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Rollup merge of #95166 - Urgau:check-cfg-values-unstable-book, r=petrochenkov

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Update the unstable book with the new `values()` form of check-cfg

Forgot to update the unstable book in https://github.com/rust-lang/rust/pull/94362

r? ``@petrochenkov``
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Matthias Krüger 2022-03-21 19:48:34 +01:00 committed by GitHub
parent 7154fa5193 c6c14a040d
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src/doc/unstable-book/src/compiler-flags/check-cfg.md
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@ -92,8 +92,6 @@ and `cfg!(name = "value")` call. It will check that the `"value"` specified is p
list of expected values. If `"value"` is not in it, then `rustc` will report an `unexpected_cfgs` list of expected values. If `"value"` is not in it, then `rustc` will report an `unexpected_cfgs`
lint diagnostic. The default diagnostic level for this lint is `Warn`. lint diagnostic. The default diagnostic level for this lint is `Warn`.
The form `values()` is an error, because it does not specify a condition name.
To enable checking of values, but to provide an empty set of valid values, use this form: To enable checking of values, but to provide an empty set of valid values, use this form:
```bash ```bash
@ -104,13 +102,17 @@ The `--check-cfg values(...)` option can be repeated, both for the same conditio
different names. If it is repeated for the same condition name, then the sets of values for that different names. If it is repeated for the same condition name, then the sets of values for that
condition are merged together. condition are merged together.
If `values()` is specified, then `rustc` will enable the checking of well-known values defined
by itself. Note that it's necessary to specify the `values()` form to enable the checking of
well known values, specifying the other forms doesn't implicitly enable it.
## Examples ## Examples
Consider this command line: Consider this command line:
```bash ```bash
rustc --check-cfg 'names(feature)' \ rustc --check-cfg 'names(feature)' \
--check-cfg 'values(feature,"lion","zebra")' \ --check-cfg 'values(feature, "lion", "zebra")' \
--cfg 'feature="lion"' -Z unstable-options \ --cfg 'feature="lion"' -Z unstable-options \
example.rs example.rs
``` ```