[sitkack]

I think the answer is that when you break compatibility for an old compiler, you rev the major version. The folks on the old compiler then get to use the old library forever. Yeah, they can pin (and should) but it makes it a little cleaner.

[MereInterest]

Except that so few people are using older compilers, that tracking exactly when features were introduced becomes burdensome to the developer. For languages that have a few large releases every decade, such as C++, this is reasonable. As a C++ developer, I can remember that `std::unique_ptr` requires C++11, `std::make_unique` requires C++14, structured bindings require C++17, etc. It's tedious, but doable, and can be validated against a specific compiler in CI.

For languages with more frequent releases, that just isn't feasible. The let-else construct [0] was stabilized in 1.65 (Nov 2022), default values for const generics were stabilized in 1.59 (Feb 2022) [1], and the const evaluation timeout [2] was removed in 1.72 (Aug 2023). These are all reasonable changes, released on a reasonable timescale. However, when writing a `struct Position<const Dim: usize = 3>`, I don't check whether my library already uses language features that post-date 1.59, and I don't consider it a backwards-compatibility breakage to do so.

Incrementing the major version for a change that is source-compatible on most compiler versions (e.g. the first use of let-else in a project) just isn't justified.

[0] https://doc.rust-lang.org/rust-by-example/flow_control/let_e...

[1] https://blog.rust-lang.org/2022/02/24/Rust-1.59.0.html#const...

[2] https://blog.rust-lang.org/2023/08/24/Rust-1.72.0.html#const...

[kmeisthax]

Is it a breaking change to not support an old compiler when the new compiler is the same major as the old one was? Transitive dependency upgrades that bump minor don't trigger major version bumps in downstream users. Why should compilers be different?

[sitkack]

You would opt in to the future, not have to pin the past. It would give the ecosystem more stability.