Preheat oven to 425 degrees Fahrenheit. Fit one pie crust into a 9-inch pie dish and trim edges.
In a bowl, mix sugar, flour, and salt. Add chopped rhubarb and toss until coated evenly.
Pour filling into crust and dot with butter pieces.
Place second crust over filling. Seal edges and crimp. Cut slits for steam.
Mix egg and water. Brush over top crust for a golden finish.
Bake at 425 degrees Fahrenheit for 15 minutes. Reduce heat to 350 degrees Fahrenheit and bake 40 to 45 minutes more.
Cool at least 2 hours before slicing