Rework and improve unstable documentation of check-cfg
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@ -4,18 +4,16 @@ The tracking issue for this feature is: [#82450](https://github.com/rust-lang/ru
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------------------------
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This feature allows you to enable complete or partial checking of configuration.
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This feature enables checking of conditional configuration.
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`rustc` accepts the `--check-cfg` option, which specifies whether to check conditions and how to
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check them. The `--check-cfg` option takes a value, called the _check cfg specification_. The
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check cfg specification is parsed using the Rust metadata syntax, just as the `--cfg` option is.
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check them. The `--check-cfg` option takes a value, called the _check cfg specification_.
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This specification has one form:
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`--check-cfg` option take one form:
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1. `--check-cfg cfg(...)` mark a configuration and it's expected values as expected.
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1. `--check-cfg cfg(...)` enables checking the values within list-valued conditions.
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NOTE: No implicit expectation is added when using `--cfg` for both forms. Users are expected to
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pass all expected names and values using `cfg(...)`.
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*No implicit expectation is added when using `--cfg`. Users are expected to
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pass all expected names and values using the _check cfg specification_.*
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## The `cfg(...)` form
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@ -23,7 +21,7 @@ The `cfg(...)` form enables checking the values within list-valued conditions. I
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basic form:
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```bash
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rustc --check-cfg 'cfg(name1, ..., nameN, values("value1", "value2", ... "valueN"))'
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rustc --check-cfg 'cfg(name, values("value1", "value2", ... "valueN"))'
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```
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where `name` is a bare identifier (has no quotes) and each `"value"` term is a quoted literal
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@ -31,162 +29,186 @@ string. `name` specifies the name of the condition, such as `feature` or `my_cfg
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When the `cfg(...)` option is specified, `rustc` will check every `#[cfg(name = "value")]`
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attribute, `#[cfg_attr(name = "value")]` attribute, `#[link(name = "a", cfg(name = "value"))]`
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and `cfg!(name = "value")` call. It will check that the `"value"` specified is present in the
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list of expected values. If `"value"` is not in it, then `rustc` will report an `unexpected_cfgs`
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lint diagnostic. The default diagnostic level for this lint is `Warn`.
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attribute and `cfg!(name = "value")` macro call. It will check that the `"value"` specified is
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present in the list of expected values. If `"value"` is not in it, then `rustc` will report an
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`unexpected_cfgs` lint diagnostic. The default diagnostic level for this lint is `Warn`.
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The command line `--cfg` arguments are currently *NOT* checked but may very well be checked in
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the future.
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*The command line `--cfg` arguments are currently *NOT* checked but may very well be checked in
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the future.*
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To enable checking of values, but to provide an empty set of expected values, use these forms:
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To enable checking of values, but to provide an *none*/empty set of expected values
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(ie. expect `#[cfg(name)]`), use these forms:
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```bash
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rustc --check-cfg 'cfg(name1, ..., nameN)'
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rustc --check-cfg 'cfg(name1, ..., nameN, values())'
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rustc --check-cfg 'cfg(name)'
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rustc --check-cfg 'cfg(name, values())'
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```
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To enable checking of name but not values (i.e. unknown expected values), use this form:
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```bash
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rustc --check-cfg 'cfg(name1, ..., nameN, values(any()))'
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rustc --check-cfg 'cfg(name, values(any()))'
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```
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To avoid repeating the same set of values, use this form:
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```bash
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rustc --check-cfg 'cfg(name1, ..., nameN, values("value1", "value2", ... "valueN"))'
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```
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The `--check-cfg cfg(...)` option can be repeated, both for the same condition name and for
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different names. If it is repeated for the same condition name, then the sets of values for that
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condition are merged together (presedence is given to `any()`).
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condition are merged together (precedence is given to `values(any())`).
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## Well known names and values
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`rustc` has a internal list of well known names and their corresponding values.
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Those well known names and values follows the same stability as what they refer to.
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Well known values checking is always enabled as long as a `--check-cfg` argument is present.
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Well known names and values checking is always enabled as long as at least one
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`--check-cfg` argument is present.
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Well known names checking is always enable as long as a `--check-cfg` argument is present
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**unless** any `cfg(any())` argument is passed.
