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

library/alloc/src/boxed.rs

@@ -1213,6 +1213,9 @@ pub const fn allocator(b: &Self) -> &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 @@ pub const fn allocator(b: &Self) -> &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]

library/alloc/src/rc.rs

@@ -1277,6 +1277,8 @@ pub unsafe fn from_raw(ptr: *const T) -> Self {
     ///
     ///     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 @@ pub fn allocator(this: &Self) -> &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 @@ pub fn strong_count(this: &Self) -> usize {
     ///
     ///     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]

library/alloc/src/string.rs

@@ -1984,6 +1984,9 @@ pub fn into_boxed_str(self) -> Box<str> {
     /// 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]

library/alloc/src/sync.rs

@@ -1414,6 +1414,8 @@ pub unsafe fn from_raw(ptr: *const T) -> Self {
     ///     // 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 @@ pub fn allocator(this: &Self) -> &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 @@ pub fn strong_count(this: &Self) -> usize {
     ///     // 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]

library/alloc/src/vec/into_iter.rs

@@ -120,10 +120,15 @@ fn as_raw_mut_slice(&mut self) -> *mut [T] {
     /// 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

library/alloc/src/vec/mod.rs

@@ -1473,6 +1473,9 @@ pub fn allocator(&self) -> &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 @@ pub fn resize_with<F>(&mut self, new_len: usize, f: F)
     /// 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]

library/core/src/mem/manually_drop.rs

@@ -62,6 +62,9 @@ impl<T> ManuallyDrop<T> {
     /// x.truncate(5); // You can still safely operate on the value
     /// assert_eq!(*x, "Hello");
     /// // But `Drop` will not be run here
+    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
+    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
+    /// # let _ = ManuallyDrop::into_inner(x);
     /// ```
     #[must_use = "if you don't need the wrapper, you can use `mem::forget` instead"]
     #[stable(feature = "manually_drop", since = "1.20.0")]

library/core/src/mem/maybe_uninit.rs

@@ -274,6 +274,8 @@ impl<T> MaybeUninit<T> {
     /// use std::mem::MaybeUninit;
     ///
     /// let v: MaybeUninit<Vec<u8>> = MaybeUninit::new(vec![42]);
+    /// # // Prevent leaks for Miri
+    /// # unsafe { let _ = MaybeUninit::assume_init(v); }
     /// ```
     ///
     /// [`assume_init`]: MaybeUninit::assume_init
@@ -446,6 +448,9 @@ pub const fn zeroed() -> MaybeUninit<T> {
     /// let mut x = MaybeUninit::<String>::uninit();
     ///
     /// x.write("Hello".to_string());
+    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
+    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
+    /// # unsafe { MaybeUninit::assume_init_drop(&mut x); }
     /// // This leaks the contained string:
     /// x.write("hello".to_string());
     /// // x is initialized now:
@@ -506,6 +511,8 @@ pub const fn write(&mut self, val: T) -> &mut T {
     /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
     /// let x_vec = unsafe { &*x.as_ptr() };
     /// assert_eq!(x_vec.len(), 3);
+    /// # // Prevent leaks for Miri
+    /// # unsafe { MaybeUninit::assume_init_drop(&mut x); }
     /// ```
     ///
     /// *Incorrect* usage of this method:
@@ -545,6 +552,8 @@ pub const fn as_ptr(&self) -> *const T {
     /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
     /// x_vec.push(3);
     /// assert_eq!(x_vec.len(), 4);
+    /// # // Prevent leaks for Miri
+    /// # unsafe { MaybeUninit::assume_init_drop(&mut x); }
     /// ```
     ///
     /// *Incorrect* usage of this method:
@@ -746,6 +755,8 @@ pub unsafe fn assume_init_drop(&mut self) {
     /// use std::mem::MaybeUninit;
     ///
     /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
+    /// # let mut x_mu = x;
+    /// # let mut x = &mut x_mu;
     /// // Initialize `x`:
     /// x.write(vec![1, 2, 3]);
     /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to
@@ -755,6 +766,8 @@ pub unsafe fn assume_init_drop(&mut self) {
     ///     x.assume_init_ref()
     /// };
     /// assert_eq!(x, &vec![1, 2, 3]);
+    /// # // Prevent leaks for Miri
+    /// # unsafe { MaybeUninit::assume_init_drop(&mut x_mu); }
     /// ```
     ///
     /// ### *Incorrect* usages of this method:
@@ -1088,6 +1101,8 @@ pub fn copy_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut
     /// let init = MaybeUninit::clone_from_slice(&mut dst, &src);
     ///
     /// assert_eq!(init, src);
+    /// # // Prevent leaks for Miri
+    /// # unsafe { std::ptr::drop_in_place(init); }
     /// ```
     ///
     /// ```

library/core/src/ptr/non_null.rs

@@ -1663,6 +1663,8 @@ pub const fn as_mut_ptr(self) -> *mut T {
     /// // Note that calling `memory.as_mut()` is not allowed here as the content may be uninitialized.
     /// # #[allow(unused_variables)]
     /// let slice: &mut [MaybeUninit<u8>] = unsafe { memory.as_uninit_slice_mut() };
+    /// # // Prevent leaks for Miri.
+    /// # unsafe { Global.deallocate(memory.cast(), Layout::new::<[u8; 32]>()); }
     /// # Ok::<_, std::alloc::AllocError>(())
     /// ```
     #[inline]

library/std/src/sync/condvar.rs

@@ -35,6 +35,7 @@ impl WaitTimeoutResult {
     /// let pair = Arc::new((Mutex::new(false), Condvar::new()));
     /// let pair2 = Arc::clone(&pair);
     ///
+    /// # let handle =
     /// thread::spawn(move || {
     ///     let (lock, cvar) = &*pair2;
     ///
@@ -58,6 +59,8 @@ impl WaitTimeoutResult {
     ///         break
     ///     }
     /// }
+    /// # // Prevent leaks for Miri.
+    /// # let _ = handle.join();
     /// ```
     #[must_use]
     #[stable(feature = "wait_timeout", since = "1.5.0")]

src/bootstrap/mk/Makefile.in

@@ -58,9 +58,8 @@ check-aux:
 		library/core \
 		library/alloc \
 		--no-doc
-	# Some doctests have intentional memory leaks.
-	# Some use file system operations to demonstrate dealing with `Result`.
-	$(Q)MIRIFLAGS="-Zmiri-ignore-leaks -Zmiri-disable-isolation" \
+	# Some doctests use file system operations to demonstrate dealing with `Result`.
+	$(Q)MIRIFLAGS="-Zmiri-disable-isolation" \
 		$(BOOTSTRAP) miri --stage 2 \
 		library/core \
 		library/alloc \
@@ -70,7 +69,7 @@ check-aux:
 		$(BOOTSTRAP) miri --stage 2 library/std \
 		--no-doc -- \
 		--skip fs:: --skip net:: --skip process:: --skip sys::pal::
-	$(Q)MIRIFLAGS="-Zmiri-ignore-leaks -Zmiri-disable-isolation" \
+	$(Q)MIRIFLAGS="-Zmiri-disable-isolation" \
 		$(BOOTSTRAP) miri --stage 2 library/std \
 		--doc -- \
 		--skip fs:: --skip net:: --skip process:: --skip sys::pal::