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Make check-cfg docs more user-friendly
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149 changes: 89 additions & 60 deletions src/doc/rustc/src/check-cfg.md
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# Checking conditional configurations

`rustc` accepts the `--check-cfg` option, which specifies whether to check conditions and how to
check them. The `--check-cfg` option takes a value, called the _check cfg specification_.
This specification has one form:
`rustc` supports checking that every _reachable_[^reachable] `#[cfg]` matches a list of the
expected config names and values.

1. `--check-cfg cfg(...)` mark a configuration and it's expected values as expected.
This can help with verifying that the crate is correctly handling conditional compilation for
different target platforms or features. It ensures that the cfg settings are consistent between
what is intended and what is used, helping to catch potential bugs or errors early in the
development process.

*No implicit expectation is added when using `--cfg`. Users are expected to
pass all expected names and values using the _check cfg specification_.*
In order to accomplish that goal, `rustc` accepts the `--check-cfg` flag, which specifies
whether to check conditions and how to check them.

## The `cfg(...)` form
> **Note:** No implicit expectation is added when using `--cfg`. Users are expected to
pass all expected names and values using the _check cfg specification_.

The `cfg(...)` form enables checking the values within list-valued conditions. It has this
basic form:
[^reachable]: `rustc` promises to at least check reachable `#[cfg]`, and while non-reachable
`#[cfg]` are not currently checked, they may well be checked in the future without it being a
breaking change.

## Specifying expected names and values

To specify expected names and values, the _check cfg specification_ provides the `cfg(...)`
option which enables specifying for an expected config name and it's expected values.

It has this basic form:

```bash
rustc --check-cfg 'cfg(name, values("value1", "value2", ... "valueN"))'
```

where `name` is a bare identifier (has no quotes) and each `"value"` term is a quoted literal
string. `name` specifies the name of the condition, such as `feature` or `my_cfg`.
`"value"` specify one of the value of that condition name.

When the `cfg(...)` option is specified, `rustc` will check every[^reachable]:
- `#[cfg(name = "value")]` attribute
- `#[cfg_attr(name = "value")]` attribute
- `#[link(name = "a", cfg(name = "value"))]` attribute
- `cfg!(name = "value")` macro call

When the `cfg(...)` option is specified, `rustc` will check every `#[cfg(name = "value")]`
attribute, `#[cfg_attr(name = "value")]` attribute, `#[link(name = "a", cfg(name = "value"))]`
attribute and `cfg!(name = "value")` macro call. It will check that the `"value"` specified is
present in the 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 command line `--cfg` arguments are currently NOT checked but may very well be checked
in the future.*

*The command line `--cfg` arguments are currently *NOT* checked but may very well be checked in
the future.*
`rustc` will check that the `"value"` specified is present in the 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`.

To check for the _none_ value (ie `#[cfg(foo)]`) one can use the `none()` predicate inside
`values()`: `values(none())`. It can be followed or preceded by any number of `"value"`.
Expand All @@ -43,12 +59,12 @@ rustc --check-cfg 'cfg(name, values(none()))'

To enable checking of name but not values, use one of these forms:

- No expected values (_will lint on every value_):
- No expected values (_will lint on every value of `name`_):
```bash
rustc --check-cfg 'cfg(name, values())'
```

- Unknown expected values (_will never lint_):
- Unknown expected values (_will never lint on value of `name`_):
```bash
rustc --check-cfg 'cfg(name, values(any()))'
```
Expand All @@ -59,17 +75,26 @@ To avoid repeating the same set of values, use this form:
rustc --check-cfg 'cfg(name1, ..., nameN, values("value1", "value2", ... "valueN"))'
```

To enable checking without specifying any names or values, use this form:

