Implement custom error formatting for BINDING_ALREADY_SET errors.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=323883237

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

@@ -81,6 +81,10 @@ protected void putBinding(BindingImpl<?> binding) {
       if (injector.getBindingData().getExplicitBinding(key) != null) {
         try {
           if (!isOkayDuplicate(original, binding, injector.getBindingData())) {
+            if (InternalFlags.enableExperimentalErrorMessages()) {
+              errors.bindingAlreadySet(binding, original);
+              return;
+            }
             errors.bindingAlreadySet(key, original.getSource());
             return;
           }

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

@@ -0,0 +1,54 @@
+package com.google.inject.internal;
+
+import com.google.common.collect.ImmutableList;
+import com.google.inject.Binding;
+import com.google.inject.spi.ErrorDetail;
+import java.util.ArrayList;
+import java.util.Formatter;
+import java.util.List;
+import java.util.stream.Collectors;
+
+/** Error reported by Guice when a key is bound at multiple places the injector. */
+final class BindingAlreadySetError extends ErrorDetail<BindingAlreadySetError> {
+  private final Binding<?> binding;
+  private final Binding<?> original;
+
+  BindingAlreadySetError(Binding<?> binding, Binding<?> original, List<Object> sources) {
+    super(
+        String.format("%s was bound multiple times.", Messages.convert(binding.getKey())),
+        sources,
+        null);
+    this.binding = binding;
+    this.original = original;
+  }
+
+  @Override
+  public boolean isMergeable(ErrorDetail<?> otherError) {
+    return otherError instanceof BindingAlreadySetError
+        && ((BindingAlreadySetError) otherError).binding.getKey().equals(binding.getKey());
+  }
+
+  @Override
+  public void format(int index, List<ErrorDetail<?>> mergeableErrors, Formatter formatter) {
+    List<List<Object>> sourcesList = new ArrayList<>();
+    sourcesList.add(ImmutableList.of(original.getSource()));
+    sourcesList.add(ImmutableList.of(binding.getSource()));
+    sourcesList.addAll(
+        mergeableErrors.stream()
+            .map(e -> ((BindingAlreadySetError) e).binding.getSource())
+            .map(ImmutableList::of)
+            .collect(Collectors.toList()));
+
+    formatter.format("%s) %s %s%n%n", index, "[Guice/BindingAlreadySet]", getMessage());
+    formatter.format("Bound at the following %s locations:%n", sourcesList.size());
+    for (int i = 0; i < sourcesList.size(); i++) {
+      ErrorFormatter.formatSources(i + 1, sourcesList.get(i), formatter);
+    }
+    formatter.format("%n");
+  }
+
+  @Override
+  public BindingAlreadySetError withSources(List<Object> newSources) {
+    return new BindingAlreadySetError(binding, original, newSources);
+  }
+}

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

@@ -511,6 +511,12 @@ public Errors recursiveBinding() {
     return addMessage(ErrorId.RECURSIVE_BINDING, "Binding points to itself.");
   }
 
+  Errors bindingAlreadySet(Binding<?> binding, Binding<?> original) {
+    BindingAlreadySetError error = new BindingAlreadySetError(binding, original, getSources());
+    return addMessage(
+        new Message(GuiceInternal.GUICE_INTERNAL, ErrorId.BINDING_ALREADY_SET, error));
+  }
+
   public Errors bindingAlreadySet(Key<?> key, Object source) {
     return addMessage(
         ErrorId.BINDING_ALREADY_SET,

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

@@ -13,7 +13,8 @@ final class MissingImplementationError extends ErrorDetail<MissingImplementation
   private final Key<?> key;
 
   public MissingImplementationError(Key<?> key, List<Object> sources) {
-    super("", sources, null);
+    super(
+        String.format("No implementation for %s was bound.", Messages.convert(key)), sources, null);
     this.key = key;
   }
 
@@ -29,16 +30,13 @@ public void format(int index, List<ErrorDetail<?>> mergeableErrors, Formatter fo
     sourcesList.add(getSources());
     sourcesList.addAll(
         mergeableErrors.stream().map(ErrorDetail::getSources).collect(Collectors.toList()));
-    formatter.format(
-        "%s) [Guice/MissingImplementation]: No implementation for %s was bound.%n",
-        index, Messages.convert(key));
-
     List<List<Object>> filteredSourcesList =
         sourcesList.stream()
             .map(this::trimSource)
             .filter(sources -> !sources.isEmpty())
             .collect(Collectors.toList());
 
+    formatter.format("%s) %s: %s%n", index, "[Guice/MissingImplementation]", getMessage());
     if (!filteredSourcesList.isEmpty()) {
       formatter.format("%n%s%n", "Requested by:");
       int sourceListIndex = 1;