How do I open files to read/write in Mojo?

I want to do the equivalent of the following Python code:

with open(file.txt, 'r') as file:
  x = file.read()

with open(file.txt, 'r') as file:
  x = file.read()

How can I do this?

How do I open files to read/write in Mojo? - Modular