Filter out /build folders on Eclipse classpath

Add `EclipseConventions` to filter out build folders contributed by
the Kotlin Gradle Plugin.

Closes gh-35101

buildSrc/src/main/java/org/springframework/boot/build/ConventionsPlugin.java

@@ -48,6 +48,7 @@ public class ConventionsPlugin implements Plugin<Project> {
 		new AsciidoctorConventions().apply(project);
 		new KotlinConventions().apply(project);
 		new WarConventions().apply(project);
+		new EclipseConventions().apply(project);
 	}
 
 }

buildSrc/src/main/java/org/springframework/boot/build/EclipseConventions.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.build;
+
+import org.gradle.api.Project;
+import org.gradle.plugins.ide.api.XmlFileContentMerger;
+import org.gradle.plugins.ide.eclipse.EclipsePlugin;
+import org.gradle.plugins.ide.eclipse.model.Classpath;
+import org.gradle.plugins.ide.eclipse.model.ClasspathEntry;
+import org.gradle.plugins.ide.eclipse.model.EclipseClasspath;
+import org.gradle.plugins.ide.eclipse.model.EclipseModel;
+import org.gradle.plugins.ide.eclipse.model.Library;
+
+/**
+ * Conventions that are applied in the presence of the {@link EclipsePlugin} to work
+ * around buildship issue {@code #1238}.
+ *
+ * @author Phillip Webb
+ */
+class EclipseConventions {
+
+	void apply(Project project) {
+		project.getPlugins().withType(EclipsePlugin.class, (eclipse) -> {
+			EclipseModel eclipseModel = project.getExtensions().getByType(EclipseModel.class);
+			eclipseModel.classpath(this::configureClasspath);
+		});
+	}
+
+	private void configureClasspath(EclipseClasspath classpath) {
+		classpath.file(this::configureClasspathFile);
+	}
+
+	private void configureClasspathFile(XmlFileContentMerger merger) {
+		merger.whenMerged((content) -> {
+			if (content instanceof Classpath classpath) {
+				classpath.getEntries().removeIf(this::isKotlinPluginContributedBuildDirectory);
+			}
+		});
+	}
+
+	private boolean isKotlinPluginContributedBuildDirectory(ClasspathEntry entry) {
+		return (entry instanceof Library library) && isKotlinPluginContributedBuildDirectory(library.getPath())
+				&& isTest(library);
+	}
+
+	private boolean isKotlinPluginContributedBuildDirectory(String path) {
+		return path.contains("/main") && path.contains("/build/classes/") || path.contains("/build/resources/");
+	}
+
+	private boolean isTest(Library library) {
+		Object value = library.getEntryAttributes().get("test");
+		return (value instanceof String string && Boolean.parseBoolean(string));
+	}
+
+}