```bash
rustc --check-cfg 'cfg()'
```

The `--check-cfg cfg(...)` option can be repeated, both for the same condition name and for
different names. If it is repeated for the same condition name, then the sets of values for that
condition are merged together (precedence is given to `values(any())`).
> To help out an equivalence table between `--cfg` arguments and `--check-cfg` is available
[down below](#equivalence-table-with---cfg).
## Well known names and values
`rustc` has a internal list of well known names and their corresponding values.
Those well known names and values follows the same stability as what they refer to.
`rustc` maintains a list of well-known names and their corresponding values in order to avoid
the need to specify them manually.
Well known names and values checking is always enabled as long as at least one
`--check-cfg` argument is present.
Well known names and values are implicitly added as long as at least one `--check-cfg` argument
is present.
As of `2024-04-06T`, the list of known names is as follows:
Expand Down Expand Up @@ -108,11 +133,9 @@ As of `2024-04-06T`, the list of known names is as follows:
Like with `values(any())`, well known names checking can be disabled by passing `cfg(any())`
as argument to `--check-cfg`.
## Examples
### Equivalence table
## Equivalence table with `--cfg`
This table describe the equivalence of a `--cfg` argument to a `--check-cfg` argument.
This table describe the equivalence between a `--cfg` argument to a `--check-cfg` argument.
| `--cfg` | `--check-cfg` |
|-------------------------------|------------------------------------------------------------|
Expand All @@ -124,40 +147,42 @@ This table describe the equivalence of a `--cfg` argument to a `--check-cfg` arg
| `--cfg foo="1" --cfg bar="2"` | `--check-cfg=cfg(foo, values("1")) --check-cfg=cfg(bar, values("2"))` |
| `--cfg foo --cfg foo="bar"` | `--check-cfg=cfg(foo, values(none(), "bar"))` |
## Examples
### Example: Cargo-like `feature` example
Consider this command line:
```bash
rustc --check-cfg 'cfg(feature, values("lion", "zebra"))' \
--cfg 'feature="lion"' -Z unstable-options example.rs
--cfg 'feature="lion"' example.rs
```
This command line indicates that this crate has two features: `lion` and `zebra`. The `lion`
> This command line indicates that this crate has two features: `lion` and `zebra`. The `lion`
feature is enabled, while the `zebra` feature is disabled.
Given the `--check-cfg` arguments, exhaustive checking of names and
values are enabled.
`example.rs`:
```rust
#[cfg(feature = "lion")] // This condition is expected, as "lion" is an expected value of `feature`
#[cfg(feature = "lion")] // This condition is expected, as "lion" is an
// expected value of `feature`
fn tame_lion(lion: Lion) {}
#[cfg(feature = "zebra")] // This condition is expected, as "zebra" is an expected value of `feature`
// but the condition will still evaluate to false
// since only --cfg feature="lion" was passed
#[cfg(feature = "zebra")] // This condition is expected, as "zebra" is an expected
// value of `feature` but the condition will evaluate
// to false since only --cfg feature="lion" was passed
fn ride_zebra(z: Zebra) {}
#[cfg(feature = "platypus")] // This condition is UNEXPECTED, as "platypus" is NOT an expected value of
// `feature` and will cause a compiler warning (by default).
#[cfg(feature = "platypus")] // This condition is UNEXPECTED, as "platypus" is NOT
// an expected value of `feature` and will cause a
// the compiler to emit the `unexpected_cfgs` lint
fn poke_platypus() {}
#[cfg(feechure = "lion")] // This condition is UNEXPECTED, as 'feechure' is NOT a expected condition
// name, no `cfg(feechure, ...)` was passed in `--check-cfg`
#[cfg(feechure = "lion")] // This condition is UNEXPECTED, as 'feechure' is NOT
// a expected condition name, no `cfg(feechure, ...)`
// was passed in `--check-cfg`
fn tame_lion() {}
#[cfg(windows = "unix")] // This condition is UNEXPECTED, as while 'windows' is a well known
// condition name, it doesn't expect any values
#[cfg(windows = "unix")] // This condition is UNEXPECTED, as the well known
// 'windows' cfg doesn't expect any values
