Zig: add documentation for Result Location Semantics

[andrewrk]

• [ ] how result locations flow across expressions
  
  
  
  • [ ] variables
  
  
  • [ ] struct initialization
  
  
  • [ ] array initialization
  
  
  • [ ] union initialization
  
  
  • [ ] if
  
  
  • [ ] while
  
  
  • [ ] switch
  
  
  • [ ] return
  
  
  • [ ] function calls - return values & parameters
  
  
  • [ ] catch
  
  
  • [ ] orelse
  
  
  • [ ] @bitCast
  
  
• [ ] mention #2761 and #2765

[vi]

What is a Result Location?

[fengb]

Here’s the original issue: https://github.com/ziglang/zig/issues/287

My best summary: when a function returns a newly created value, it will automatically put it into the function’s return location instead of making a copy. Previously, this was only a potential optimization from LLVM, but now it’s guaranteed by Zig.