Auto merge of #8982 - ehuss:fix-alt-target-tests, r=alexcrichton

Fix tests not working with a different CARGO_TARGET_DIR.

Fixes #8980

tests/testsuite/standard_lib.rs

@@ -7,7 +7,7 @@
 use cargo_test_support::registry::{Dependency, Package};
 use cargo_test_support::ProjectBuilder;
 use cargo_test_support::{is_nightly, paths, project, rustc_host, Execs};
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 struct Setup {
     rustc_wrapper: PathBuf,
@@ -131,15 +131,7 @@ fn setup() -> Option<Setup> {
 fn enable_build_std(e: &mut Execs, setup: &Setup) {
     // First up, force Cargo to use our "mock sysroot" which mimics what
     // libstd looks like upstream.
-    let root = paths::root();
-    let root = root
-        .parent() // chop off test name
-        .unwrap()
-        .parent() // chop off `citN`
-        .unwrap()
-        .parent() // chop off `target`
-        .unwrap()
-        .join("tests/testsuite/mock-std");
+    let root = Path::new(env!("CARGO_MANIFEST_DIR")).join("tests/testsuite/mock-std");
     e.env("__CARGO_TESTS_ONLY_SRC_ROOT", &root);
 
     e.masquerade_as_nightly_cargo();