What is the performance of the Last() extension method for List<T>?

I really like Last() and would use it all the time for List<T>s. But since it seems to be defined for IEnumerable<T>, I guess it enumerates the enumeration first. This should be O(n), as opposed to O(1) for directly indexing the last element of a List<T>.

Are the standard (LINQ) extension methods aware of this?

The STL in C++ is aware of this by virtue of a whole "inheritance tree" for iterators and whatnot.