Rollup merge of #127446 - zachs18:miri-stdlib-leaks-core-alloc, r=Mark-Simulacrum

Remove memory leaks in doctests in `core`, `alloc`, and `std`

cc `@RalfJung`  https://github.com/rust-lang/rust/issues/126067 https://github.com/rust-lang/miri/issues/3670

Should be no actual *documentation* changes[^1], all added/modified lines in the doctests are hidden with `#`,

This PR splits the existing memory leaks in doctests in `core`, `alloc`, and `std` into two general categories:

1. "Non-focused" memory leaks that are incidental to the thing being documented, and/or are easy to remove, i.e. they are only there because preventing the leak would make the doctest less clear and/or concise.
    - These doctests simply have a comment like `# // Prevent leaks for Miri.` above the added line that removes the memory leak.
    - [^2]Some of these would perhaps be better as part of the public documentation part of the doctest, to clarify that a memory leak can happen if it is not otherwise mentioned explicitly in the documentation  (specifically the ones in `(A)Rc::increment_strong_count(_in)`).
2. "Focused" memory leaks that are intentional and documented, and/or are possibly fragile to remove.
    - These doctests have a `# // FIXME` comment above the line that removes the memory leak, with a note that once `-Zmiri-disable-leak-check` can be applied at test granularity, these tests should be "un-unleakified" and have `-Zmiri-disable-leak-check` enabled.
    - Some of these are possibly fragile (e.g. unleaking the result of `Vec::leak`) and thus should definitely not be made part of the documentation.

This should be all of the leaks currently in `core` and `alloc`. I only found one leak in `std`, and it was in the first category (excluding the modules `@RalfJung` mentioned in https://github.com/rust-lang/rust/issues/126067 , and reducing the number of iterations of [one test](https://github.com/rust-lang/rust/blob/master/library/std/src/sync/once_lock.rs#L49-L94) from 1000 to 10)

