diff --git a/mlir/docs/Interfaces.md b/mlir/docs/Interfaces.md
--- a/mlir/docs/Interfaces.md
+++ b/mlir/docs/Interfaces.md
@@ -393,6 +393,18 @@
         These declarations are _not_ implicitly visible in default
         implementations of interface methods, but static declarations may be
         accessed with full name qualification.
+*   Extra Shared Class Declarations (Optional: `extraSharedClassDeclaration`)
+    -   Additional C++ code that is injected into the declarations of both the
+        interface and trait class. This allows for defining methods and more
+        that are exposed on both the interface and trait class, e.g. to inject
+        utilties on both the interface and the derived entity implementing the
+        interface (e.g. attribute, operation, etc.).
+    -   In non-static methods, `$_attr`/`$_op`/`$_type`
+        (depending on the type of interface) may be used to refer to an
+        instance of the IR entity. In the interface declaration, the type of
+        the instance is the interface class. In the trait declaration, the
+        type of the instance is the concrete entity class
+        (e.g. `IntegerAttr`, `FuncOp`, etc.).
 
 `OpInterface` classes may additionally contain the following:
 
diff --git a/mlir/include/mlir/IR/OpBase.td b/mlir/include/mlir/IR/OpBase.td
--- a/mlir/include/mlir/IR/OpBase.td
+++ b/mlir/include/mlir/IR/OpBase.td
@@ -2226,6 +2226,10 @@
   // An optional code block containing extra declarations to place in the
   // interface declaration.
   code extraClassDeclaration = "";
+
+  // An optional code block containing extra declarations to place in both
+  // the interface and trait declaration.
+  code extraSharedClassDeclaration = "";
 }
 
 // AttrInterface represents an interface registered to an attribute.
diff --git a/mlir/include/mlir/TableGen/Interfaces.h b/mlir/include/mlir/TableGen/Interfaces.h
--- a/mlir/include/mlir/TableGen/Interfaces.h
+++ b/mlir/include/mlir/TableGen/Interfaces.h
@@ -91,6 +91,10 @@
   // Return the traits extra class declaration code.
   llvm::Optional<StringRef> getExtraTraitClassDeclaration() const;
 
+  // Return the extra class declaration code shared between the interface and
+  // trait classes.
+  llvm::Optional<StringRef> getExtraSharedClassDeclaration() const;
+
   // Return the verify method body if it has one.
   llvm::Optional<StringRef> getVerify() const;
 
diff --git a/mlir/lib/TableGen/Interfaces.cpp b/mlir/lib/TableGen/Interfaces.cpp
--- a/mlir/lib/TableGen/Interfaces.cpp
+++ b/mlir/lib/TableGen/Interfaces.cpp
@@ -110,6 +110,12 @@
   return value.empty() ? llvm::Optional<StringRef>() : value;
 }
 
+// Return the shared extra class declaration code.
+llvm::Optional<StringRef> Interface::getExtraSharedClassDeclaration() const {
+  auto value = def->getValueAsString("extraSharedClassDeclaration");
+  return value.empty() ? llvm::Optional<StringRef>() : value;
+}
+
 // Return the body for this method if it has one.
 llvm::Optional<StringRef> Interface::getVerify() const {
   // Only OpInterface supports the verify method.
diff --git a/mlir/test/mlir-tblgen/op-interface.td b/mlir/test/mlir-tblgen/op-interface.td
--- a/mlir/test/mlir-tblgen/op-interface.td
+++ b/mlir/test/mlir-tblgen/op-interface.td
@@ -3,6 +3,24 @@
 
 include "mlir/IR/OpBase.td"
 
+def ExtraShardDeclsInterface : OpInterface<"ExtraShardDeclsInterface"> {
+  let extraSharedClassDeclaration = [{
+    bool sharedMethodDeclaration() {
+      return $_op.someOtherMethod();
+    }
+  }];
+}
+
+// DECL: class ExtraShardDeclsInterface
+// DECL:      bool sharedMethodDeclaration() {
+// DECL-NEXT:   return (*this).someOtherMethod();
+// DECL-NEXT: }
+
+// DECL: struct ExtraShardDeclsInterfaceTrait
+// DECL:      bool sharedMethodDeclaration() {
+// DECL-NEXT:   return (*static_cast<ConcreteOp *>(this)).someOtherMethod();
+// DECL-NEXT: }
+
 def TestOpInterface : OpInterface<"TestOpInterface"> {
   let description = [{some op interface description}];
 
diff --git a/mlir/tools/mlir-tblgen/OpInterfacesGen.cpp b/mlir/tools/mlir-tblgen/OpInterfacesGen.cpp
--- a/mlir/tools/mlir-tblgen/OpInterfacesGen.cpp
+++ b/mlir/tools/mlir-tblgen/OpInterfacesGen.cpp
@@ -104,6 +104,7 @@
   /// The format context to use for methods.
   tblgen::FmtContext nonStaticMethodFmt;
   tblgen::FmtContext traitMethodFmt;
+  tblgen::FmtContext extraDeclsFmt;
 };
 
 /// A specialized generator for attribute interfaces.
@@ -118,6 +119,7 @@
     nonStaticMethodFmt.addSubst("_attr", castCode).withSelf(castCode);
     traitMethodFmt.addSubst("_attr",
                             "(*static_cast<const ConcreteAttr *>(this))");
+    extraDeclsFmt.addSubst("_attr", "(*this)");
   }
 };
 /// A specialized generator for operation interfaces.
@@ -132,6 +134,7 @@
         .withOp(castCode)
         .withSelf(castCode);
     traitMethodFmt.withOp("(*static_cast<ConcreteOp *>(this))");
+    extraDeclsFmt.withOp("(*this)");
   }
 };
 /// A specialized generator for type interfaces.
@@ -146,6 +149,7 @@
     nonStaticMethodFmt.addSubst("_type", castCode).withSelf(castCode);
     traitMethodFmt.addSubst("_type",
                             "(*static_cast<const ConcreteType *>(this))");
+    extraDeclsFmt.addSubst("_type", "(*this)");
   }
 };
 } // namespace
@@ -415,6 +419,8 @@
   }
   if (auto extraTraitDecls = interface.getExtraTraitClassDeclaration())
     os << tblgen::tgfmt(*extraTraitDecls, &traitMethodFmt) << "\n";
+  if (auto extraTraitDecls = interface.getExtraSharedClassDeclaration())
+    os << tblgen::tgfmt(*extraTraitDecls, &traitMethodFmt) << "\n";
 
   os << "  };\n";
 }
@@ -469,6 +475,9 @@
   // Emit any extra declarations.
   if (Optional<StringRef> extraDecls = interface.getExtraClassDeclaration())
     os << *extraDecls << "\n";
+  if (Optional<StringRef> extraDecls =
+          interface.getExtraSharedClassDeclaration())
+    os << tblgen::tgfmt(*extraDecls, &extraDeclsFmt);
 
   os << "};\n";