Stop using the CompilerClassLoader in sbt-bridge

sbt-bridge/src/xsbt/CompilerClassLoader.java

@@ -119,4 +119,4 @@ private static ClassLoader computeFixedLoader(ClassLoader bridgeLoader) {
       throw new RuntimeException(e);
     }
   }
-}
+}

sbt-bridge/src/xsbt/CompilerInterface.java

@@ -6,36 +6,50 @@
 import xsbti.AnalysisCallback;
 import xsbti.Logger;
 import xsbti.Reporter;
-import xsbti.Severity;
 import xsbti.compile.*;
 
-import java.io.File;
-
-import dotty.tools.dotc.core.Contexts.ContextBase;
 import dotty.tools.dotc.Main;
-import dotty.tools.dotc.interfaces.*;
 
 import java.lang.reflect.InvocationTargetException;
-import java.net.URLClassLoader;
+import java.io.File;
 
 /**
  * The new compiler interface is [[dotty.tools.xsbt.CompilerBridge]] that extends the new `xsbti.CompilerInterface2`.
  * This interface is kept for compatibility with Mill and the sbt 1.3.x series.
  */
 public final class CompilerInterface {
-  public CachedCompiler newCompiler(String[] options, Output output, Logger initialLog, Reporter initialDelegate) {
-    // The classloader that sbt uses to load the compiler bridge is broken
-    // (see CompilerClassLoader#fixBridgeLoader for details). To workaround
-    // this we construct our own ClassLoader and then run the following code
-    // with it:
-    //   new CachedCompilerImpl(options, output)
+  public CachedCompiler newCompiler(String[] options, Output output, Logger initialLog, Reporter initialDelegate) throws java.lang.Exception {
+    if(isClassLoaderValid()) {
+      return new CachedCompilerImpl(options, output);
+    } else {
+      initialLog.warn(() ->
+        "The compiler class loader is badly configured.\n" +
+        "Consider using a more recent version of your build tool:\n" +
+        " - sbt >= 1.4.0 and sbt-dotty >= 1.5.0\n" +
+        " - Mill >= 0.9.3-21-002361\n" +
+        " - Bloop >= 1.4.6-23-20a501bc"
+      );
+      // To workaround the wrong class loader, we construct our own and run
+      // the following code with it:
+      //   new CachedCompilerImpl(options, output)
+      try {
+        ClassLoader bridgeLoader = this.getClass().getClassLoader();
+        ClassLoader fixedLoader = CompilerClassLoader.fixBridgeLoader(bridgeLoader);
+        Class<?> cciClass = fixedLoader.loadClass("xsbt.CachedCompilerImpl");
+        return (CachedCompiler) cciClass.getConstructors()[0].newInstance(options, output);
+      } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | InvocationTargetException e) {
+        throw new RuntimeException(e);
+      }
+    }
+  }
 
+  private boolean isClassLoaderValid() {
+    // Check that the `xsbti.*` classes are loaded from the same class loader in the compiler and the bridge
+    ClassLoader compilerClassLoader = Main.class.getClassLoader();
+    Class<Logger> clazz = Logger.class;
     try {
-      ClassLoader bridgeLoader = this.getClass().getClassLoader();
-      ClassLoader fixedLoader = CompilerClassLoader.fixBridgeLoader(bridgeLoader);
-      Class<?> cciClass = fixedLoader.loadClass("xsbt.CachedCompilerImpl");
-      return (CachedCompiler) cciClass.getConstructors()[0].newInstance(options, output);
-    } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | InvocationTargetException e) {
+      return compilerClassLoader.loadClass("xsbti.Logger") == clazz;
+    } catch (ClassNotFoundException e) {
       throw new RuntimeException(e);
     }
   }