Expost host and target sysroot to build scripts.

src/cargo/core/compiler/custom_build.rs

@@ -32,7 +32,7 @@
 //! [instructions]: https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script
 
 use super::{fingerprint, Context, Job, Unit, Work};
-use crate::core::compiler::artifact;
+use crate::core::compiler::{artifact, CompileKind};
 use crate::core::compiler::context::Metadata;
 use crate::core::compiler::job_queue::JobState;
 use crate::core::{profiles::ProfileRoot, PackageId, Target};
@@ -316,6 +316,15 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
         cmd.env("CARGO_TRIM_PATHS", trim_paths.to_string());
     }
 
+    // Pass along sysroots used by the host/target.
+    let host_sysroot = &bcx.target_data.info(CompileKind::Host).sysroot;
+    let target_sysroot = match unit.kind {
+        CompileKind::Host => host_sysroot,
+        CompileKind::Target(_) => &bcx.target_data.info(unit.kind).sysroot
+    };
+    cmd.env("RUSTC_HOST_SYSROOT", host_sysroot);
+    cmd.env("RUSTC_TARGET_SYSROOT", target_sysroot);
+
     // Be sure to pass along all enabled features for this package, this is the
     // last piece of statically known information that we have.
     for feat in &unit.features {

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

@@ -392,6 +392,8 @@ let out_dir = env::var("OUT_DIR").unwrap();
                    changed by editing `.cargo/config.toml`; see the documentation
                    about [cargo configuration][cargo-config] for more
                    information.
+* `RUSTC_HOST_SYSROOT` --- The path to the `rustc` sysroot for the host.
+* `RUSTC_TARGET_SYSROOT` --- The path to the `rustc` sysroot for the target.
 * `CARGO_ENCODED_RUSTFLAGS` --- extra flags that Cargo invokes `rustc` with,
   separated by a `0x1f` character (ASCII Unit Separator). See
   [`build.rustflags`]. Note that since Rust 1.55, `RUSTFLAGS` is removed from