[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...