1 files changed, 35 insertions, 0 deletions
diff --git a/src/bootstrap/bin/rustc.rs b/src/bootstrap/bin/rustc.rs
index 1d5260228..9611c866d 100644
--- a/src/bootstrap/bin/rustc.rs
+++ b/src/bootstrap/bin/rustc.rs
@@ -154,6 +154,41 @@ fn main() {
         cmd.arg("-Z").arg("force-unstable-if-unmarked");
     }
 
+    // allow-features is handled from within this rustc wrapper because of
+    // issues with build scripts. Some packages use build scripts to
+    // dynamically detect if certain nightly features are available.
+    // There are different ways this causes problems:
+    //
+    // * rustix runs `rustc` on a small test program to see if the feature is
+    //   available (and sets a `cfg` if it is). It does not honor
+    //   CARGO_ENCODED_RUSTFLAGS.
+    // * proc-macro2 detects if `rustc -vV` says "nighty" or "dev" and enables
+    //   nightly features. It will scan CARGO_ENCODED_RUSTFLAGS for
+    //   -Zallow-features. Unfortunately CARGO_ENCODED_RUSTFLAGS is not set
+    //   for build-dependencies when --target is used.
+    //
+    // The issues above means we can't just use RUSTFLAGS, and we can't use
+    // `cargo -Zallow-features=…`. Passing it through here ensures that it
+    // always gets set. Unfortunately that also means we need to enable more
+    // features than we really want (like those for proc-macro2), but there
+    // isn't much of a way around it.
+    //
+    // I think it is unfortunate that build scripts are doing this at all,
+    // since changes to nightly features can cause crates to break even if the
+    // user didn't want or care about the use of the nightly features. I think
+    // nightly features should be opt-in only. Unfortunately the dynamic
+    // checks are now too wide spread that we just need to deal with it.
+    //
+    // If you want to try to remove this, I suggest working with the crate
+    // authors to remove the dynamic checking. Another option is to pursue
+    // https://github.com/rust-lang/cargo/issues/11244 and
+    // https://github.com/rust-lang/cargo/issues/4423, which will likely be
+    // very difficult, but could help expose -Zallow-features into build
+    // scripts so they could try to honor them.
+    if let Ok(allow_features) = env::var("RUSTC_ALLOW_FEATURES") {
+        cmd.arg(format!("-Zallow-features={allow_features}"));
+    }
+
     if let Ok(flags) = env::var("MAGIC_EXTRA_RUSTFLAGS") {
         for flag in flags.split(' ') {
             cmd.arg(flag);