fix: avoid panicing on platforms with no sane default tempdir

fixes #365

Author: Stebalien

src/env.rs

@@ -1,12 +1,18 @@
-use std::env;
 use std::ffi::{OsStr, OsString};
 use std::path::{Path, PathBuf};
 use std::sync::{LazyLock, OnceLock};
+use std::{env, io};
 
 #[cfg(doc)]
 use crate::{Builder, tempdir_in, tempfile_in};
 
-static ENV_TEMPDIR: LazyLock<PathBuf> = LazyLock::new(env::temp_dir);
+static ENV_TEMPDIR: LazyLock<Option<PathBuf>> =
+    // Only call env::temp_dir() on platforms known to not panic.
+    LazyLock::new(if cfg!(any(unix, windows, target_os = "hermit")) {
+            || Some(env::temp_dir())
+        } else {
+            || None
+        });
 static DEFAULT_TEMPDIR: OnceLock<PathBuf> = OnceLock::new();
 static DEFAULT_PREFIX: OnceLock<OsString> = OnceLock::new();
 
@@ -36,16 +42,22 @@ pub fn override_temp_dir(path: impl Into<PathBuf>) -> Result<&'static Path, &'st
 /// Returns the default temporary directory, used for both temporary directories and files if no
 /// directory is explicitly specified.
 ///
-/// This function simply delegates to [`std::env::temp_dir`] unless the default temporary directory
-/// has been overridden by a call to [`override_temp_dir`].
+/// Unless the default temporary directory has been overridden by a call to [`override_temp_dir`],
+/// this function delegates to [`std::env::temp_dir`] (when supported by the platform). It returns
+/// an error on platforms with no default temporary directory (e.g., WASI/WASM) unless
+/// [`override_temp_dir`] has already been called to set the temporary directory.
 ///
 /// **NOTE:**
 ///
 /// 1. This function does not check if the returned directory exists and/or is writable.
 /// 2. This function caches the result of [`std::env::temp_dir`]. Any future changes to, e.g., the
 ///    `TMPDIR` environment variable won't have any effect.
-pub fn temp_dir() -> &'static Path {
-    DEFAULT_TEMPDIR.get().unwrap_or_else(|| &ENV_TEMPDIR)
+pub fn temp_dir() -> io::Result<&'static Path> {
+    DEFAULT_TEMPDIR
+        .get()
+        .or_else(|| ENV_TEMPDIR.as_ref())
+        .map(|a| &**a)
+        .ok_or_else(|| io::Error::other("no temporary directory configured"))
 }
 
 /// Override the default prefix for new temporary files (defaults to "tmp"). This function changes

src/file/mod.rs

@@ -49,7 +49,7 @@ mod imp;
 /// # Ok::<(), std::io::Error>(())
 /// ```
 pub fn tempfile() -> io::Result<File> {
-    tempfile_in(env::temp_dir())
+    tempfile_in(env::temp_dir()?)
 }
 
 /// Create a new temporary file in the specified directory. Also see [`tempfile`].

src/lib.rs

@@ -491,7 +491,7 @@ impl<'a> Builder<'a> {
     /// [security]: struct.NamedTempFile.html#security
     /// [resource-leaking]: struct.NamedTempFile.html#resource-leaking
     pub fn tempfile(&self) -> io::Result<NamedTempFile> {
-        self.tempfile_in(env::temp_dir())
+        self.tempfile_in(env::temp_dir()?)
     }
 
     /// Create the named temporary file in the specified directory.
@@ -560,7 +560,7 @@ impl<'a> Builder<'a> {
     ///
     /// [resource-leaking]: struct.TempDir.html#resource-leaking
     pub fn tempdir(&self) -> io::Result<TempDir> {
-        self.tempdir_in(env::temp_dir())
+        self.tempdir_in(env::temp_dir()?)
     }
 
     /// Attempts to make a temporary directory inside of `dir`.
@@ -684,7 +684,7 @@ impl<'a> Builder<'a> {
     where
         F: FnMut(&MakeParams<'_>) -> io::Result<R>,
     {
-        self.make_in(env::temp_dir(), f)
+        self.make_in(env::temp_dir()?, f)
     }
 
     /// This is the same as [`Builder::make`], except `dir` is used as the base

tests/env.rs

@@ -5,11 +5,11 @@ use std::path::Path;
 #[test]
 fn test_override_temp_dir() {
     #[cfg(not(target_os = "wasi"))]
-    assert_eq!(tempfile::env::temp_dir(), std::env::temp_dir());
+    assert_eq!(tempfile::env::temp_dir().unwrap(), std::env::temp_dir());
 
     let new_tmp = Path::new("/tmp/override");
     tempfile::env::override_temp_dir(new_tmp).unwrap();
-    assert_eq!(tempfile::env::temp_dir(), new_tmp);
+    assert_eq!(tempfile::env::temp_dir().unwrap(), new_tmp);
 
     let new_tmp2 = Path::new("/tmp/override2");
     tempfile::env::override_temp_dir(new_tmp2).expect_err("override should only be possible once");

tests/namedtempfile.rs

@@ -85,7 +85,7 @@ fn test_persist() {
 
     let mut tmpfile = NamedTempFile::new().unwrap();
     let old_path = tmpfile.path().to_path_buf();
-    let persist_path = env::temp_dir().join("persisted_temporary_file");
+    let persist_path = env::temp_dir().unwrap().join("persisted_temporary_file");
     write!(tmpfile, "abcde").unwrap();
     {
         assert!(exists(&old_path));
@@ -233,7 +233,7 @@ fn test_temppath_persist() {
     let tmppath = tmpfile.into_temp_path();
 
     let old_path = tmppath.to_path_buf();
-    let persist_path = env::temp_dir().join("persisted_temppath_file");
+    let persist_path = env::temp_dir().unwrap().join("persisted_temppath_file");
 
     {
         assert!(exists(&old_path));

tests/spooled.rs

@@ -43,7 +43,7 @@ fn test_custom_dir() {
     configure_wasi_temp_dir();
 
     {
-        let mut t = spooled_tempfile_in(10, env::temp_dir());
+        let mut t = spooled_tempfile_in(10, env::temp_dir().unwrap());
         t.roll()
             .expect("failed to roll temp file in a specified directory");
     }