Add minimum supported AGP version

This is a new policy of support. This target will keep moving as AGP
releases new versions.

Author: hfmehmed

gradle-plugin/src/main/kotlin/com/google/devtools/ksp/gradle/KspSubplugin.kt

@@ -16,12 +16,15 @@
  */
 package com.google.devtools.ksp.gradle
 
+import com.android.build.api.AndroidPluginVersion
 import com.android.build.api.variant.Component
 import com.google.devtools.ksp.KspExperimental
 import com.google.devtools.ksp.gradle.model.builder.KspModelBuilder
+import com.google.devtools.ksp.gradle.utils.MINIMUM_SUPPORTED_AGP_VERSION
 import com.google.devtools.ksp.gradle.utils.canUseGeneratedKotlinApi
 import com.google.devtools.ksp.gradle.utils.canUseInternalKspApis
 import com.google.devtools.ksp.gradle.utils.enableProjectIsolationCompatibleCodepath
+import com.google.devtools.ksp.gradle.utils.getAgpVersion
 import com.google.devtools.ksp.gradle.utils.isAgpBuiltInKotlinUsed
 import com.google.devtools.ksp.gradle.utils.useLegacyVariantApi
 import org.gradle.api.Project
@@ -116,6 +119,7 @@ class KspGradleSubplugin @Inject internal constructor(private val registry: Tool
         registry.register(KspModelBuilder())
 
         target.plugins.withId("com.android.base") {
+            checkMinimumAgpVersion(target.getAgpVersion())
             val androidComponents =
                 target.extensions.findByType(com.android.build.api.variant.AndroidComponentsExtension::class.java)!!
 
@@ -130,6 +134,15 @@ class KspGradleSubplugin @Inject internal constructor(private val registry: Tool
         }
     }
 
+    private fun checkMinimumAgpVersion(agpVersion: AndroidPluginVersion?) {
+        if (agpVersion != null && agpVersion < MINIMUM_SUPPORTED_AGP_VERSION) {
+            throw RuntimeException(
+                "The minimum support AGP version is ${MINIMUM_SUPPORTED_AGP_VERSION.version}. " +
+                    "Please upgrade the AGP version in your project."
+            )
+        }
+    }
+
     override fun isApplicable(kotlinCompilation: KotlinCompilation<*>): Boolean {
         val project = kotlinCompilation.target.project
         val useKsp2 = project.extensions.getByType(KspExtension::class.java).useKsp2.get()

gradle-plugin/src/main/kotlin/com/google/devtools/ksp/gradle/utils/agpUtils.kt

@@ -42,3 +42,11 @@ fun Project.canUseInternalKspApis(): Boolean {
     val agpVersion = project.getAgpVersion() ?: return false
     return agpVersion >= AndroidPluginVersion(9, 0, 0).alpha(14)
 }
+
+/**
+ * Defines the minimum supported Android Gradle Plugin (AGP) version.
+ *
+ * KSP aims to support AGP versions released approximately within the last year
+ * from the current KSP release date.
+ */
+val MINIMUM_SUPPORTED_AGP_VERSION = AndroidPluginVersion(8, 3, 0)