Inform build scripts of rustc compiler context

src/cargo/core/compiler/build_context/target_info.rs

@@ -548,6 +548,7 @@ fn output_err_info(cmd: &ProcessBuilder, stdout: &str, stderr: &str) -> String {
 ///
 /// The locations are:
 ///
+///  - the `CARGO_ENCODED_RUSTFLAGS` environment variable
 ///  - the `RUSTFLAGS` environment variable
 ///
 /// then if this was not found
@@ -595,7 +596,16 @@ fn env_args(
         return Ok(Vec::new());
     }
 
-    // First try RUSTFLAGS from the environment
+    // First try CARGO_ENCODED_RUSTFLAGS from the environment.
+    // Prefer this over RUSTFLAGS since it's less prone to encoding errors.
+    if let Ok(a) = env::var(format!("CARGO_ENCODED_{}", name)) {
+        if a.is_empty() {
+            return Ok(Vec::new());
+        }
+        return Ok(a.split('\x1f').map(str::to_string).collect());
+    }
+
+    // Then try RUSTFLAGS from the environment
     if let Ok(a) = env::var(name) {
         let args = a
             .split(' ')

src/cargo/core/compiler/custom_build.rs

@@ -249,6 +249,24 @@ 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_remove("RUSTC_WORKSPACE_WRAPPER");
+    if cx.bcx.ws.is_member(&unit.pkg) {
+        if let Some(wrapper) = bcx.rustc().workspace_wrapper.as_ref() {
+            cmd.env("RUSTC_WORKSPACE_WRAPPER", wrapper);
+        }
+    }
+    cmd.env(
+        "CARGO_ENCODED_RUSTFLAGS",
+        bcx.rustflags_args(unit).join("\x1f"),
+    );
+    cmd.env_remove("RUSTFLAGS");
+
     // 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

@@ -35,11 +35,17 @@ system:
 * `RUSTDOCFLAGS` — A space-separated list of custom flags to pass to all `rustdoc`
   invocations that Cargo performs. In contrast with [`cargo rustdoc`], this is
   useful for passing a flag to *all* `rustdoc` instances. See
-  [`build.rustdocflags`] for some more ways to set flags.
+  [`build.rustdocflags`] for some more ways to set flags. This string is
+  split by whitespace; for a more robust encoding of multiple arguments,
+  set `CARGO_ENCODED_RUSTDOCFLAGS` instead with arguments separated by
+  `0x1f` (ASCII Unit Separator).
 * `RUSTFLAGS` — A space-separated list of custom flags to pass to all compiler
   invocations that Cargo performs. In contrast with [`cargo rustc`], this is
   useful for passing a flag to *all* compiler instances. See
-  [`build.rustflags`] for some more ways to set flags.
+  [`build.rustflags`] for some more ways to set flags. This string is
+  split by whitespace; for a more robust encoding of multiple arguments,
+  set `CARGO_ENCODED_RUSTFLAGS` instead with arguments separated by
+  `0x1f` (ASCII Unit Separator).
 * `CARGO_INCREMENTAL` — If this is set to 1 then Cargo will force [incremental
   compilation] to be enabled for the current compilation, and when set to 0 it
   will force disabling it. If this env var isn't present then cargo's defaults
@@ -334,11 +340,20 @@ 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_WORKSPACE_WRAPPER` — the `rustc` wrapper, if any, that Cargo is
+			      using for workspace members. See
+			      [`build.rustc-workspace-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.
+* `CARGO_ENCODED_RUSTFLAGS` — extra flags that Cargo invokes `rustc`
+			      with, separated by a `0x1f` character
+			      (ASCII Unit Separator). 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].
 
 [unix-like platforms]: ../../reference/conditional-compilation.html#unix-and-windows

tests/testsuite/build_script.rs

@@ -3,6 +3,7 @@
 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_util::paths::remove_dir_all;
@@ -82,7 +83,6 @@ fn custom_build_env_vars() {
     let file_content = format!(
         r#"
             use std::env;
-            use std::io::prelude::*;
             use std::path::Path;
 
             fn main() {{
@@ -115,21 +115,197 @@ 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_WORKSPACE_WRAPPER").is_err());
+
                 assert!(env::var("RUSTC_LINKER").is_err());
+
+                assert!(env::var("RUSTFLAGS").is_err());
+                let rustflags = env::var("CARGO_ENCODED_RUSTFLAGS").unwrap();
+                assert_eq!(rustflags, "");
             }}
         "#,
         p.root()
             .join("target")
             .join("debug")
             .join("build")
-            .display()
+            .display(),
     );
 
     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.
+                    assert!(env::var("RUSTFLAGS").is_err());
+                    let x;
+                    #[cfg(special)]
+                    {{ assert_eq!(env::var("CARGO_ENCODED_RUSTFLAGS").unwrap(), "{}"); x = String::new(); }}
+                    #[cfg(notspecial)]
+                    {{ assert_eq!(env::var("CARGO_ENCODED_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_encoded_rustflags() {
+    // NOTE: We use "-Clink-arg=-B nope" here rather than, say, "-A missing_docs", since for the
+    // latter it won't matter if the whitespace accidentally gets split, as rustc will do the right
+    // thing either way.
+    let p = project()
+        .file(
+            ".cargo/config",
+            r#"
+            [build]
+            rustflags = ["-Clink-arg=-B nope", "--cfg=foo"]
+            "#,
+        )
+        .file(
+            "build.rs",
+            r#"
+                use std::env;
+
+                fn main() {{
+                    assert_eq!(env::var("CARGO_ENCODED_RUSTFLAGS").unwrap(), "-Clink-arg=-B nope\x1f--cfg=foo");
+                }}
+                "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check").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_workspace_wrapper() {
+    let wrapper = tools::echo_wrapper();
+
+    // Workspace wrapper should be set for any crate we're operating directly on.
+    let p = project()
+        .file(
+            "build.rs",
+            r#"
+            use std::env;
+
+            fn main() {{
+                assert_eq!(
+                    env::var("RUSTC_WORKSPACE_WRAPPER").unwrap(),
+                    env::var("CARGO_RUSTC_WORKSPACE_WRAPPER_CHECK").unwrap()
+                );
+            }}
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check")
+        .env("CARGO_BUILD_RUSTC_WORKSPACE_WRAPPER", &wrapper)
+        .env("CARGO_RUSTC_WORKSPACE_WRAPPER_CHECK", &wrapper)
+        .run();
+
+    // But should not be set for a crate from the registry, as then it's not in a workspace.
+    Package::new("bar", "0.1.0")
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "bar"
+            version = "0.1.0"
+            links = "a"
+            "#,
+        )
+        .file(
+            "build.rs",
+            r#"
+            use std::env;
+
+            fn main() {{
+                assert!(env::var("RUSTC_WORKSPACE_WRAPPER").is_err());
+            }}
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .publish();
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.1.0"
+
+            [dependencies]
+            bar = "0.1"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check")
+        .env("CARGO_BUILD_RUSTC_WORKSPACE_WRAPPER", &wrapper)
+        .run();
+}
+
 #[cargo_test]
 fn custom_build_env_var_rustc_linker() {
     if cross_compile::disabled() {