The LINQ provider currently only registers the char[] overloads of string.Trim, string.TrimStart and string.TrimEnd in StringMethod.cs. We should also register the parameterless and single-char overloads so queries like: 

collection.AsQueryable().Where(x => x.Str.TrimStart() == "abc")
collection.AsQueryable().Where(x => x.Str.TrimStart(' ') == "abc")

don't throw ExpressionNotSupportedException.