[mlir][python] Expose PyInsertionPoint's reference operation (#69082)

The reason I want this is that I am writing my own Python bindings and
would like to use the insertion point from
`PyThreadContextEntry::getDefaultInsertionPoint()` to call C++ functions
that take an `OpBuilder` (I don't need to expose it in Python but it
also seems appropriate). AFAICT, there is currently no way to translate
a `PyInsertionPoint` into an `OpBuilder` because the operation is
inaccessible.

Author: tlongeri

mlir/lib/Bindings/Python/IRCore.cpp

@@ -3207,7 +3207,18 @@ void mlir::python::populateIRCore(py::module &m) {
            "Inserts an operation.")
       .def_property_readonly(
           "block", [](PyInsertionPoint &self) { return self.getBlock(); },
-          "Returns the block that this InsertionPoint points to.");
+          "Returns the block that this InsertionPoint points to.")
+      .def_property_readonly(
+          "ref_operation",
+          [](PyInsertionPoint &self) -> py::object {
+            auto ref_operation = self.getRefOperation();
+            if (ref_operation)
+              return ref_operation->getObject();
+            return py::none();
+          },
+          "The reference operation before which new operations are "
+          "inserted, or None if the insertion point is at the end of "
+          "the block");
 
   //----------------------------------------------------------------------------
   // Mapping of PyAttribute.

mlir/lib/Bindings/Python/IRModule.h

@@ -833,6 +833,7 @@ class PyInsertionPoint {
                    const pybind11::object &excTb);
 
   PyBlock &getBlock() { return block; }
+  std::optional<PyOperationRef> &getRefOperation() { return refOperation; }
 
 private:
   // Trampoline constructor that avoids null initializing members while

mlir/python/mlir/_mlir_libs/_mlir/ir.pyi

@@ -755,6 +755,8 @@ class InsertionPoint:
     def __exit__(self, arg0: object, arg1: object, arg2: object) -> None: ...
     @property
     def block(self) -> Block: ...
+    @property
+    def ref_operation(self) -> Optional[_OperationBase]: ...
 
 # TODO: Auto-generated. Audit and fix.
 class IntegerAttr(Attribute):

mlir/test/python/ir/insertion_point.py

@@ -27,6 +27,8 @@ def test_insert_at_block_end():
         )
         entry_block = module.body.operations[0].regions[0].blocks[0]
         ip = InsertionPoint(entry_block)
+        assert ip.block == entry_block
+        assert ip.ref_operation is None
         ip.insert(Operation.create("custom.op2"))
         # CHECK: "custom.op1"
         # CHECK: "custom.op2"
@@ -51,6 +53,8 @@ def test_insert_before_operation():
         )
         entry_block = module.body.operations[0].regions[0].blocks[0]
         ip = InsertionPoint(entry_block.operations[1])
+        assert ip.block == entry_block
+        assert ip.ref_operation == entry_block.operations[1]
         ip.insert(Operation.create("custom.op3"))
         # CHECK: "custom.op1"
         # CHECK: "custom.op3"
@@ -75,6 +79,8 @@ def test_insert_at_block_begin():
         )
         entry_block = module.body.operations[0].regions[0].blocks[0]
         ip = InsertionPoint.at_block_begin(entry_block)
+        assert ip.block == entry_block
+        assert ip.ref_operation == entry_block.operations[0]
         ip.insert(Operation.create("custom.op1"))
         # CHECK: "custom.op1"
         # CHECK: "custom.op2"
@@ -108,6 +114,8 @@ def test_insert_at_terminator():
         )
         entry_block = module.body.operations[0].regions[0].blocks[0]
         ip = InsertionPoint.at_block_terminator(entry_block)
+        assert ip.block == entry_block
+        assert ip.ref_operation == entry_block.operations[1]
         ip.insert(Operation.create("custom.op2"))
         # CHECK: "custom.op1"
         # CHECK: "custom.op2"