Add support for CARGO_TARGET_DIR_PREFIX

This change adds support for a new environment variable,
CARGO_TARGET_DIR_PREFIX, to cargo. This variable, when set, is treated
as a prefix to the target directory.
This change addresses rust-lang#5544.

TODO: Definitely not finished. This patch needs more tests and may need
      additional config.toml support (?).

Author: d-e-s-o

src/cargo/util/config.rs

@@ -28,6 +28,7 @@ use core::{CliUnstable, Shell, SourceId, Workspace};
 use ops;
 use url::Url;
 use util::errors::{internal, CargoResult, CargoResultExt};
+use util::important_paths::find_root_manifest_for_wd;
 use util::paths;
 use util::toml as cargo_toml;
 use util::Filesystem;
@@ -290,6 +291,25 @@ impl Config {
             Ok(Some(dir.clone()))
         } else if let Some(dir) = env::var_os("CARGO_TARGET_DIR") {
             Ok(Some(Filesystem::new(self.cwd.join(dir))))
+        } else if let Some(dir) = env::var_os("CARGO_TARGET_DIR_PREFIX") {
+            let prefix = Path::new(&dir);
+            if !prefix.is_absolute() {
+                bail!("CARGO_TARGET_DIR_PREFIX must describe an absolute path");
+            }
+            // We need to take into consideration that `self.cwd` may
+            // point to a directory other than the one containing
+            // Cargo.toml. We definitely want to stay relative to the
+            // latter.
+            let mut cwd = find_root_manifest_for_wd(&self.cwd)?;
+            let result = cwd.pop();
+            assert!(result);
+
+            match cwd.strip_prefix("/") {
+                Ok(cwd) => Ok(Some(Filesystem::new(prefix.join(&cwd).join("target")))),
+                // FIXME: This logic is probably not safe on Windows. Not sure how
+                //        to make a path relative there.
+                Err(_) => bail!("Current directory must be an absolute path"),
+            }
         } else if let Some(val) = self.get_path("build.target-dir")? {
             let val = self.cwd.join(val.val);
             Ok(Some(Filesystem::new(val)))

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

@@ -13,6 +13,10 @@ system:
   checkouts of crates.  By default these are stored under `$HOME/.cargo`, but
   this variable overrides the location of this directory. Once a crate is cached
   it is not removed by the clean command.
+* `CARGO_TARGET_DIR_PREFIX` — Prefix to the location where to place all
+  generated artifacts. The current working directory will be appended to this
+  prefix to form the final path for generated artifacts. Note that
+  `CARGO_TARGET_DIR`, if set, takes precedence over this variable.
 * `CARGO_TARGET_DIR` — Location of where to place all generated artifacts,
   relative to the current working directory.
 * `RUSTC` — Instead of running `rustc`, Cargo will execute this specified

tests/testsuite/build.rs

@@ -3006,6 +3006,63 @@ fn explicit_color_config_is_propagated_to_rustc() {
         ).run();
 }
 
+#[test]
+fn custom_target_dir_prefix() {
+    fn test(cwd: &str) {
+        let tmpdir = tempfile::Builder::new()
+            .tempdir()
+            .unwrap()
+            .path()
+            .to_path_buf();
+
+        let p = project()
+            .file(
+                "Cargo.toml",
+                r#"
+                [package]
+                name = "foo"
+                version = "0.0.1"
+                authors = []
+            "#,
+            )
+            .file("src/main.rs", "fn main() {}")
+            .build();
+
+        let root = p.root();
+        let root_suffix = root.strip_prefix("/").unwrap();
+        let exe_name = format!("foo{}", env::consts::EXE_SUFFIX);
+
+        p.cargo("build")
+            .env("CARGO_TARGET_DIR_PREFIX", tmpdir.clone())
+            .cwd(p.root().join(cwd))
+            .run();
+
+        assert!(
+            tmpdir
+                .clone()
+                .join(root_suffix)
+                .join("target/debug")
+                .join(&exe_name)
+                .is_file()
+        );
+        assert!(!&p.root().join("target/debug").join(&exe_name).is_file());
+
+        p.cargo("build").run();
+        assert!(
+            tmpdir
+                .clone()
+                .join(root_suffix)
+                .join("target/debug")
+                .join(&exe_name)
+                .is_file()
+        );
+        assert!(&p.root().join("target/debug").join(&exe_name).is_file())
+    };
+
+    test(".");
+    test("src");
+}
+
 #[test]
 fn compiler_json_error_format() {
     let p = project()

tests/testsuite/support/mod.rs

@@ -1468,6 +1468,7 @@ fn _process(t: &OsStr) -> cargo::util::ProcessBuilder {
      .env_remove("GIT_AUTHOR_EMAIL")
      .env_remove("GIT_COMMITTER_NAME")
      .env_remove("GIT_COMMITTER_EMAIL")
+     .env_remove("CARGO_TARGET_DIR_PREFIX") // we assume no prefix
      .env_remove("CARGO_TARGET_DIR")     // we assume 'target'
      .env_remove("MSYSTEM"); // assume cmd.exe everywhere on windows
     p