Identification: What is this fruit?

My neighbor started growing a fruit tree near our fence recently, and some of its fruit fell into our yard.

I've never seen this fruit before and I can't find it at any nearby grocery store. Can anyone recognize it?

Here are some scans I took of it.

 

Looks like a Feijoa, or possibly a Guava. Cut it open - Feijoa is white inside, and Guava is pink.

 

Ah, so it's likely a feijoa then. Thanks!

 

What does the foliage look like? Acca (Feijoa) has pretty distinctive leaves, guavas (Psidium) produce different ones.