feat(trim-paths): emit CARGO_TRIM_PATHS_REMAP for build.rs

src/cargo/core/compiler/custom_build.rs

@@ -399,7 +399,14 @@ fn build_work(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResul
     }
 
     if let Some(trim_paths) = unit.profile.trim_paths.as_ref() {
-        cmd.env("CARGO_TRIM_PATHS", trim_paths.to_string());
+        cmd.env("CARGO_TRIM_PATHS_SCOPE", trim_paths.to_string());
+        if !trim_paths.is_none() {
+            let pairs = super::trim_paths_remap(build_runner, unit);
+            cmd.env(
+                "CARGO_TRIM_PATHS_REMAP",
+                paths::join_paths(&pairs, "CARGO_TRIM_PATHS_REMAP")?,
+            );
+        }
     }
 
     // Be sure to pass along all enabled features for this package, this is the

src/cargo/core/compiler/mod.rs

@@ -1511,11 +1511,11 @@ fn trim_paths_args_rustdoc(
     // feature gate was checked during manifest/config parsing.
     cmd.arg("-Zunstable-options");
 
-    // Order of `--remap-path-prefix` flags is important for `-Zbuild-std`.
-    // We want to show `/rustc/<hash>/library/std` instead of `std-0.0.0`.
-    cmd.arg(package_remap(build_runner, unit));
-    cmd.arg(build_dir_remap(build_runner));
-    cmd.arg(sysroot_remap(build_runner, unit));
+    for pair in trim_paths_remap(build_runner, unit) {
+        let mut arg = OsString::from("--remap-path-prefix=");
+        arg.push(pair);
+        cmd.arg(arg);
+    }
 
     Ok(())
 }
@@ -1538,21 +1538,35 @@ fn trim_paths_args(
     // feature gate was checked during manifest/config parsing.
     cmd.arg(format!("--remap-path-scope={trim_paths}"));
 
-    // Order of `--remap-path-prefix` flags is important for `-Zbuild-std`.
-    // We want to show `/rustc/<hash>/library/std` instead of `std-0.0.0`.
-    cmd.arg(package_remap(build_runner, unit));
-    cmd.arg(build_dir_remap(build_runner));
-    cmd.arg(sysroot_remap(build_runner, unit));
+    for pair in trim_paths_remap(build_runner, unit) {
+        let mut arg = OsString::from("--remap-path-prefix=");
+        arg.push(pair);
+        cmd.arg(arg);
+    }
 
     Ok(())
 }
 
+/// Computes the `<from>=<to>` path remap pairs for [RFC 3127] trim-paths.
+///
+/// Order of `--remap-path-prefix` flags is important for `-Zbuild-std`.
+/// We want to show `/rustc/<hash>/library/std` instead of `std-0.0.0`.
+///
+/// [RFC 3127]: https://rust-lang.github.io/rfcs/3127-trim-paths.html
+pub(crate) fn trim_paths_remap(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> [OsString; 3] {
+    [
+        package_remap(build_runner, unit),
+        build_dir_remap(build_runner),
+        sysroot_remap(build_runner, unit),
+    ]
+}
+
 /// Path prefix remap rules for sysroot.
 ///
 /// This remap logic aligns with rustc:
 /// <https://github.com/rust-lang/rust/blob/c2ef3516/src/bootstrap/src/lib.rs#L1113-L1116>
 fn sysroot_remap(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> OsString {
-    let mut remap = OsString::from("--remap-path-prefix=");
+    let mut remap = OsString::new();
     remap.push({
         // See also `detect_sysroot_src_path()`.
         let mut sysroot = build_runner.bcx.target_data.info(unit.kind).sysroot.clone();
@@ -1582,7 +1596,7 @@ fn sysroot_remap(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> OsString {
 fn package_remap(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> OsString {
     let pkg_root = unit.pkg.root();
     let ws_root = build_runner.bcx.ws.root();
-    let mut remap = OsString::from("--remap-path-prefix=");
+    let mut remap = OsString::new();
     let source_id = unit.pkg.package_id().source_id();
     if source_id.is_git() {
         remap.push(
@@ -1629,7 +1643,7 @@ fn package_remap(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> OsString {
 ///   files (dwp and dwo).
 fn build_dir_remap(build_runner: &BuildRunner<'_, '_>) -> OsString {
     let build_dir = build_runner.bcx.ws.build_dir();
-    let mut remap = OsString::from("--remap-path-prefix=");
+    let mut remap = OsString::new();
     remap.push(build_dir.as_path_unlocked());
     remap.push("=/cargo/build-dir");
     remap

src/doc/src/reference/unstable.md

@@ -1566,14 +1566,20 @@ This will not affect any hard-coded paths in the source code, such as in strings
 
 *as a new entry of ["Environment variables Cargo sets for build scripts"](./environment-variables.md#environment-variables-cargo-sets-for-crates)*
 
-* `CARGO_TRIM_PATHS` --- The value of `trim-paths` profile option.
+* `CARGO_TRIM_PATHS_SCOPE` --- The value of `trim-paths` profile option.
     `false`, `"none"`, and empty arrays would be converted to `none`.
     `true` and `"all"` become `all`.
     Values in a non-empty array would be joined into a comma-separated list.
     If the build script introduces absolute paths to built artifacts (such as by invoking a compiler),
     the user may request them to be sanitized in different types of artifacts.
     Common paths requiring sanitization include `OUT_DIR`, `CARGO_MANIFEST_DIR` and `CARGO_MANIFEST_PATH`,
     plus any other introduced by the build script, such as include directories.
+* `CARGO_TRIM_PATHS_REMAP` --- The `<from>=<to>` path remap pairs Cargo passes to the compiler,
+    joined by the platform path separator.
+    Only set when `trim-paths` profile is active.
+    Build scripts can forward these mappings to C/C++ compilers and other tools,
+    for example via `cc`'s `-ffile-prefix-map`,
+    to sanitize paths consistently with the rest of the build.
 
 ## gc
 

tests/testsuite/profile_trim_paths.rs

@@ -766,10 +766,24 @@ fn custom_build_env_var_trim_paths() {
             &format!(
                 r#"
                 fn main() {{
-                    assert_eq!(
-                        std::env::var("CARGO_TRIM_PATHS").unwrap().as_str(),
-                        "{expected}",
-                    );
+                    let scope = std::env::var("CARGO_TRIM_PATHS_SCOPE").unwrap();
+                    assert_eq!(scope.as_str(), "{expected}");
+
+                    let remap = std::env::var_os("CARGO_TRIM_PATHS_REMAP");
+                    if scope == "none" {{
+                        assert_eq!(remap, None);
+                    }} else {{
+                        let remap = remap.unwrap();
+                        let pairs: Vec<String> = std::env::split_paths(&remap)
+                            .map(|p| p.into_os_string().into_string().unwrap())
+                            .collect();
+                        // package, build-dir, sysroot
+                        assert_eq!(pairs.len(), 3, "remap = {{remap:?}}");
+                        // The package lives at the workspace root, remapped to `.`.
+                        assert!(pairs[0].ends_with("=."), "remap = {{remap:?}}");
+                        assert!(pairs[1].ends_with("=/cargo/build-dir"), "remap = {{remap:?}}");
+                        assert!(pairs[2].contains("=/rustc/"), "remap = {{remap:?}}");
+                    }}
                 }}
                 "#
             ),