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

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Loïc BRANSTETT 2022-03-21 12:59:46 +01:00
parent 3b84829750
commit c6c14a040d
1 changed files with 5 additions and 3 deletions
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8
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`
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:
```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
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
Consider this command line:
```bash
rustc --check-cfg 'names(feature)' \
--check-cfg 'values(feature,"lion","zebra")' \
--check-cfg 'values(feature, "lion", "zebra")' \
--cfg 'feature="lion"' -Z unstable-options \
example.rs
```