diff --git a/gen/build/src/cargo.rs b/gen/build/src/cargo.rs
new file mode 100644
index 000000000..9d9f548d1
--- /dev/null
+++ b/gen/build/src/cargo.rs
@@ -0,0 +1,153 @@
+use crate::gen::{CfgEvaluator, CfgResult};
+use once_cell::sync::OnceCell;
+use std::borrow::Borrow;
+use std::cmp::Ordering;
+use std::collections::{BTreeMap as Map, BTreeSet as Set};
+use std::env;
+
+static ENV: OnceCell<CargoEnv> = OnceCell::new();
+
+struct CargoEnv {
+    features: Set<Name>,
+    cfgs: Map<Name, String>,
+}
+
+pub(super) struct CargoEnvCfgEvaluator;
+
+impl CfgEvaluator for CargoEnvCfgEvaluator {
+    fn eval(&self, name: &str, query_value: Option<&str>) -> CfgResult {
+        let env = ENV.get_or_init(CargoEnv::load);
+        if name == "feature" {
+            if let Some(query_value) = query_value {
+                CfgResult::from(env.features.contains(Lookup::new(query_value)))
+            } else {
+                let msg = "expected `feature = \"...\"`".to_owned();
+                CfgResult::Undetermined { msg }
+            }
+        } else {
+            match env.cfgs.get(Lookup::new(name)) {
+                Some(cargo_value) => {
+                    if let Some(query_value) = query_value {
+                        CfgResult::from(cargo_value.split(',').any(|value| value == query_value))
+                    } else {
+                        CfgResult::True
+                    }
+                }
+                None => CfgResult::False,
+            }
+        }
+    }
+}
+
+impl CargoEnv {
+    fn load() -> Self {
+        const CARGO_FEATURE_PREFIX: &str = "CARGO_FEATURE_";
+        const CARGO_CFG_PREFIX: &str = "CARGO_CFG_";
+
+        let mut features = Set::new();
+        let mut cfgs = Map::new();
+        for (k, v) in env::vars_os() {
+            let k = match k.to_str() {
+                Some(k) => k,
+                None => continue,
+            };
+            let v = match v.into_string() {
+                Ok(v) => v,
+                Err(_) => continue,
+            };
+            if let Some(feature_name) = k.strip_prefix(CARGO_FEATURE_PREFIX) {
+                let feature_name = Name(feature_name.to_owned());
+                features.insert(feature_name);
+            } else if let Some(cfg_name) = k.strip_prefix(CARGO_CFG_PREFIX) {
+                let cfg_name = Name(cfg_name.to_owned());
+                cfgs.insert(cfg_name, v);
+            }
+        }
+        CargoEnv { features, cfgs }
+    }
+}
+
+struct Name(String);
+
+impl Ord for Name {
+    fn cmp(&self, rhs: &Self) -> Ordering {
+        Lookup::new(&self.0).cmp(Lookup::new(&rhs.0))
+    }
+}
+
+impl PartialOrd for Name {
+    fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
+        Some(self.cmp(rhs))
+    }
+}
+
+impl Eq for Name {}
+
+impl PartialEq for Name {
+    fn eq(&self, rhs: &Self) -> bool {
+        Lookup::new(&self.0).eq(Lookup::new(&rhs.0))
+    }
+}
+
+#[repr(transparent)]
+struct Lookup(str);
+
+impl Lookup {
+    fn new(name: &str) -> &Self {
+        unsafe { &*(name as *const str as *const Self) }
+    }
+}
+
+impl Borrow<Lookup> for Name {
+    fn borrow(&self) -> &Lookup {
+        Lookup::new(&self.0)
+    }
+}
+
+impl Ord for Lookup {
+    fn cmp(&self, rhs: &Self) -> Ordering {
+        self.0
+            .bytes()
+            .map(CaseAgnosticByte)
+            .cmp(rhs.0.bytes().map(CaseAgnosticByte))
+    }
+}
+
+impl PartialOrd for Lookup {
+    fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
+        Some(self.cmp(rhs))
+    }
+}
+
+impl Eq for Lookup {}
+
+impl PartialEq for Lookup {
+    fn eq(&self, rhs: &Self) -> bool {
+        self.0
+            .bytes()
+            .map(CaseAgnosticByte)
+            .eq(rhs.0.bytes().map(CaseAgnosticByte))
+    }
+}
+
+struct CaseAgnosticByte(u8);
+
+impl Ord for CaseAgnosticByte {
+    fn cmp(&self, rhs: &Self) -> Ordering {
+        self.0.to_ascii_lowercase().cmp(&rhs.0.to_ascii_lowercase())
+    }
+}
+
+impl PartialOrd for CaseAgnosticByte {
+    fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
+        Some(self.cmp(rhs))
+    }
+}
+
+impl Eq for CaseAgnosticByte {}
+
+impl PartialEq for CaseAgnosticByte {
+    fn eq(&self, rhs: &Self) -> bool {
+        self.cmp(rhs) == Ordering::Equal
+    }
+}
diff --git a/gen/build/src/lib.rs b/gen/build/src/lib.rs
index b8a463c3a..bdb43a159 100644
--- a/gen/build/src/lib.rs
+++ b/gen/build/src/lib.rs
@@ -78,6 +78,7 @@
     clippy::wrong_self_convention
 )]
 
+mod cargo;
 mod cfg;
 mod deps;
 mod error;
@@ -89,6 +90,7 @@ mod syntax;
 mod target;
 mod vec;
 
+use crate::cargo::CargoEnvCfgEvaluator;
 use crate::deps::{Crate, HeaderDir};
 use crate::error::{Error, Result};
 use crate::gen::error::report;
@@ -393,6 +395,7 @@ fn make_include_dir(prj: &Project) -> Result<PathBuf> {
 fn generate_bridge(prj: &Project, build: &mut Build, rust_source_file: &Path) -> Result<()> {
     let opt = Opt {
         allow_dot_includes: false,
+        cfg_evaluator: Box::new(CargoEnvCfgEvaluator),
         ..Opt::default()
     };
     let generated = gen::generate_from_path(rust_source_file, &opt);