1  /// Print to `stdout` a build-script instruction for Cargo.
 2  ///
 3  /// # Panics
 4  /// If either argument is an empty string.
 5  ///
 6  /// (Actually private to the crate, not part of public API.  Is only `pub` for old Rust versions.)
 7  pub fn emit_cargo_instruction(
 8      instruction: &str,
 9      arg: Option<&str>,
10  )
11  {
12      assert!(!instruction.is_empty());
13      if let Some(arg) = arg {
14          assert!(!arg.is_empty());
15      }
16      println!(
17          "cargo:{}{}",
18          instruction,
19          arg.map(|s| format!("={}", s)).unwrap_or_else(String::new)
20      );
21  }
22  
23  /// Tell Cargo to display the given warning message after a build script has finished running.
24  pub fn emit_warning(message: &str)
25  {
26      emit_cargo_instruction("warning", Some(message));
27  }
28  
29  /// Tell Cargo to pass a key-value configuration option to the compiler to be set for conditional
30  /// compilation, for features of the Rust compiler, language, or standard library.
31  ///
32  /// This enables using [the standard conditional-compilation
33  /// forms](https://doc.rust-lang.org/reference/conditional-compilation.html) (i.e. the `cfg`
34  /// attribute, et al) for features of Rust itself, in a way that is more similar to Cargo package
35  /// features.
36  ///
37  /// `category`: One of `"comp"`, `"lang"`, or `"lib"`.
38  ///
39  /// `value`: The feature name, which should follow [The Unstable
40  /// Book](https://doc.rust-lang.org/nightly/unstable-book/index.html) where appropriate.
41  ///
42  /// # Examples
43  ///
44  /// Doing `emit_rust_feature("lib", "step_trait")` in a package's build script enables the
45  /// package's source code to use `#[cfg(rust_lib_feature = "step_trait")]`.
46  ///
47  /// # Panics
48  ///
49  /// If `category` is not one of the acceptable categories.
50  ///
51  /// (Actually private to the crate, not part of public API.  Is only `pub` for old Rust versions.)
52  pub fn emit_rust_feature(
53      category: &str,
54      name: &str,
55  )
56  {
57      assert!(["comp", "lang", "lib"].contains(&category));
58      emit_cargo_instruction("rustc-cfg", Some(&format!("rust_{}_feature={:?}", category, name)));
59  }