Introduce ra_cfg to parse and evaluate CfgExpr

crates/ra_cfg/src/lib.rs

@@ -0,0 +1,43 @@
+//! ra_cfg defines conditional compiling options, `cfg` attibute parser and evaluator
+use ra_syntax::SmolStr;
+use rustc_hash::{FxHashMap, FxHashSet};
+
+mod cfg_expr;
+
+pub use cfg_expr::{parse_cfg, CfgExpr};
+
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub struct CfgOptions {
+    atoms: FxHashSet<SmolStr>,
+    features: FxHashSet<SmolStr>,
+    options: FxHashMap<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) if key == "feature" => self.features.contains(value),
+            Some(value) => self.options.get(key).map_or(false, |v| v == value),
+        })
+    }
+
+    pub fn is_cfg_enabled(&self, attr: &tt::Subtree) -> Option<bool> {
+        self.check(&parse_cfg(attr))
+    }
+
+    pub fn atom(mut self, name: SmolStr) -> CfgOptions {
+        self.atoms.insert(name);
+        self
+    }
+
+    pub fn feature(mut self, name: SmolStr) -> CfgOptions {
+        self.features.insert(name);
+        self
+    }
+
+    pub fn option(mut self, key: SmolStr, value: SmolStr) -> CfgOptions {
+        self.options.insert(key, value);
+        self
+    }
+}