Rewrote the "child binding already set" error message for better clarity.

core/src/com/google/inject/internal/ChildBindingAlreadySetError.java

@@ -21,8 +21,10 @@ final class ChildBindingAlreadySetError extends InternalErrorDetail<ChildBinding
     super(
         ErrorId.CHILD_BINDING_ALREADY_SET,
         String.format(
-            "Unable to create binding for %s because it was already configured on one or more"
-                + " child injectors or private modules.",
+            "A binding for %s already exists in one or more child injectors or private modules."
+                + " Bindings from children can only be used by a parent if the child was a"
+                + " PrivateModule and the binding was exposed. Parent injectors cannot create"
+                + " bindings that already exist in a child.",
             Messages.convert(key)),
         sources,
         null);

core/test/com/google/inject/JitBindingsTest.java

@@ -53,10 +53,12 @@ private String jitInParentFailed(TypeLiteral<?> clazz) {
   }
 
   private String inChildMessage(Class<?> clazz) {
-    return "Unable to create binding for "
-        + "JitBindingsTest$"
+    return "A binding for JitBindingsTest$"
         + clazz.getSimpleName()
-        + " because it was already configured on one or more child injectors or private modules";
+        + " already exists in one or more child"
+        + " injectors or private modules. Bindings from children can only be used by a parent if"
+        + " the child was a PrivateModule and the binding was exposed. Parent injectors cannot"
+        + " create bindings that already exist in a child.";
   }
 
   public void testLinkedBindingWorks() {

core/test/com/google/inject/ParentInjectorTest.java

@@ -41,7 +41,9 @@
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
 
-/** @author jessewilson@google.com (Jesse Wilson) */
+/**
+ * @author jessewilson@google.com (Jesse Wilson)
+ */
 @RunWith(JUnit4.class)
 public class ParentInjectorTest {
 
@@ -70,8 +72,10 @@ public void testParentJitBindingWontClobberChildBinding() {
     } catch (ConfigurationException e) {
       assertContains(
           e.getMessage(),
-          "Unable to create binding for ParentInjectorTest$A because it was already configured on"
-              + " one or more child injectors or private modules.",
+          "A binding for ParentInjectorTest$A already exists in one or more child injectors or"
+              + " private modules. Bindings from children can only be used by a parent if the child"
+              + " was a PrivateModule and the binding was exposed. Parent injectors cannot create"
+              + " bindings that already exist in a child.",
           "ParentInjectorTest$9.configure");
     }
   }

core/test/com/google/inject/PrivateModuleTest.java

@@ -32,7 +32,9 @@
 import java.util.List;
 import junit.framework.TestCase;
 
-/** @author jessewilson@google.com (Jesse Wilson) */
+/**
+ * @author jessewilson@google.com (Jesse Wilson)
+ */
 public class PrivateModuleTest extends TestCase {
 
   public void testBasicUsage() {
@@ -589,8 +591,10 @@ public void testParentBindsSomethingInPrivate() {
       assertEquals(1, expected.getErrorMessages().size());
       assertContains(
           expected.toString(),
-          "Unable to create binding for List<String> ",
-          "because it was already configured on one or more child injectors or private modules.",
+          "A binding for List<String> already exists in one or more child injectors",
+          " or private modules. Bindings from children can only be used by a parent if the child",
+          " was a PrivateModule and the binding was exposed. Parent injectors cannot create",
+          " bindings that already exist in a child.",
           "1 : PrivateModuleTest$FailingPrivateModule.configure",
           "PrivateModuleTest$FailingModule -> PrivateModuleTest$ManyPrivateModules ->"
               + " PrivateModuleTest$FailingPrivateModule",
@@ -610,8 +614,10 @@ public void testParentBindingToPrivateLinkedJitBinding() {
       assertEquals(1, expected.getErrorMessages().size());
       assertContains(
           expected.toString(),
-          "Unable to create binding for List<String> because it was already configured on one or"
-              + " more child injectors or private modules",
+          "A binding for List<String> already exists in one or more child injectors",
+          " or private modules. Bindings from children can only be used by a parent if the child",
+          " was a PrivateModule and the binding was exposed. Parent injectors cannot create",
+          " bindings that already exist in a child.",
           "1 : PrivateModuleTest$FailingPrivateModule.configure",
           "PrivateModuleTest$ManyPrivateModules -> PrivateModuleTest$FailingPrivateModule",
           "2 : PrivateModuleTest$SecondFailingPrivateModule.configure",
@@ -628,8 +634,10 @@ public void testParentBindingToPrivateJitBinding() {
       assertEquals(1, expected.getErrorMessages().size());
       assertContains(
           expected.toString(),
-          "Unable to create binding for PrivateModuleTest$PrivateFoo because it was already"
-              + " configured on one or more child injectors or private modules.",
+          "A binding for PrivateModuleTest$PrivateFoo already exists in one or more child",
+          " injectors or private modules. Bindings from children can only be used by a parent if",
+          " the child was a PrivateModule and the binding was exposed. Parent injectors cannot",
+          " create bindings that already exist in a child.",
           "as a just-in-time binding");
     }
   }

