Generate MSRV in Kotlin file (#3869)

Currently, when generating Rust crates, the MSRV is read from
`gradle.properties` by determining the project root using `git
rev-parse`. However, some build environments smithy-rs needs to build on
don't have `git` available.

This commit uses Gradle's `rootDir` to determine the project root, and
generates a Kotlin file exposing the MSRV as part of a Gradle build task
that is then published as part of `codegen-core.jar`. This also has the
benefit that consumers of smithy-rs can read the MSRV from there.

----

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

codegen-core/build.gradle.kts

@@ -5,6 +5,7 @@
 
 import org.gradle.api.tasks.testing.logging.TestExceptionFormat
 import java.io.ByteArrayOutputStream
+import java.util.Properties
 
 plugins {
     kotlin("jvm")
@@ -53,6 +54,44 @@ fun gitCommitHash(): String {
     }
 }
 
+// Define the directory where the generated Kotlin file will be placed
+val generatedSrcDir = layout.buildDirectory.dir("generated/src/main/kotlin")
+
+sourceSets {
+    main {
+        kotlin {
+            srcDir(generatedSrcDir)
+        }
+    }
+}
+
+val generateBuildEnvironmentConstants = tasks.register("generateBuildEnvironmentConstants") {
+    // Specify that the task generates sources.
+    val outputDir = generatedSrcDir.get().asFile
+    outputs.dir(outputDir)
+
+    doLast {
+        // Load properties from `gradle.properties`.
+        val properties = Properties()
+        val gradlePropertiesFile = file("${rootDir}/gradle.properties")
+        properties.load(gradlePropertiesFile.inputStream())
+
+        val rustMsrv = properties.getProperty("rust.msrv")
+
+        // Generate the Kotlin file.
+        val generatedFile = file("$outputDir/BuildEnvironment.kt")
+        generatedFile.writeText("""
+            // This file is automatically generated. Do not modify manually.
+            package software.amazon.smithy.rust.codegen.core.generated
+
+            object BuildEnvironment {
+                const val MSRV: String = "$rustMsrv"
+                const val PROJECT_DIR: String = "$rootDir"
+            }
+        """.trimIndent())
+    }
+}
+
 val generateSmithyRuntimeCrateVersion by tasks.registering {
     // Generate the version of the runtime to use as a resource.
     // This keeps us from having to manually change version numbers in multiple places.
@@ -125,6 +164,7 @@ java {
 tasks.compileKotlin {
     kotlinOptions.jvmTarget = "11"
     dependsOn(generateSmithyRuntimeCrateVersion)
+    dependsOn(generateBuildEnvironmentConstants)
 }
 
 // Reusable license copySpec

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

@@ -15,6 +15,7 @@ import software.amazon.smithy.model.loader.ModelAssembler
 import software.amazon.smithy.model.node.Node
 import software.amazon.smithy.model.node.ObjectNode
 import software.amazon.smithy.model.shapes.ShapeId
+import software.amazon.smithy.rust.codegen.core.generated.BuildEnvironment
 import software.amazon.smithy.rust.codegen.core.rustlang.Attribute
 import software.amazon.smithy.rust.codegen.core.rustlang.CargoDependency
 import software.amazon.smithy.rust.codegen.core.rustlang.DependencyScope
@@ -40,11 +41,10 @@ import software.amazon.smithy.rust.codegen.core.util.letIf
 import software.amazon.smithy.rust.codegen.core.util.orNullIfEmpty
 import software.amazon.smithy.rust.codegen.core.util.runCommand
 import java.io.File
-import java.io.FileInputStream
 import java.nio.file.Files
 import java.nio.file.Files.createTempDirectory
 import java.nio.file.Path
-import java.util.Properties
+import kotlin.io.path.Path
 import kotlin.io.path.absolutePathString
 import kotlin.io.path.writeText
 
@@ -56,8 +56,6 @@ val TestModuleDocProvider =
             }
     }
 
-val projectRootDir by lazy { File("git rev-parse --show-toplevel".runCommand().replace("\n", "")) }
-
 /**
  * Waiting for Kotlin to stabilize their temp directory functionality
  */
@@ -73,23 +71,14 @@ private fun tempDir(directory: File? = null): File {
  * This function returns the minimum supported Rust version, as specified in the `gradle.properties` file
  * located at the root of the project.
  */
-fun msrv(): String {
-    val properties = Properties()
-    val propertiesFilePath = projectRootDir.resolve("gradle.properties")
-
-    FileInputStream(propertiesFilePath).use { inputStream ->
-        properties.load(inputStream)
-    }
-
-    return properties.getProperty("rust.msrv")
-}
+fun msrv(): String = BuildEnvironment.MSRV
 
 /**
  * Generates the `rust-toolchain.toml` file in the specified directory.
  *
  * The compiler version is set in `gradle.properties` under the `rust.msrv` property.
- * The Gradle task `GenerateMsrvTask` generates the Kotlin class
- * `software.amazon.smithy.rust.codegen.core.Msrv` and writes the value of `rust.msrv` into it.
+ * The Gradle task `generateRustMsrvFile` generates the Kotlin class
+ * `software.amazon.smithy.rust.codegen.core.generated.RustMsrv.kt` and writes the value of `rust.msrv` into it.
  */
 private fun File.generateRustToolchainToml() {
     resolve("rust-toolchain.toml").writeText(
@@ -123,7 +112,7 @@ object TestWorkspace {
     private val subprojects = mutableListOf<String>()
 
     private val cargoLock: File by lazy {
-        projectRootDir.resolve("aws/sdk/Cargo.lock")
+        File(BuildEnvironment.PROJECT_DIR).resolve("aws/sdk/Cargo.lock")
     }
 
     init {

codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/util/RustToolChainTomlTest.kt

@@ -9,9 +9,10 @@ import io.kotest.matchers.booleans.shouldBeTrue
 import io.kotest.matchers.paths.shouldExist
 import io.kotest.matchers.shouldNotBe
 import org.junit.jupiter.api.Test
+import software.amazon.smithy.rust.codegen.core.generated.BuildEnvironment
 import software.amazon.smithy.rust.codegen.core.testutil.asSmithyModel
 import software.amazon.smithy.rust.codegen.core.testutil.generatePluginContext
-import software.amazon.smithy.rust.codegen.core.testutil.projectRootDir
+import java.io.File
 import java.nio.file.Files.createTempDirectory
 import java.util.regex.Pattern
 
@@ -48,7 +49,7 @@ internal class RustToolChainTomlTest {
 
         // Read the MSRV written in `gradle.properties` file.
         val msrvPattern = Pattern.compile("rust\\.msrv=(.+)")
-        val gradlePropertiesPath = projectRootDir.resolve("gradle.properties")
+        val gradlePropertiesPath = File(BuildEnvironment.PROJECT_DIR).resolve("gradle.properties")
         val msrv =
             gradlePropertiesPath.useLines { lines ->
                 lines.firstNotNullOfOrNull { line ->
@@ -75,12 +76,11 @@ internal class RustToolChainTomlTest {
 
                 // There should be a [toolchain] table, and it must have a key called 'channel' whose value must
                 // match the `rust.msrv` specified in gradle.properties.
-                toolchainSection != null &&
-                    toolchainSection.any { line ->
-                        channelPattern.matcher(line).let { matcher ->
-                            matcher.find() && matcher.group(1) == msrv
-                        }
+                toolchainSection.any { line ->
+                    channelPattern.matcher(line).let { matcher ->
+                        matcher.find() && matcher.group(1) == msrv
                     }
+                }
             }
 
         channelMatches.shouldBeTrue()