cargo_emit/
rustc_cfg.rs

1/// Tells Cargo to enable a `$feature`.
2///
3/// This is equivalent to:
4///
5/// ```
6/// println!("cargo:rustc-cfg=$feature");
7/// ```
8///
9/// # Examples
10///
11/// Useful for conditionally enabling certain code to run.
12///
13/// ```
14/// # struct Cargo;
15/// # impl Cargo {
16/// #   fn can_bench(&self) -> bool { true }
17/// # }
18/// # let cargo = Cargo;
19/// if cargo.can_bench() {
20///     cargo_emit::rustc_cfg!("bench");
21/// }
22/// ```
23///
24/// or, in case you want it to emit to a custom stream:
25///
26/// ```
27/// # struct Cargo;
28/// # impl Cargo {
29/// #   fn can_bench(&self) -> bool { true }
30/// # }
31/// # let cargo = Cargo;
32/// let mut stdout = std::io::stdout();    
33/// if cargo.can_bench() {
34///     cargo_emit::rustc_cfg!(
35///         to: stdout,
36///         "bench"
37///     );
38/// }
39/// ```
40///
41/// Then outside of `build.rs`:
42///
43/// ```
44/// #[cfg(bench)]
45/// mod benches {
46///     // ...
47/// }
48/// ```
49#[macro_export]
50macro_rules! rustc_cfg {
51    (to: $stream:expr, $feature:expr $(, $($args:tt)*)?) => {
52        $crate::pair!(to: $stream, "rustc-cfg", $feature $(, $($args)+)?);
53    };
54    ($feature:expr $(, $($args:tt)*)?) => {
55        $crate::rustc_cfg!(to: std::io::stdout(), $feature $(, $($args)+)?);
56    };
57}
58
59#[cfg(test)]
60mod tests {
61    #[test]
62    fn literal() {
63        insta::assert_display_snapshot!(
64            crate::capture_output(|output| {
65                crate::rustc_cfg!(
66                    to: output,
67                    "CFG"
68                );
69            }),
70            @"cargo:rustc-cfg=CFG\n"
71        );
72    }
73
74    #[test]
75    fn formatted() {
76        insta::assert_display_snapshot!(
77            crate::capture_output(|output| {
78                crate::rustc_cfg!(
79                    to: output,
80                    "{}", "CFG"
81                );
82            }),
83            @"cargo:rustc-cfg=CFG\n"
84        );
85    }
86}
cargo_emit/rustc_cfg.rs

cargo_emit/
rustc_cfg.rs
