test harness informs tests about suitable temp dir

tests/compiletest.rs

@@ -110,6 +110,8 @@ fn get_target() -> String {
 fn test_runner(_tests: &[&()]) {
     // Add a test env var to do environment communication tests.
     std::env::set_var("MIRI_ENV_VAR_TEST", "0");
+    // Let the tests know where to store temp files (they might run for a different target, which can make this hard to find).
+    std::env::set_var("MIRI_TEMP", std::env::temp_dir());
 
     let target = get_target();
     miri_pass("tests/run-pass", &target);

tests/run-pass/fs.rs

@@ -16,19 +16,21 @@ fn main() {
     test_directory();
 }
 
+fn tmp() -> PathBuf {
+    std::env::var("MIRI_TEMP").map(PathBuf::from).unwrap_or_else(|_| std::env::temp_dir())
+}
+
 /// Prepare: compute filename and make sure the file does not exist.
 fn prepare(filename: &str) -> PathBuf {
-    let tmp = std::env::temp_dir();
-    let path = tmp.join(filename);
+    let path = tmp().join(filename);
     // Clean the paths for robustness.
     remove_file(&path).ok();
     path
 }
 
 /// Prepare directory: compute directory name and make sure it does not exist.
 fn prepare_dir(dirname: &str) -> PathBuf {
-    let tmp = std::env::temp_dir();
-    let path = tmp.join(&dirname);
+    let path = tmp().join(&dirname);
     // Clean the directory for robustness.
     remove_dir_all(&path).ok();
     path

tests/run-pass/libc.rs

@@ -2,19 +2,24 @@
 // compile-flags: -Zmiri-disable-isolation
 
 #![feature(rustc_private)]
+#![allow(unused)] // necessary on macos due to conditional compilation
+
+use std::path::PathBuf;
 
-#[allow(unused)] // necessary on macos due to conditional compilation
 extern crate libc;
 
+fn tmp() -> PathBuf {
+    std::env::var("MIRI_TEMP").map(PathBuf::from).unwrap_or_else(|_| std::env::temp_dir())
+}
+
 #[cfg(not(target_os = "macos"))]
 fn test_posix_fadvise() {
     use std::convert::TryInto;
-    use std::env::temp_dir;
     use std::fs::{File, remove_file};
     use std::io::Write;
     use std::os::unix::io::AsRawFd;
 
-    let path = temp_dir().join("miri_test_libc.txt");
+    let path = tmp().join("miri_test_libc.txt");
     // Cleanup before test
     remove_file(&path).ok();