---
Following #1216:
If a value is known to be immutable, then it doesn't need to be considered
'captured' since no mutation should occur.
Couldn't figure out a unit test in which this specific fix matters, but we need
this to fix test output of #1273
cc. @gsathya, would love some feedback / eyes on this. This makes sense for
Primitives in particular (which are always read / copied in rval position), but
I'm not as familiar with edge cases for other immutable values especially around
lambdas.