Add CARGO_TARGET_TMPDIR env var for integration tests & benches

The variable is set to $target_dir/$config/tmp
This is a directory where tests & benches can place testcasei-related data
for use by the tests.
cargo makes sure the directory exists when building tests/benches.

src/cargo/core/compiler/layout.rs

@@ -125,6 +125,8 @@ pub struct Layout {
     examples: PathBuf,
     /// The directory for rustdoc output: `$root/doc`
     doc: PathBuf,
+    /// The directory for temporary data of integration tests and benches: `$dest/tmp`
+    tmp: PathBuf,
     /// The lockfile for a build (`.cargo-lock`). Will be unlocked when this
     /// struct is `drop`ped.
     _lock: FileLock,
@@ -170,6 +172,7 @@ impl Layout {
             fingerprint: dest.join(".fingerprint"),
             examples: dest.join("examples"),
             doc: root.join("doc"),
+            tmp: dest.join("tmp"),
             root,
             dest,
             _lock: lock,
@@ -219,4 +222,9 @@ impl Layout {
     pub fn build(&self) -> &Path {
         &self.build
     }
+    /// Create and return the tmp path.
+    pub fn prepare_tmp(&self) -> CargoResult<&Path> {
+        paths::create_dir_all(&self.tmp)?;
+        Ok(&self.tmp)
+    }
 }

src/cargo/core/compiler/mod.rs

@@ -581,6 +581,11 @@ fn prepare_rustc(
         base.env("CARGO_PRIMARY_PACKAGE", "1");
     }
 
+    if unit.target.is_test() || unit.target.is_bench() {
+        let tmp = cx.files().layout(unit.kind).prepare_tmp()?;
+        base.env("CARGO_TARGET_TMPDIR", tmp.display().to_string());
+    }
+
     if cx.bcx.config.cli_unstable().jobserver_per_rustc {
         let client = cx.new_jobserver()?;
         base.inherit_jobserver(&client);

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

@@ -221,6 +221,12 @@ corresponding environment variable is set to the empty string, `""`.
   on the current directory and the default workspace members. This environment
   variable will not be set when building dependencies. This is only set when
   compiling the package (not when running binaries or tests).
+* `CARGO_TARGET_TMPDIR` — Only set when building [integration test] or benchmark code.
+  This is a path to a directory inside the target directory
+  where integration tests or benchmarks are free to put any data needed by
+  the tests/benches. Cargo initially creates this directory but doesn't
+  manage its content in any way, this is the responsibility of the test code.
+  There are separate directories for `debug` and `release` profiles.
 
 [integration test]: cargo-targets.md#integration-tests
 [`env` macro]: ../../std/macro.env.html

tests/testsuite/build.rs

@@ -1334,12 +1334,18 @@ fn crate_env_vars() {
                     let s = format!("{}.{}.{}-{}", VERSION_MAJOR,
                                     VERSION_MINOR, VERSION_PATCH, VERSION_PRE);
                     assert_eq!(s, VERSION);
+
+                    // Verify CARGO_TARGET_TMPDIR isn't set for bins
+                    assert!(option_env!("CARGO_TARGET_TMPDIR").is_none());
                 }
             "#,
         )
         .file(
             "src/lib.rs",
             r#"
+                use std::env;
+                use std::path::PathBuf;
+
                 pub fn version() -> String {
                     format!("{}-{}-{} @ {} in {}",
                             env!("CARGO_PKG_VERSION_MAJOR"),
@@ -1348,9 +1354,60 @@ fn crate_env_vars() {
                             env!("CARGO_PKG_VERSION_PRE"),
                             env!("CARGO_MANIFEST_DIR"))
                 }
+
+                pub fn check_no_int_test_env() {
+                    env::var("CARGO_TARGET_DIR").unwrap_err();
+                }
+
+                pub fn check_tmpdir(tmp: Option<&'static str>) {
+                    let tmpdir: PathBuf = tmp.unwrap().into();
+
+                    let exe: PathBuf = env::current_exe().unwrap().into();
+                    let mut expected: PathBuf = exe.parent().unwrap().parent().unwrap().into();
+                    expected.push("tmp");
+                    assert_eq!(tmpdir, expected);
+
+                    // Check that CARGO_TARGET_TMPDIR isn't set for lib code
+                    assert!(option_env!("CARGO_TARGET_TMPDIR").is_none());
+                    env::var("CARGO_TARGET_TMPDIR").unwrap_err();
+                }
+
+                #[test]
+                fn env() {
+                    // Check that CARGO_TARGET_TMPDIR isn't set for unit tests
+                    assert!(option_env!("CARGO_TARGET_TMPDIR").is_none());
+                    env::var("CARGO_TARGET_TMPDIR").unwrap_err();
+                }
             "#,
         )
-        .build();
+        .file(
+            "tests/env.rs",
+            r#"
+                #[test]
+                fn env() {
+                    foo::check_tmpdir(option_env!("CARGO_TARGET_TMPDIR"));
+                }
+            "#,
+        );
+
+    let p = if is_nightly() {
+        p.file(
+            "benches/env.rs",
+            r#"
+                #![feature(test)]
+                extern crate test;
+                use test::Bencher;
+
+                #[bench]
+                fn env(_: &mut Bencher) {
+                    foo::check_tmpdir(option_env!("CARGO_TARGET_TMPDIR"));
+                }
+            "#,
+        )
+        .build()
+    } else {
+        p.build()
+    };
 
     println!("build");
     p.cargo("build -v").run();
@@ -1362,6 +1419,11 @@ fn crate_env_vars() {
 
     println!("test");
     p.cargo("test -v").run();
+
+    if is_nightly() {
+        println!("bench");
+        p.cargo("bench -v").run();
+    }
 }
 
 #[cargo_test]