docs: document how semver-exempt APIs are enabled on Span::eq - #541
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Span::eq is semver-exempt and deliberately not a Cargo feature; it is enabled via `--cfg procmacro2_semver_exempt` in RUSTFLAGS from the top-level crate (README "Unstable features"). The method docs said only "not exposed by default", which led to a downstream failure trying to enable it as a feature (dtolnay#538). Document the actual enablement mechanism on the method itself.
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Closes #538.
Background
Span::eqis#[cfg(procmacro2_semver_exempt)]and its docs say only"semver exempt and not exposed by default". As kevinmehall clarified in
#538, this is deliberate: semver-exempt APIs are not a Cargo feature and
must be enabled by passing
--cfg procmacro2_semver_exemptinRUSTFLAGSfrom the top-level crate (documented in the README's "Unstable features"
section). But nothing on the method itself points there, so downstream
users reasonably try
features = ["procmacro2_semver_exempt"]and get aversion-selection error.
Change (docs-only)
Span::eq's doc comment now states the actual enablement mechanism andlinks to the "Unstable features" section of the crate documentation.
No code behavior is changed.
Verification
#[cfg(procmacro2_semver_exempt)]so thedefault build and tests are untouched