Stateless feature probing

src/lib.rs

@@ -378,6 +378,42 @@ impl AutoCfg {
             emit(cfg);
         }
     }
+
+    /// Tests whether the given features can be used.
+    ///
+    /// To ensure the features work as expected, it is highly recommended to
+    /// attempt to use the features.
+    ///
+    /// The test code is subject to change, but currently looks like:
+    ///
+    /// ```ignore
+    /// #![feature(FEATURES)]
+    /// CODE
+    /// ```
+    pub fn probe_features(&self, features: &[&str], code: &str) -> bool {
+        use std::fmt::Write;
+        let probe = &mut String::new();
+        write!(probe, "#![feature(").unwrap();
+        for feature in features {
+            write!(probe, "{},", feature).unwrap();
+        }
+        write!(probe, ")] {}", code).unwrap();
+        self.probe(probe).unwrap_or(false)
+    }
+
+    /// Emits a config value `has_FEATURE` if `probe_features` returns true.
+    pub fn emit_has_feature(&self, feature: &str, code: &str) {
+        if self.probe_features(&[feature], code) {
+            emit(&format!("has_{}", feature));
+        }
+    }
+
+    /// Emits the given `cfg` value if `probe_features` returns true.
+    pub fn emit_features_cfg(&self, features: &[&str], code: &str, cfg: &str) {
+        if self.probe_features(features, code) {
+            emit(cfg)
+        }
+    }
 }
 
 fn mangle(s: &str) -> String {

src/tests.rs

@@ -1,6 +1,8 @@
 use super::AutoCfg;
 use std::env;
 use std::path::Path;
+use std::process::Command;
+use std::str;
 
 impl AutoCfg {
     fn core_std(&self, path: &str) -> String {
@@ -22,6 +24,32 @@ impl AutoCfg {
             None => Self::with_dir("target"),
         }
     }
+
+    fn assert_nightly(&self, probe_result: bool) {
+        // Get rustc's verbose version
+        let output = Command::new(&self.rustc)
+            .args(&["--version", "--verbose"])
+            .output()
+            .unwrap();
+        if !output.status.success() {
+            panic!("could not execute rustc")
+        }
+        let output = str::from_utf8(&output.stdout).unwrap();
+
+        // Find the release line in the verbose version output.
+        let release = match output.lines().find(|line| line.starts_with("release: ")) {
+            Some(line) => &line["release: ".len()..],
+            None => panic!("could not find rustc release"),
+        };
+
+        // Check for nightly channel info, e.g. "-nightly", "-dev"
+        let nightly = match release.find('-') {
+            Some(i) => &release[i..] == "-nightly" || &release[i..] == "-dev",
+            None => false,
+        };
+
+        assert_eq!(nightly, probe_result);
+    }
 }
 
 #[test]
@@ -133,6 +161,17 @@ fn probe_constant() {
     ac.assert_min(1, 39, ac.probe_constant(r#""test".len()"#));
 }
 
+#[test]
+fn prope_feature() {
+    let ac = AutoCfg::for_test().unwrap();
+    // an empty #![features()] has no effect
+    assert!(ac.probe_features(&[], ""));
+    // stabilized feature succeeds
+    ac.assert_nightly(ac.probe_features(&["rust1"], ""));
+    // fake feature fails
+    ac.assert_nightly(!ac.probe_features(&["RUSTC_DONT_MAKE_ME_A_LIAR"], ""));
+}
+
 #[test]
 fn dir_does_not_contain_target() {
     assert!(!super::dir_contains_target(