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As of `2024-01-09T`, the list of known names is as follows:
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To disable checking of well known names, use this form:
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<!--- See CheckCfg::fill_well_known in compiler/rustc_session/src/config.rs -->
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```bash
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rustc --check-cfg 'cfg(any())'
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```
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- `debug_assertions`
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- `doc`
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- `doctest`
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- `miri`
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- `overflow_checks`
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- `panic`
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- `proc_macro`
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- `relocation_model`
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- `sanitize`
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- `sanitizer_cfi_generalize_pointers`
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- `sanitizer_cfi_normalize_integers`
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- `target_abi`
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- `target_arch`
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- `target_endian`
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- `target_env`
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- `target_family`
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- `target_feature`
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- `target_has_atomic`
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- `target_has_atomic_equal_alignment`
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- `target_has_atomic_load_store`
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- `target_os`
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- `target_pointer_width`
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- `target_thread_local`
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- `target_vendor`
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- `test`
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- `unix`
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- `windows`
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NOTE: If one want to enable values and names checking without having any cfg to declare, one
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can use an empty `cfg()` argument.
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Like with `values(any())`, well known names checking can be disabled by passing `cfg(any())`
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as argument to `--check-cfg`.
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## Examples
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### Equivalence table
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This table describe the equivalence of a `--cfg` argument to a `--check-cfg` argument.
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| `--cfg` | `--check-cfg` |
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|-----------------------------|----------------------------------------------------------|
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| *nothing* | *nothing* or `--check-cfg=cfg()` (to enable the checking) |
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| `--cfg foo` | `--check-cfg=cfg(foo) or --check-cfg=cfg(foo, values())` |
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| `--cfg foo=""` | `--check-cfg=cfg(foo, values(""))` |
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| `--cfg foo="bar"` | `--check-cfg=cfg(foo, values("bar"))` |
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| `--cfg foo="1" --cfg foo="2"` | `--check-cfg=cfg(foo, values("1", "2"))` |
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| `--cfg foo="1" --cfg bar="2"` | `--check-cfg=cfg(foo, values("1")) --check-cfg=cfg(bar, values("2"))` |
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| `--cfg foo --cfg foo="bar"` | `--check-cfg=cfg(foo) --check-cfg=cfg(foo, values("bar"))` |
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NOTE: There is (currently) no way to express that a condition name is expected but no (!= none)
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values are expected. Passing an empty `values()` means *(none)* in the sense of `#[cfg(foo)]`
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with no value. Users are expected to NOT pass a `--check-cfg` with that condition name.
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### Example: Cargo-like `feature` example
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Consider this command line:
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```bash
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rustc --check-cfg 'cfg(feature, values("lion", "zebra"))' \
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--cfg 'feature="lion"' -Z unstable-options \
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example.rs
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--cfg 'feature="lion"' -Z unstable-options example.rs
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```
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This command line indicates that this crate has two features: `lion` and `zebra`. The `lion`
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feature is enabled, while the `zebra` feature is disabled. Exhaustive checking of names and
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values are enabled by default. Consider compiling this code:
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feature is enabled, while the `zebra` feature is disabled.
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Given the `--check-cfg` arguments, exhaustive checking of names and
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values are enabled.
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`example.rs`:
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```rust
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// This is expected, and tame_lion() will be compiled
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#[cfg(feature = "lion")]
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#[cfg(feature = "lion")] // This condition is expected, as "lion" is an expected value of `feature`
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fn tame_lion(lion: Lion) {}
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// This is expected, and ride_zebra() will NOT be compiled.
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#[cfg(feature = "zebra")]
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fn ride_zebra(zebra: Zebra) {}
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#[cfg(feature = "zebra")] // This condition is expected, as "zebra" is an expected value of `feature`
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// but the condition will still evaluate to false
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// since only --cfg feature="lion" was passed
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fn ride_zebra(z: Zebra) {}
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// This is UNEXPECTED, and will cause a compiler warning (by default).
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#[cfg(feature = "platypus")]
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#[cfg(feature = "platypus")] // This condition is UNEXPECTED, as "platypus" is NOT an expected value of
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// `feature` and will cause a compiler warning (by default).
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fn poke_platypus() {}
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// This is UNEXPECTED, because 'feechure' is not a known condition name,
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// and will cause a compiler warning (by default).
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#[cfg(feechure = "lion")]
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#[cfg(feechure = "lion")] // This condition is UNEXPECTED, as 'feechure' is NOT a expected condition
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// name, no `cfg(feechure, ...)` was passed in `--check-cfg`
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fn tame_lion() {}
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// This is UNEXPECTED, because 'windows' is a well known condition name,
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// and because 'windows' doesn't take any values,
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// and will cause a compiler warning (by default).
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#[cfg(windows = "unix")]
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#[cfg(windows = "unix")] // This condition is UNEXPECTED, as while 'windows' is a well known
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// condition name, it doens't expect any values
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fn tame_windows() {}
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```
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### Example: Checking condition names, but not values
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### Example: Multiple names and values
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```bash
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# This turns on checking for condition names, but not values, such as 'feature' values.
