add rustc build flags reflection

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/info/mod.rs

@@ -2,4 +2,7 @@
 //! etc information which can be queried by admins or used by developers.
 
 pub mod cargo;
+pub mod rustc;
 pub mod version;
+
+pub use conduit_macros::rustc_flags_capture;

src/core/info/rustc.rs

@@ -0,0 +1,53 @@
+//! Information about the build related to rustc. This is a frontend interface
+//! informed by proc-macros at build time. Since the project is split into
+//! several crates, lower-level information is supplied from each crate during
+//! static initialization.
+
+use std::{
+	collections::BTreeMap,
+	sync::{Mutex, OnceLock},
+};
+
+use crate::utils::exchange;
+
+/// Raw capture of rustc flags used to build each crate in the project. Informed
+/// by rustc_flags_capture macro (one in each crate's mod.rs). This is
+/// done during static initialization which is why it's mutex-protected and pub.
+/// Should not be written to by anything other than our macro.
+pub static FLAGS: Mutex<BTreeMap<&str, &[&str]>> = Mutex::new(BTreeMap::new());
+
+/// Processed list of enabled features across all project crates. This is
+/// generated from the data in FLAGS.
+static FEATURES: OnceLock<Vec<&'static str>> = OnceLock::new();
+
+/// List of features enabled for the project.
+pub fn features() -> &'static Vec<&'static str> { FEATURES.get_or_init(init_features) }
+
+fn init_features() -> Vec<&'static str> {
+	let mut features = Vec::new();
+	FLAGS
+		.lock()
+		.expect("locked")
+		.iter()
+		.for_each(|(_, flags)| append_features(&mut features, flags));
+
+	features.sort_unstable();
+	features.dedup();
+	features
+}
+
+fn append_features(features: &mut Vec<&'static str>, flags: &[&'static str]) {
+	let mut next_is_cfg = false;
+	for flag in flags {
+		let is_cfg = *flag == "--cfg";
+		let is_feature = flag.starts_with("feature=");
+		if exchange(&mut next_is_cfg, is_cfg) && is_feature {
+			if let Some(feature) = flag
+				.split_once('=')
+				.map(|(_, feature)| feature.trim_matches('"'))
+			{
+				features.push(feature);
+			}
+		}
+	}
+}