Macro assert_cfg::exactly_one

macro_rules! exactly_one {
    ($($args:tt)*) => { ... };
}

Asserts that exactly one of the passed-in features is enabled.

Example

No enabled features

This example demonstrates the error message when no features are enabled.

assert_cfg::exactly_one!{
    feature = "std",
    feature = "no_std",
}

When no feature is enabled, the above code produces this compile-time error:

error[E0080]: evaluation of constant value failed
 --> src/lib.rs:10:1
  |
4 | / assert_cfg::exactly_one!{
5 | |     feature = "std",
6 | |     feature = "no_std",
7 | | }
  | |_^ the evaluated program panicked at '
no features were enabled, expected one of these to be enabled:
- `feature = "std"`
- `feature = "no_std"`

Too many enabled features

This example demonstrates the error message when more than one feature is enabled.

assert_cfg::exactly_one!{
    feature = "foo",
    not(feature = "qux"),
    feature = "hello",
}

When the "foo" and "bar" features are enabled, the above code produces this compile-time error:

error[E0080]: evaluation of constant value failed
 --> src/assert_exactly_one.rs:55:1
  |
4 | / assert_cfg::exactly_one!{
5 | |     feature = "foo",
6 | |     not(feature = "qux"),
7 | |     feature = "hello",
8 | | }
  | |_^ the evaluated program panicked at '
too many features were enabled, only one of them can be enabled:
- `feature = "foo"`
- `not (feature = "qux")`
', src/assert_exactly_one.rs:4:1
  |