IDEA-56656 IDEA does not highlight not well-formed XML: comment before XML declaration

platform/platform-resources-en/src/messages/XmlErrorMessages.properties

@@ -74,3 +74,4 @@ xml.parsing.closing.tag.name.missing=Closing tag name missing
 xml.parsing.closing.tag.is.not.done=Closing tag is not done
 attribute.should.be.preceded.with.space=There should be a space between attribute and previous attribute
 cdata.end.should.not.appear.in.content.unless.to.mark.end.of.cdata.section=Character sequence ']]>' must not appear in content unless used to mark the end of a CDATA section
+xml.declaration.should.precede.all.document.content=Xml declaration should precede all document content

xml/impl/src/com/intellij/codeInsight/daemon/impl/analysis/XmlHighlightVisitor.java

@@ -167,6 +167,30 @@ public class XmlHighlightVisitor extends XmlElementVisitor implements HighlightV
     bindMessageToAstNode(childByRole, warning, 0, messageLength, localizedMessage, quickFixActions);
   }
 
+
+  @Override
+  public void visitXmlProcessingInstruction(XmlProcessingInstruction processingInstruction) {
+    super .visitXmlProcessingInstruction(processingInstruction);
+    PsiElement parent = processingInstruction.getParent();
+
+    if (parent instanceof XmlProlog && processingInstruction.getText().startsWith("<?xml")) {
+      for(PsiElement e = PsiTreeUtil.prevLeaf(processingInstruction); e != null; e = PsiTreeUtil.prevLeaf(e)) {
+        if (e instanceof PsiWhiteSpace && PsiTreeUtil.prevLeaf(e) != null ||
+            e instanceof OuterLanguageElement) {
+          continue;
+        }
+        PsiElement eParent = e.getParent();
+        if (eParent instanceof PsiComment) e = eParent;
+  
+        addToResults(HighlightInfo.createHighlightInfo(
+          HighlightInfoType.ERROR,
+          e,
+          XmlErrorMessages.message("xml.declaration.should.precede.all.document.content")
+        ));
+      }
+    }
+  }
+
   private void bindMessageToAstNode(final PsiElement childByRole,
                                     final HighlightInfoType warning,
                                     final int offset,

xml/impl/src/com/intellij/psi/impl/source/xml/XmlProcessingInstructionImpl.java

@@ -40,7 +40,7 @@ public class XmlProcessingInstructionImpl extends XmlElementImpl implements XmlP
 
   public void accept(@NotNull PsiElementVisitor visitor) {
     if (visitor instanceof XmlElementVisitor) {
-      ((XmlElementVisitor)visitor).visitXmlElement(this);
+      ((XmlElementVisitor)visitor).visitXmlProcessingInstruction(this);
     }
     else {
       visitor.visitElement(this);

xml/openapi/src/com/intellij/psi/XmlElementVisitor.java

@@ -65,4 +65,8 @@ public abstract class XmlElementVisitor extends PsiElementVisitor {
   public void visitXmlDoctype(XmlDoctype xmlDoctype) {
     visitXmlElement(xmlDoctype);
   }
+
+  public void visitXmlProcessingInstruction(XmlProcessingInstruction processingInstruction) {
+    visitXmlElement(processingInstruction);
+  }
 }