core/test/com/google/inject/errors/testdata/child_binding_already_set_by_just_in_time_binding.txt

@@ -1,6 +1,6 @@
 Guice configuration errors:
 
-1) [Guice/ChildBindingAlreadySet]: Unable to create binding for ChildBindingAlreadySetErrorTest$DependsOnFoo because it was already configured on one or more child injectors or private modules.
+1) [Guice/ChildBindingAlreadySet]: A binding for ChildBindingAlreadySetErrorTest$DependsOnFoo already exists in one or more child injectors or private modules. Bindings from children can only be used by a parent if the child was a PrivateModule and the binding was exposed. Parent injectors cannot create bindings that already exist in a child.
 
 Bound at:
 1 : ChildBindingAlreadySetErrorTest$ChildModule4.configure(ChildBindingAlreadySetErrorTest.java:105)

core/test/com/google/inject/errors/testdata/child_binding_already_set_error.txt

@@ -1,6 +1,6 @@
 Guice configuration errors:
 
-1) [Guice/ChildBindingAlreadySet]: Unable to create binding for ChildBindingAlreadySetErrorTest$Foo because it was already configured on one or more child injectors or private modules.
+1) [Guice/ChildBindingAlreadySet]: A binding for ChildBindingAlreadySetErrorTest$Foo already exists in one or more child injectors or private modules. Bindings from children can only be used by a parent if the child was a PrivateModule and the binding was exposed. Parent injectors cannot create bindings that already exist in a child.
 
 Bound at:
 1 : ChildBindingAlreadySetErrorTest$ChildModule.configure(ChildBindingAlreadySetErrorTest.java:34)

core/test/com/google/inject/errors/testdata/child_binding_already_set_multiple_times_error.txt

@@ -1,6 +1,6 @@
 Guice configuration errors:
 
-1) [Guice/ChildBindingAlreadySet]: Unable to create binding for ChildBindingAlreadySetErrorTest$Foo because it was already configured on one or more child injectors or private modules.
+1) [Guice/ChildBindingAlreadySet]: A binding for ChildBindingAlreadySetErrorTest$Foo already exists in one or more child injectors or private modules. Bindings from children can only be used by a parent if the child was a PrivateModule and the binding was exposed. Parent injectors cannot create bindings that already exist in a child.
 
 Bound at:
 1 : ChildBindingAlreadySetErrorTest$ChildModule.configure(ChildBindingAlreadySetErrorTest.java:34)

core/test/com/google/inject/errors/testdata/multiple_child_binding_already_set_errors.txt

@@ -1,6 +1,6 @@
 Unable to create injector, see the following errors:
 
-1) [Guice/ChildBindingAlreadySet]: Unable to create binding for ChildBindingAlreadySetErrorTest$Foo because it was already configured on one or more child injectors or private modules.
+1) [Guice/ChildBindingAlreadySet]: A binding for ChildBindingAlreadySetErrorTest$Foo already exists in one or more child injectors or private modules. Bindings from children can only be used by a parent if the child was a PrivateModule and the binding was exposed. Parent injectors cannot create bindings that already exist in a child.
 
 Bound at:
 1 : ChildBindingAlreadySetErrorTest$ChildModule.configure(ChildBindingAlreadySetErrorTest.java:34)