[steveklabnik]

Basically, the interpreter interpret's rustc's internal IR, so it can theoretically support the entire language. That's not a good idea for various reasons, though, so its capabilities are effectively on an allowlist, that we expand over time as we're sure we want to enable a given feature.

[tines]

That seems like a really good design, as opposed to having an AST interpreter like I might do otherwise. But would this indeed support a "much larger set of features" than constexpr as was claimed?

[steveklabnik]

I think that the earlier const evaluator was an AST interpreter? It's been a long time and I don't work on this code, so I'm not 100% sure.

I don't know constexpr well enough to comment on that claim.

[dodobirdlord]

It has been a while, and I’m remembering from a discussion of new features in C++20, but I recall that constexpr isn’t capable of fully supporting memory allocations made during evaluation, while Rust’s const fn will eventually be able to.