Make create_dir_all_bare an std integration test

Moving `create_dir_all` out of `ui-fulldeps` is complicated by the fact it sets the current directory. This means it can't be a unit test. Instead, move it to its own integration test.

Author: ChrisDenton

Diff for: library/std/tests/common/mod.rs

@@ -0,0 +1,58 @@
+#![allow(unused)]
+
+use std::env;
+use std::fs;
+use std::path::{Path, PathBuf};
+use std::thread;
+use rand::RngCore;
+
+/// Copied from `std::test_helpers::test_rng`, since these tests rely on the
+/// seed not being the same for every RNG invocation too.
+#[track_caller]
+pub(crate) fn test_rng() -> rand_xorshift::XorShiftRng {
+    use core::hash::{BuildHasher, Hash, Hasher};
+    let mut hasher = std::collections::hash_map::RandomState::new().build_hasher();
+    core::panic::Location::caller().hash(&mut hasher);
+    let hc64 = hasher.finish();
+    let seed_vec = hc64.to_le_bytes().into_iter().chain(0u8..8).collect::<Vec<u8>>();
+    let seed: [u8; 16] = seed_vec.as_slice().try_into().unwrap();
+    rand::SeedableRng::from_seed(seed)
+}
+
+// Copied from std::sys_common::io
+pub struct TempDir(PathBuf);
+
+impl TempDir {
+    pub fn join(&self, path: &str) -> PathBuf {
+        let TempDir(ref p) = *self;
+        p.join(path)
+    }
+
+    pub fn path(&self) -> &Path {
+        let TempDir(ref p) = *self;
+        p
+    }
+}
+
+impl Drop for TempDir {
+    fn drop(&mut self) {
+        // Gee, seeing how we're testing the fs module I sure hope that we
+        // at least implement this correctly!
+        let TempDir(ref p) = *self;
+        let result = fs::remove_dir_all(p);
+        // Avoid panicking while panicking as this causes the process to
+        // immediately abort, without displaying test results.
+        if !thread::panicking() {
+            result.unwrap();
+        }
+    }
+}
+
+#[track_caller] // for `test_rng`
+pub fn tmpdir() -> TempDir {
+    let p = env::temp_dir();
+    let mut r = test_rng();
+    let ret = p.join(&format!("rust-{}", r.next_u32()));
+    fs::create_dir(&ret).unwrap();
+    TempDir(ret)
+}

Diff for: library/std/tests/create_dir_all_bare.rs

@@ -0,0 +1,37 @@
+//! Note that this test changes the current directory so
+//! should not be in the same process as other tests.
+use std::env;
+use std::fs;
+use std::path::{Path, PathBuf};
+
+mod common;
+
+// On some platforms, setting the current directory will prevent deleting it.
+// So this helper ensures the current directory is reset.
+struct CurrentDir(PathBuf);
+impl CurrentDir {
+    fn new() -> Self {
+        Self(env::current_dir().unwrap())
+    }
+    fn set(&self, path: &Path) {
+        env::set_current_dir(path).unwrap();
+    }
+    fn with(path: &Path, f: impl FnOnce()) {
+        let current_dir = Self::new();
+        current_dir.set(path);
+        f();
+    }
+}
+impl Drop for CurrentDir {
+    fn drop(&mut self) {
+        env::set_current_dir(&self.0).unwrap();
+    }
+}
+
+#[test]
+fn create_dir_all_bare() {
+    let tmpdir = common::tmpdir();
+    CurrentDir::with(tmpdir.path(), || {
+        fs::create_dir_all("create-dir-all-bare").unwrap();
+    });
+}

Diff for: library/std/tests/env.rs

@@ -3,18 +3,8 @@ use std::ffi::{OsStr, OsString};
 
 use rand::distributions::{Alphanumeric, DistString};
 
-/// Copied from `std::test_helpers::test_rng`, since these tests rely on the
-/// seed not being the same for every RNG invocation too.
-#[track_caller]
-pub(crate) fn test_rng() -> rand_xorshift::XorShiftRng {
-    use core::hash::{BuildHasher, Hash, Hasher};
-    let mut hasher = std::collections::hash_map::RandomState::new().build_hasher();
-    core::panic::Location::caller().hash(&mut hasher);
-    let hc64 = hasher.finish();
-    let seed_vec = hc64.to_le_bytes().into_iter().chain(0u8..8).collect::<Vec<u8>>();
-    let seed: [u8; 16] = seed_vec.as_slice().try_into().unwrap();
-    rand::SeedableRng::from_seed(seed)
-}
+mod common;
+use common::test_rng;
 
 #[track_caller]
 fn make_rand_name() -> OsString {