[^1]: assuming [^2] is not added
[^2]: backlink

6 files changed, 33 insertions, 1 deletions

diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index 65bcb241e4a..f299aa0124d 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -1213,6 +1213,9 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// let static_ref: &'static mut usize = Box::leak(x);
     /// *static_ref += 1;
     /// assert_eq!(*static_ref, 42);
+    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
+    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
+    /// # drop(unsafe { Box::from_raw(static_ref) });
     /// ```
     ///
     /// Unsized data:
@@ -1222,6 +1225,9 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// let static_ref = Box::leak(x);
     /// static_ref[0] = 4;
     /// assert_eq!(*static_ref, [4, 2, 3]);
+    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
+    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
+    /// # drop(unsafe { Box::from_raw(static_ref) });
     /// ```
     #[stable(feature = "box_leak", since = "1.26.0")]
     #[inline]
diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index aa8dd9f79c3..9982c8ea6dc 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -1277,6 +1277,8 @@ impl<T: ?Sized> Rc<T> {
     ///
     ///     let five = Rc::from_raw(ptr);
     ///     assert_eq!(2, Rc::strong_count(&five));
+    /// #   // Prevent leaks for Miri.
+    /// #   Rc::decrement_strong_count(ptr);
     /// }
     /// ```
     #[inline]
@@ -1345,6 +1347,8 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
     /// let x = Rc::new("hello".to_owned());
     /// let x_ptr = Rc::into_raw(x);
     /// assert_eq!(unsafe { &*x_ptr }, "hello");
+    /// # // Prevent leaks for Miri.
+    /// # drop(unsafe { Rc::from_raw(x_ptr) });
     /// ```
     #[must_use = "losing the pointer will leak memory"]
     #[stable(feature = "rc_raw", since = "1.17.0")]
@@ -1572,6 +1576,8 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
     ///
     ///     let five = Rc::from_raw_in(ptr, System);
     ///     assert_eq!(2, Rc::strong_count(&five));
+    /// #   // Prevent leaks for Miri.
+    /// #   Rc::decrement_strong_count_in(ptr, System);
     /// }
     /// ```
     #[inline]
diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index 36078da7c35..07ffd3e1519 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -1984,6 +1984,9 @@ impl String {
     /// let x = String::from("bucket");
     /// let static_ref: &'static mut str = x.leak();
     /// assert_eq!(static_ref, "bucket");
+    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
+    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
+    /// # drop(unsafe { Box::from_raw(static_ref) });
     /// ```
     #[stable(feature = "string_leak", since = "1.72.0")]
     #[inline]
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index 57ac20ba323..a905a1e6b7e 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -1414,6 +1414,8 @@ impl<T: ?Sized> Arc<T> {
     ///     // the `Arc` between threads.
     ///     let five = Arc::from_raw(ptr);
     ///     assert_eq!(2, Arc::strong_count(&five));
+    /// #   // Prevent leaks for Miri.
+    /// #   Arc::decrement_strong_count(ptr);
     /// }
     /// ```
     #[inline]
@@ -1484,6 +1486,8 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
     /// let x = Arc::new("hello".to_owned());
     /// let x_ptr = Arc::into_raw(x);
     /// assert_eq!(unsafe { &*x_ptr }, "hello");
+    /// # // Prevent leaks for Miri.
+    /// # drop(unsafe { Arc::from_raw(x_ptr) });
     /// ```
     #[must_use = "losing the pointer will leak memory"]
     #[stable(feature = "rc_raw", since = "1.17.0")]
@@ -1766,6 +1770,8 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
     ///     // the `Arc` between threads.
     ///     let five = Arc::from_raw_in(ptr, System);
     ///     assert_eq!(2, Arc::strong_count(&five));
+    /// #   // Prevent leaks for Miri.
+    /// #   Arc::decrement_strong_count_in(ptr, System);
     /// }
     /// ```
     #[inline]
diff --git a/library/alloc/src/vec/into_iter.rs b/library/alloc/src/vec/into_iter.rs
index 3bd89eaa6cb..10f62e4bb62 100644
--- a/library/alloc/src/vec/into_iter.rs
+++ b/library/alloc/src/vec/into_iter.rs
@@ -120,10 +120,15 @@ impl<T, A: Allocator> IntoIter<T, A> {
     /// This is roughly equivalent to the following, but more efficient
     ///
     /// ```
-    /// # let mut into_iter = Vec::<u8>::with_capacity(10).into_iter();
+    /// # let mut vec = Vec::<u8>::with_capacity(10);
+    /// # let ptr = vec.as_mut_ptr();
+    /// # let mut into_iter = vec.into_iter();
     /// let mut into_iter = std::mem::replace(&mut into_iter, Vec::new().into_iter());
     /// (&mut into_iter).for_each(drop);
     /// std::mem::forget(into_iter);
+    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
+    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
+    /// # drop(unsafe { Vec::<u8>::from_raw_parts(ptr, 0, 10) });
     /// ```
     ///
     /// This method is used by in-place iteration, refer to the vec::in_place_collect
diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index 6e9b017ad75..729d5dd4fe4 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -1473,6 +1473,9 @@ impl<T, A: Allocator> Vec<T, A> {
     /// // 2. `0 <= capacity` always holds whatever `capacity` is.
     /// unsafe {
     ///     vec.set_len(0);
+    /// #   // FIXME(https://github.com/rust-lang/miri/issues/3670):
+    /// #   // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
+    /// #   vec.set_len(3);
     /// }
     /// ```
     ///
@@ -2391,6 +2394,9 @@ impl<T, A: Allocator> Vec<T, A> {
     /// let static_ref: &'static mut [usize] = x.leak();
     /// static_ref[0] += 1;
     /// assert_eq!(static_ref, &[2, 2, 3]);
+    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
+    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
+    /// # drop(unsafe { Box::from_raw(static_ref) });
     /// ```
     #[stable(feature = "vec_leak", since = "1.47.0")]
     #[inline]