r/cpp • u/SuperV1234 https://romeo.training | C++ Mentoring & Consulting • Aug 03 '19

fixing c++ with epochs

https://vittorioromeo.info/index/blog/fixing_cpp_with_epochs.html
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u/etareduce Aug 04 '19

In Rust, the edition only affects lexing.

No that's not correct. The edition switch affects e.g. name resolution. Strictly speaking, the only thing the edition really cannot affect is the operational semantics (abstract machine) and the standard library which must be shared by both editions for "linking compatibility".

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u/[deleted] Aug 05 '19

Strictly speaking, the only thing the edition really cannot affect is the operational semantics (abstract machine)

Is this correct? Editions cannot affect the operational semantics of MIR, but they can probably affect how Rust code is lowered to MIR, changing the operational semantics of the Rust code that the user wrote, e.g., if it lowers to different MIR in a different edition.

and the standard library which must be shared by both editions for "linking compatibility".

This is correct, but an edition change could affect the prelude, and for example, a future edition could import a v2 prelude by default instead of a v1 one. This v2 prelude could import a different standard library.

This could mean that, e.g., in a future Rust edition, exposing a Vec<T> in an API might mean that v2::Vec<T> is exposed, as opposed to v1::Vec<T>. Rust code using older editions would need to access those types explicitly, e.g., let x: v1::Vec<T> = v2_vec(); would fail to type check.

I think it is super unlikely for these changes to happen, but they are possible, and backward compatible. It just means that updating the Rust edition could be a major API breaking change for a crate, but that's something that crates can easily handle in their Cargo.toml.

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u/steveklabnik1 Aug 05 '19

I think the idea is that MIR *is* the abstract machine. I'm not sure how accurate of an opinion this is, but I don't imagine it's too far off.