Require an opt-in for enabling min_specialization rustc feature
#238
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alexcrichton
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Jul 8, 2024
This commit switches away from implicitly enabling the `min_specialize` Rust nightly feature whenever a Nightly compiler is used to instead requiring an explicit opt-in with a new Cargo feature `nightly-specialize`. The goal of this commit is to fix tkaitchuck#238 and has two primary motivations: 1. In tkaitchuck#238 I'm trying to build something that depends on this crate as part of the Rust bootstrap process but this crate fails to build due to `min_specialize` not being allowed but a nightly compiler is in use. This is due to the fact that the way `-Zallow-features` is managed in the bootstrap is different than the standard Cargo way of doing so. 2. This removes a failure mode where if one day the `min_specialize` feature changes this crate won't break when built on nightly. Users of Nightly compilers will be able to continue using this crate if the feature was not explicitly opted-in to.
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I'm in the process of adding a new tool to the Rust compiler but it's failing to build on CI. The root of the reason appears to be that this crate will automatically enable usage of the
min_specializationfeature on Nightly-looking Rust channels but the way that Rust is compiling crates makes it so themin_specializationfeature isn't detected but this crate doesn't catch it. This is distinct from 76dd8d2 in that-Zallow-featuresis used as part of the rustc build but it's used in a non-default way. Overall the end-state is that this crate is causing a build failure as a transitive dependency of the tool I'm adding.Given that context I'd like to pose a question, how would you feel about making usage of
min_specializationrequire an opt-in from users rather than implicitly enabling it on supporting Nightly channels? That would look like a crate feature, a requiredRUSTFLAGSentry, or perhaps an environment variable at build time.In addition to the rationale of working as part of rust-lang/rust I'd also offer the rationale that Nightly Rust features can theoretically break at any time and users using a Nightly toolchain may not expect breakage from crates because unstable features change. Requiring an explicit opt-in would help with that since then only those opting-in would be broken which would be expected.
I'm happy to make a PR for this as well, but wanted to gauge interest in such a change before doing so to make sure I wouldn't step on any toes.
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