From the article "Lighting and Meaning in Kurosawa's Rashomon" by Asa Fitch (1998):
The effect of pointing the camera right at the sun in this scene and in others is an innovation in cinematography. Until Rashomon was made, pointing the camera directly at the sun was unheard of. It did not occur to anyone that pointing the camera at the sun would do anything more than burn the eyes. Rashomon proved this wrong. "These days it is not uncommon to point the camera directly at the sun, but at the time Rashomon was being made it was still one of the taboos of cinematography." (Autobiography, 185)
Autobiography refers to Akira Kurosawa. Something Like an Autobiography. Trans. Audie Bock. New York: Random House, 1983.
(Upon further research) Here is a fuller quote from Kurosawa:
I had to be sure that this huge gate looked huge to the camera. And I had to figure out how to use the sun itself. This was a major concern because of the decision to use the light and shadows of the forest as the keynote of the whole film. I determined to solve the problem by actually filming the sun. These days it is not uncommon to point the camera directly at the sun, but at the time Rashomon was being made it was still one of the taboos of cinematography. It was even thought that the sun’s rays shining directly into your lens would burn the film in your camera. But my cameraman, Miyagawa Kazuo, boldly defied this convention and created superb images.