fn tame_windows() {}
```
Expand All @@ -166,50 +191,54 @@ fn tame_windows() {}
```bash
rustc --check-cfg 'cfg(is_embedded, has_feathers)' \
--check-cfg 'cfg(feature, values("zapping", "lasers"))' \
--cfg has_feathers --cfg 'feature="zapping"' -Z unstable-options
--cfg has_feathers --cfg 'feature="zapping"'
```
```rust
#[cfg(is_embedded)] // This condition is expected, as 'is_embedded' was provided in --check-cfg
fn do_embedded() {} // and doesn't take any value
#[cfg(is_embedded)] // This condition is expected, as 'is_embedded' was
// provided in --check-cfg and doesn't take any value
fn do_embedded() {}
#[cfg(has_feathers)] // This condition is expected, as 'has_feathers' was provided in --check-cfg
fn do_features() {} // and doesn't take any value
#[cfg(has_feathers)] // This condition is expected, as 'has_feathers' was
// provided in --check-cfg and doesn't take any value
fn do_features() {}
#[cfg(has_mumble_frotz)] // This condition is UNEXPECTED, as 'has_mumble_frotz' was NEVER provided
// in any --check-cfg arguments
#[cfg(has_mumble_frotz)] // This condition is UNEXPECTED, as 'has_mumble_frotz'
// was NEVER provided in any --check-cfg arguments
fn do_mumble_frotz() {}
#[cfg(feature = "lasers")] // This condition is expected, as "lasers" is an expected value of `feature`
#[cfg(feature = "lasers")] // This condition is expected, as "lasers" is an
// expected value of `feature`
fn shoot_lasers() {}
#[cfg(feature = "monkeys")] // This condition is UNEXPECTED, as "monkeys" is NOT an expected value of
// `feature`
#[cfg(feature = "monkeys")] // This condition is UNEXPECTED, as "monkeys" is NOT
// an expected value of `feature`
fn write_shakespeare() {}
```
### Example: Condition names without values
```bash
rustc --check-cfg 'cfg(is_embedded, has_feathers, values(any()))' \
--cfg has_feathers -Z unstable-options
--cfg has_feathers
```
```rust
#[cfg(is_embedded)] // This condition is expected, as 'is_embedded' was provided in --check-cfg
// as condition name
#[cfg(is_embedded)] // This condition is expected, as 'is_embedded' was
// provided in --check-cfg as condition name
fn do_embedded() {}
#[cfg(has_feathers)] // This condition is expected, as "has_feathers" was provided in --check-cfg
// as condition name
#[cfg(has_feathers)] // This condition is expected, as "has_feathers" was
// provided in --check-cfg as condition name
fn do_features() {}
#[cfg(has_feathers = "zapping")] // This condition is expected, as "has_feathers" was provided in
// and because *any* values is expected for 'has_feathers' no
#[cfg(has_feathers = "zapping")] // This condition is expected, as "has_feathers"
// was provided and because *any* values is
// expected for 'has_feathers' no
// warning is emitted for the value "zapping"
fn do_zapping() {}
#[cfg(has_mumble_frotz)] // This condition is UNEXPECTED, as 'has_mumble_frotz' was not provided
// in any --check-cfg arguments
#[cfg(has_mumble_frotz)] // This condition is UNEXPECTED, as 'has_mumble_frotz'
// was not provided in any --check-cfg arguments
fn do_mumble_frotz() {}
```
10 changes: 8 additions & 2 deletions src/doc/rustc/src/command-line-arguments.md
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Expand Up @@ -19,9 +19,15 @@ For examples, `--cfg 'verbose'` or `--cfg 'feature="serde"'`. These correspond
to `#[cfg(verbose)]` and `#[cfg(feature = "serde")]` respectively.

<a id="option-check-cfg"></a>
## `--check-cfg`: enables checking conditional configurations
## `--check-cfg`: configure compile-time checking of conditional compilation

This flag enables checking conditional configurations of the crate at compile-time,
specifically it helps configure the set of expected cfg names and values, in order
to check that every _reachable_ `#[cfg]` matches the expected config names and values.

This is different from the `--cfg` flag above which activates some config but do
not expect them. This is useful to prevent stalled conditions, typos, ...

This flag will enable checking conditional configurations.
Refer to the [Checking conditional configurations](check-cfg.md) of this book
for further details and explanation.

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