1 files changed, 51 insertions, 0 deletions

diff --git a/crates/cfg/src/lib.rs b/crates/cfg/src/lib.rs
new file mode 100644
index 000000000..a9d50e698
--- /dev/null
+++ b/crates/cfg/src/lib.rs
@@ -0,0 +1,51 @@
+//! cfg defines conditional compiling options, `cfg` attibute parser and evaluator
+
+mod cfg_expr;
+
+use rustc_hash::FxHashSet;
+use tt::SmolStr;
+
+pub use cfg_expr::CfgExpr;
+
+/// Configuration options used for conditional compilition on items with `cfg` attributes.
+/// We have two kind of options in different namespaces: atomic options like `unix`, and
+/// key-value options like `target_arch="x86"`.
+///
+/// Note that for key-value options, one key can have multiple values (but not none).
+/// `feature` is an example. We have both `feature="foo"` and `feature="bar"` if features
+/// `foo` and `bar` are both enabled. And here, we store key-value options as a set of tuple
+/// of key and value in `key_values`.
+///
+/// See: https://doc.rust-lang.org/reference/conditional-compilation.html#set-configuration-options
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub struct CfgOptions {
+    atoms: FxHashSet<SmolStr>,
+    key_values: FxHashSet<(SmolStr, SmolStr)>,
+}
+
+impl CfgOptions {
+    pub fn check(&self, cfg: &CfgExpr) -> Option<bool> {
+        cfg.fold(&|key, value| match value {
+            None => self.atoms.contains(key),
+            Some(value) => self.key_values.contains(&(key.clone(), value.clone())),
+        })
+    }
+
+    pub fn insert_atom(&mut self, key: SmolStr) {
+        self.atoms.insert(key);
+    }
+
+    pub fn insert_key_value(&mut self, key: SmolStr, value: SmolStr) {
+        self.key_values.insert((key, value));
+    }
+
+    pub fn append(&mut self, other: &CfgOptions) {
+        for atom in &other.atoms {
+            self.atoms.insert(atom.clone());
+        }
+
+        for (key, value) in &other.key_values {
+            self.key_values.insert((key.clone(), value.clone()));
+        }
+    }
+}