Inform build scripts of rustc compiler context

crates/cargo-test-support/src/lib.rs

@@ -1041,6 +1041,14 @@ pub fn is_nightly() -> bool {
         && (vv.contains("-nightly") || vv.contains("-dev"))
 }
 
+pub fn rustc_release() -> &'static str {
+    RUSTC_INFO
+        .verbose_version
+        .lines()
+        .find_map(|line| line.strip_prefix("release: "))
+        .expect("verbose version has release: field")
+}
+
 pub fn process<T: AsRef<OsStr>>(t: T) -> ProcessBuilder {
     _process(t.as_ref())
 }

src/cargo/core/compiler/custom_build.rs

@@ -249,6 +249,22 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
         }
     }
 
+    // Also inform the build script of the rustc compiler context.
+    if let Some(wrapper) = bcx.rustc().wrapper.as_ref() {
+        cmd.env("RUSTC_WRAPPER", wrapper);
+    } else {
+        cmd.env_remove("RUSTC_WRAPPER");
+    }
+    cmd.env("RUSTFLAGS", bcx.rustflags_args(unit).join(" "));
+    let version = &bcx.rustc().version;
+    cmd.env(
+        "RUSTC_VERSION",
+        format!("{}.{}.{}", version.major, version.minor, version.patch),
+    );
+    cmd.env("RUSTC_VERSION_MAJOR", version.major.to_string());
+    cmd.env("RUSTC_VERSION_MINOR", version.minor.to_string());
+    cmd.env("RUSTC_VERSION_PATCH", version.patch.to_string());
+
     // Gather the set of native dependencies that this package has along with
     // some other variables to close over.
     //

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

@@ -334,12 +334,20 @@ let out_dir = env::var("OUT_DIR").unwrap();
 * `RUSTC`, `RUSTDOC` — the compiler and documentation generator that Cargo has
                        resolved to use, passed to the build script so it might
                        use it as well.
+* `RUSTC_WRAPPER` — the `rustc` wrapper, if any, that Cargo is using.
+                    See [`build.rustc-wrapper`].
 * `RUSTC_LINKER` — The path to the linker binary that Cargo has resolved to use
                    for the current target, if specified. The linker can be
                    changed by editing `.cargo/config.toml`; see the documentation
                    about [cargo configuration][cargo-config] for more
                    information.
+* `RUSTFLAGS` — the `RUSTFLAGS` that Cargo invokes `rustc` with.
+                See [`build.rustflags`].
 * `CARGO_PKG_<var>` - The package information variables, with the same names and values as are [provided during crate building][variables set for crates].
+* `RUSTC_VERSION` - The version of rustc used by the cargo that invokes
+		    the build script. Its constituent parts are also
+		    available as `CARGO_VERSION_MAJOR`, `_MINOR`, and
+		    `_PATCH`.
 
 [unix-like platforms]: ../../reference/conditional-compilation.html#unix-and-windows
 [windows-like platforms]: ../../reference/conditional-compilation.html#unix-and-windows

tests/testsuite/build_script.rs

@@ -3,8 +3,11 @@
 use cargo_test_support::compare::assert_match_exact;
 use cargo_test_support::paths::CargoPathExt;
 use cargo_test_support::registry::Package;
+use cargo_test_support::tools;
 use cargo_test_support::{basic_manifest, cross_compile, is_coarse_mtime, project};
-use cargo_test_support::{rustc_host, sleep_ms, slow_cpu_multiplier, symlink_supported};
+use cargo_test_support::{
+    rustc_host, rustc_release, sleep_ms, slow_cpu_multiplier, symlink_supported,
+};
 use cargo_util::paths::remove_dir_all;
 use std::env;
 use std::fs;
@@ -79,10 +82,10 @@ fn custom_build_env_vars() {
         )
         .file("bar/src/lib.rs", "pub fn hello() {}");
 
+    let rustc_version = semver::Version::parse(rustc_release()).unwrap();
     let file_content = format!(
         r#"
             use std::env;
-            use std::io::prelude::*;
             use std::path::Path;
 
             fn main() {{
@@ -115,21 +118,107 @@ fn custom_build_env_vars() {
                 let rustdoc = env::var("RUSTDOC").unwrap();
                 assert_eq!(rustdoc, "rustdoc");
 
+                assert!(env::var("RUSTC_WRAPPER").is_err());
+
                 assert!(env::var("RUSTC_LINKER").is_err());
+
+                let rustflags = env::var("RUSTFLAGS").unwrap();
+                assert_eq!(rustflags, "");
+
+                let version = env::var("RUSTC_VERSION").unwrap();
+                assert_eq!(version, "{1}.{2}.{3}", "bad rust version");
+                let version = env::var("RUSTC_VERSION_MAJOR").unwrap();
+                assert_eq!(version, "{1}");
+                let version = env::var("RUSTC_VERSION_MINOR").unwrap();
+                assert_eq!(version, "{2}");
+                let version = env::var("RUSTC_VERSION_PATCH").unwrap();
+                assert_eq!(version, "{3}");
             }}
         "#,
         p.root()
             .join("target")
             .join("debug")
             .join("build")
-            .display()
+            .display(),
+        rustc_version.major,
+        rustc_version.minor,
+        rustc_version.patch,
     );
 
     let p = p.file("bar/build.rs", &file_content).build();
 
     p.cargo("build --features bar_feat").run();
 }
 
+#[cargo_test]
+fn custom_build_env_var_rustflags() {
+    let rustflags = "--cfg=special";
+    let rustflags_alt = "--cfg=notspecial";
+    let p = project()
+        .file(
+            ".cargo/config",
+            &format!(
+                r#"
+                [build]
+                rustflags = ["{}"]
+                "#,
+                rustflags
+            ),
+        )
+        .file(
+            "build.rs",
+            &format!(
+                r#"
+                use std::env;
+
+                fn main() {{
+                    // Static assertion that exactly one of the cfg paths is always taken.
+                    let x;
+                    #[cfg(special)]
+                    {{ assert_eq!(env::var("RUSTFLAGS").unwrap(), "{}"); x = String::new(); }}
+                    #[cfg(notspecial)]
+                    {{ assert_eq!(env::var("RUSTFLAGS").unwrap(), "{}"); x = String::new(); }}
+                    let _ = x;
+                }}
+                "#,
+                rustflags, rustflags_alt,
+            ),
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check").run();
+
+    // RUSTFLAGS overrides build.rustflags, so --cfg=special shouldn't be passed
+    p.cargo("check").env("RUSTFLAGS", rustflags_alt).run();
+}
+
+#[cargo_test]
+fn custom_build_env_var_rustc_wrapper() {
+    let wrapper = tools::echo_wrapper();
+    let p = project()
+        .file(
+            "build.rs",
+            r#"
+            use std::env;
+
+            fn main() {{
+                assert_eq!(
+                    env::var("RUSTC_WRAPPER").unwrap(),
+                    env::var("CARGO_RUSTC_WRAPPER_CHECK").unwrap()
+                );
+            }}
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check")
+        .env("CARGO_BUILD_RUSTC_WRAPPER", &wrapper)
+        .env("CARGO_RUSTC_WRAPPER_CHECK", &wrapper)
+        .run();
+}
+
 #[cargo_test]
 fn custom_build_env_var_rustc_linker() {
     if cross_compile::disabled() {