Add check for adding #[non_exhaustive] causing breakage. #4
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obi1kenobi
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Document that adding `#[non_exhaustive]` on existing items is breaking. ### What does this PR try to resolve? Adding `#[non_exhaustive]` to an existing struct, enum, or variant is almost always a breaking change and requires a major version bump for semver purposes. This PR adds a section to the semver reference page that describes this and provides examples showing how `#[non_exhaustive]` can break code. ### Additional information Adding `#[non_exhaustive]` to a unit struct currently has no effect on whether that struct can be constructed in downstream crates. This is inconsistent with the behavior of `#[non_exhaustive]` on unit enum variants, which may not be constructed outside their own crate. This might be due to a similar underlying cause as: rust-lang/rust#78586 The confusing "variant is private" error messages for non-exhaustive unit and tuple variants are a known issue tracked in: rust-lang/rust#82788 Checking for the struct portion of this semver rule is done in: obi1kenobi/cargo-semver-checks#4
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Per this page in the Rust language reference, it seems that adding
#[non_exhaustive]to a non-unit struct that has no private fields is a major breaking change.This is because structs with no private fields can have struct literal be used to construct them outside their own crate, but the addition of
#[non_exhaustive]will prevent struct literals from being used to construct them outside their own crate. Unit structs are not affected since, based on a quick test onrustc 1.62.0 (a8314ef7d 2022-06-27),#[non_exhaustive]is ignored on unit structs and they can still be constructed externally using a struct literal.Will link this to a PR against the cargo semver reference page when I've had a chance to write a PR for it.