun `generate code and check member constrained shapes are in the right modules`() {
-        serverIntegrationTest(sampleModel, IntegrationTestParams(service = "constrainedMemberShape#ConstrainedService")) { codegenContext, rustCrate ->
+        serverIntegrationTest(
+            sampleModel,
+            IntegrationTestParams(
+                service = "constrainedMemberShape#ConstrainedService",
+            ),
+        ) { _, rustCrate ->
             fun RustWriter.testTypeExistsInBuilderModule(typeName: String) {
                 unitTest(
                     "builder_module_has_${typeName.toSnakeCase()}",
@@ -347,6 +361,27 @@ class ConstraintsMemberShapeTest {
         }
     }
 
+    @Test
+    fun `merging docs should not produce extra empty lines`() {
+        val docWriter =
+            object : ModuleDocProvider {
+                override fun docsWriter(module: RustModule.LeafModule): Writable? = null
+            }
+        val innerModule = InnerModule(docWriter, false)
+        innerModule.mergeDocumentation("\n\n", "") shouldBe ""
+        innerModule.mergeDocumentation(null, null) shouldBe null
+        innerModule.mergeDocumentation(null, "some docs\n") shouldBe "some docs"
+        innerModule.mergeDocumentation("some docs\n", null) shouldBe "some docs"
+        innerModule.mergeDocumentation(null, "some docs") shouldBe "some docs"
+        innerModule.mergeDocumentation("some docs", null) shouldBe "some docs"
+        innerModule.mergeDocumentation(null, "some docs\n\n") shouldBe "some docs"
+        innerModule.mergeDocumentation("some docs\n\n", null) shouldBe "some docs"
+        innerModule.mergeDocumentation(null, "some docs\n\n") shouldBe "some docs"
+        innerModule.mergeDocumentation("left side", "right side") shouldBe "left side\nright side"
+        innerModule.mergeDocumentation("left side\n", "right side\n") shouldBe "left side\nright side"
+        innerModule.mergeDocumentation("left side\n\n\n", "right side\n\n") shouldBe "left side\nright side"
+    }
+
     /**
      *  Checks that the given member shape:
      *  1. Has been changed to a new shape