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reference: clarify #[cfg] section

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Alex Burka 2017-01-28 16:53:06 -05:00 committed by Alex Burka
parent a797b6e2fe
commit 620074df6a
1 changed files with 13 additions and 5 deletions
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src/doc/reference.md
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@ -2114,10 +2114,15 @@ Sometimes one wants to have different compiler outputs from the same code,
depending on build target, such as targeted operating system, or to enable
release builds.
There are two kinds of configuration options, one that is either defined or not
(`#[cfg(foo)]`), and the other that contains a string that can be checked
against (`#[cfg(bar = "baz")]`). Currently, only compiler-defined configuration
options can have the latter form.
Configuration options are boolean (on or off) and are named either with a
single identifier (e.g. `foo`) or an identifier and a string (e.g. `foo = "bar"`;
the quotes are required and spaces around the `=` are unimportant). Note that
similarly-named options, such as `foo`, `foo="bar"` and `foo="baz"` may each be set
or unset independently.
Configuration options are either provided by the compiler or passed in on the
command line using `--cfg` (e.g. `rustc main.rs --cfg foo --cfg 'bar="baz"'`).
Rust code then checks for their presence using the `#[cfg(...)]` attribute:
```
// The function is only included in the build when compiling for OSX
@ -2196,7 +2201,10 @@ You can also set another attribute based on a `cfg` variable with `cfg_attr`:
#[cfg_attr(a, b)]
```
Will be the same as `#[b]` if `a` is set by `cfg`, and nothing otherwise.
This is the same as `#[b]` if `a` is set by `cfg`, and nothing otherwise.
Lastly, configuration options can be used in expressions by invoking the `cfg!`
macro: `cfg!(a)` evaluates to `true` if `a` is set, and `false` otherwise.
### Lint check attributes