feat(build-script): Pass CARGO_CFG_FEATURE

crates/build-rs-test-lib/build.rs

@@ -9,6 +9,7 @@ fn main() {
 fn smoke_test_inputs() {
     use build_rs::input::*;
     dbg!(cargo());
+    dbg!(cargo_cfg_feature());
     dbg!(cargo_cfg("careful"));
     #[cfg(feature = "unstable")]
     dbg!(cargo_cfg_fmt_debug());

crates/build-rs/src/input.rs

@@ -102,6 +102,13 @@ mod cfg {
     // those disabled with #[cfg(any())] don't seem meaningfully useful
     // but we list all cfg that are default known to check-cfg
 
+    /// Each activated feature of the package being built
+    #[doc = requires_msrv!("1.85")]
+    #[track_caller]
+    pub fn cargo_cfg_feature() -> Vec<String> {
+        to_strings(var_or_panic(&cargo_cfg_var("target_feature")), ',')
+    }
+
     #[cfg(any())]
     #[track_caller]
     pub fn cargo_cfg_clippy() -> bool {

crates/build-rs/src/lib.rs

@@ -40,6 +40,20 @@ MSRV: Respected as of "#,
     };
 }
 
+macro_rules! requires_msrv {
+    ($ver:literal) => {
+        concat!(
+            r#"<div class="warning">
+
+MSRV: Requires "#,
+            $ver,
+            r#".
+
+</div>"#
+        )
+    };
+}
+
 mod ident;
 
 pub mod input;

src/cargo/core/compiler/custom_build.rs

@@ -336,6 +336,10 @@ fn build_work(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResul
     }
 
     let mut cfg_map = HashMap::new();
+    cfg_map.insert(
+        "feature",
+        unit.features.iter().map(|s| s.as_str()).collect::<Vec<_>>(),
+    );
     for cfg in bcx.target_data.cfg(unit.kind) {
         match *cfg {
             Cfg::Name(ref n) => {

src/doc/src/reference/environment-variables.md

@@ -342,6 +342,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
   values built-in to the compiler (which can be seen with `rustc --print=cfg`)
   and values set by build scripts and extra flags passed to `rustc` (such as
   those defined in `RUSTFLAGS`). Some examples of what these variables are:
+    * `CARGO_CFG_FEATURE` --- Each activated feature of the package being built.
     * `CARGO_CFG_UNIX` --- Set on [unix-like platforms].
     * `CARGO_CFG_WINDOWS` --- Set on [windows-like platforms].
     * `CARGO_CFG_TARGET_FAMILY=unix,wasm` --- The [target family].
@@ -356,8 +357,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
   > Note that different [target triples][Target Triple] have different sets of `cfg` values,
   > hence variables present in one target triple might not be available in the other.
   >
-  > Some cfg values like `debug_assertions`, `test`, and Cargo features like
-  > `feature="foo"` are not available.
+  > Some cfg values like `debug_assertions` and `test` are not available.
 * `OUT_DIR` --- the folder in which all output and intermediate artifacts should
   be placed. This folder is inside the build directory for the package being built,
   and it is unique for the package in question.

tests/testsuite/build_script.rs

@@ -156,7 +156,7 @@ fn custom_build_env_vars() {
                 authors = ["wycats@example.com"]
 
                 [features]
-                bar_feat = ["bar/foo"]
+                bar_feat = ["bar/foo", "bar/other-feature"]
 
                 [dependencies.bar]
                 path = "bar"
@@ -176,6 +176,7 @@ fn custom_build_env_vars() {
 
                 [features]
                 foo = []
+                other-feature = []
             "#,
         )
         .file("bar/src/lib.rs", "pub fn hello() {}");
@@ -213,6 +214,8 @@ fn custom_build_env_vars() {
                 let _host = env::var("HOST").unwrap();
 
                 let _feat = env::var("CARGO_FEATURE_FOO").unwrap();
+                let feat = env::var("CARGO_CFG_FEATURE").unwrap();
+                assert_eq!(feat, "foo,other-feature");
 
                 let cargo = env::var("CARGO").unwrap();
                 if env::var_os("CHECK_CARGO_IS_RUSTC").is_some() {{