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rustc --check-cfg 'cfg(is_embedded, has_feathers, values(any()))' \
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--cfg has_feathers -Z unstable-options
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```
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```rust
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#[cfg(is_embedded)] // This is expected as "is_embedded" was provided in cfg()
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fn do_embedded() {} // and because names exhaustiveness was not disabled
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#[cfg(has_feathers)] // This is expected as "has_feathers" was provided in cfg()
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fn do_features() {} // and because names exhaustiveness was not disabled
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#[cfg(has_feathers = "zapping")] // This is expected as "has_feathers" was provided in cfg()
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// and because no value checking was enable for "has_feathers"
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// no warning is emitted for the value "zapping"
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fn do_zapping() {}
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#[cfg(has_mumble_frotz)] // This is UNEXPECTED because names checking is enable and
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// "has_mumble_frotz" was not provided in cfg()
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fn do_mumble_frotz() {}
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```
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### Example: Checking feature values, but not condition names
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```bash
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# This turns on checking for feature values, but not for condition names.
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rustc --check-cfg 'cfg(feature, values("zapping", "lasers"))' \
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--check-cfg 'cfg(any())' \
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--cfg 'feature="zapping"' -Z unstable-options
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```
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```rust
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#[cfg(is_embedded)] // This is doesn't raise a warning, because names checking was
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// disabled by 'cfg(any())'
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fn do_embedded() {}
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#[cfg(has_feathers)] // Same as above, 'cfg(any())' was provided so no name
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// checking is performed
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fn do_features() {}
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#[cfg(feature = "lasers")] // This is expected, "lasers" is in the cfg(feature) list
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fn shoot_lasers() {}
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#[cfg(feature = "monkeys")] // This is UNEXPECTED, because "monkeys" is not in the
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// cfg(feature) list
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fn write_shakespeare() {}
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```
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### Example: Checking both condition names and feature values
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```bash
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# This turns on checking for feature values and for condition names.
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rustc --check-cfg 'cfg(is_embedded, has_feathers)' \
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--check-cfg 'cfg(feature, values("zapping", "lasers"))' \
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--cfg has_feathers --cfg 'feature="zapping"' -Z unstable-options
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```
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```rust
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#[cfg(is_embedded)] // This is expected because "is_embedded" was provided in cfg()
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#[cfg(is_embedded)] // This condition is expected, as 'is_embedded' was provided in --check-cfg
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fn do_embedded() {} // and doesn't take any value
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#[cfg(has_feathers)] // This is expected because "has_feathers" was provided in cfg()
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fn do_features() {} // and deosn't take any value
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#[cfg(has_feathers)] // This condition is expected, as 'has_feathers' was provided in --check-cfg
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fn do_features() {} // and doesn't take any value
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#[cfg(has_mumble_frotz)] // This is UNEXPECTED, because "has_mumble_frotz" was never provided
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#[cfg(has_mumble_frotz)] // This condition is UNEXPECTED, as 'has_mumble_frotz' was NEVER provided
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// in any --check-cfg arguments
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fn do_mumble_frotz() {}
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#[cfg(feature = "lasers")] // This is expected, "lasers" is in the cfg(feature) list
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#[cfg(feature = "lasers")] // This condition is expected, as "lasers" is an expected value of `feature`
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fn shoot_lasers() {}
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#[cfg(feature = "monkeys")] // This is UNEXPECTED, because "monkeys" is not in
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// the cfg(feature) list
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#[cfg(feature = "monkeys")] // This condition is UNEXPECTED, as "monkeys" is NOT an expected value of
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// `feature`
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fn write_shakespeare() {}
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```
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### Example: Condition names without values
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```bash
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rustc --check-cfg 'cfg(is_embedded, has_feathers, values(any()))' \
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--cfg has_feathers -Z unstable-options
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```
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```rust
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#[cfg(is_embedded)] // This condition is expected, as 'is_embedded' was provided in --check-cfg
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// as condition name
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fn do_embedded() {}
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#[cfg(has_feathers)] // This condition is expected, as "has_feathers" was provided in --check-cfg
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// as condition name
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fn do_features() {}
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#[cfg(has_feathers = "zapping")] // This condition is expected, as "has_feathers" was provided in
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// and because *any* values is expected for 'has_feathers' no
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// warning is emitted for the value "zapping"
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fn do_zapping() {}
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#[cfg(has_mumble_frotz)] // This condition is UNEXPECTED, as 'has_mumble_frotz' was not provided
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// in any --check-cfg arguments
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fn do_mumble_frotz() {}
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```
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