In rustc 1.90 the feature[let_chains] does not work in editions prior to 2024.

[berkus]

See ericlagergren/hctr2-rs#6 (comment)

The older edition = 2021 code which uses feature[let_chains] now stopped compiling in rustc 1.90 saying

error: let chains are only allowed in Rust 2024 or later

Although the feature flag was supported before and there are crates depending on it.

Intentional?

[theemathas]

This is intentional. See #143214

Rust does not guarantee that unstable features will stay usable across different versions of rust. Because of this, projects that require unstable features will often pin the specific version of rust used.

[theemathas]

The reason that let chains are only supported on edition 2024 is because making let chains work requires reordering when destructors are ran, which is done in edition 2024 by changing how if let works.

[jieyouxu]

Yeah, not-a-bug.