feat: allow const spooled temporary files

Cursors are stable in rust as of 1.79. This change lets us keep a global
scratch file behind a Mutex.

Author: Stebalien

src/spooled.rs

@@ -64,7 +64,7 @@ pub struct SpooledTempFile {
 /// # Ok::<(), std::io::Error>(())
 /// ```
 #[inline]
-pub fn spooled_tempfile(max_size: usize) -> SpooledTempFile {
+pub const fn spooled_tempfile(max_size: usize) -> SpooledTempFile {
     SpooledTempFile::new(max_size)
 }
 
@@ -94,7 +94,7 @@ fn cursor_to_tempfile(cursor: &Cursor<Vec<u8>>, p: &Option<PathBuf>) -> io::Resu
 impl SpooledTempFile {
     /// Construct a new [`SpooledTempFile`].
     #[must_use]
-    pub fn new(max_size: usize) -> SpooledTempFile {
+    pub const fn new(max_size: usize) -> SpooledTempFile {
         SpooledTempFile {
             max_size,
             dir: None,
@@ -247,3 +247,18 @@ impl Seek for SpooledTempFile {
         }
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::SpooledTempFile;
+    use std::io::Write;
+    use std::sync::Mutex;
+
+    // This is fine because `tempfile` doesn't rely on drop.
+    static TEMPFILE: Mutex<SpooledTempFile> = Mutex::new(SpooledTempFile::new(1024));
+
+    #[test]
+    fn test_tempfile() {
+        TEMPFILE.lock().unwrap().write(b"foo").unwrap();
+    }
+}