[humanrebar]

> const-by-default

const isn't the same as immutable. 'const' implies a (mostly) read-only view, not read-only data. That is, C++ does not guarantee that the memory does not change, just that some parts of the code will not change it (and even then, there are ways to get around 'const').

It would be marginally better to be const by default, because there would be less code to consider when analyzing changes to data, but the cost of this feature is much larger than the benefits we'd see from it.

And, to top it all off, dealing with a lot of immutables really requires different syntax and different style. Read through the rust book and then try to do the examples in C++. You can do pattern matching and variant types in C++, but they are ugly and probably slower than what we do now.