Remove hidden foo functions from doc examples; use Termination trait.

Fixes #49233.

Author: frewsxcv

src/libstd/env.rs

@@ -552,17 +552,17 @@ pub fn home_dir() -> Option<PathBuf> {
 ///
 /// [msdn]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa364992(v=vs.85).aspx
 ///
-/// ```
+/// ```no_run
 /// use std::env;
 /// use std::fs::File;
 ///
-/// # fn foo() -> std::io::Result<()> {
-/// let mut dir = env::temp_dir();
-/// dir.push("foo.txt");
+/// fn main() -> std::io::Result<()> {
+///     let mut dir = env::temp_dir();
+///     dir.push("foo.txt");
 ///
-/// let f = File::create(dir)?;
-/// # Ok(())
-/// # }
+///     let f = File::create(dir)?;
+///     Ok(())
+/// }
 /// ```
 #[stable(feature = "env", since = "1.0.0")]
 pub fn temp_dir() -> PathBuf {