add SmallVec test

tests/pass/both_borrows/smallvec.rs

@@ -0,0 +1,99 @@
+//! This test represents a core part of `SmallVec`'s `extend_impl`.
+//! What makes it interesting as a test is that it relies on Stacked Borrow's "quirk"
+//! in a fundamental, hard-to-fix-without-full-trees way.
+
+//@revisions: stack tree
+//@[tree]compile-flags: -Zmiri-tree-borrows
+
+use std::marker::PhantomData;
+use std::mem::{ManuallyDrop, MaybeUninit};
+use std::ptr::NonNull;
+
+#[repr(C)]
+pub union RawSmallVec<T, const N: usize> {
+    inline: ManuallyDrop<MaybeUninit<[T; N]>>,
+    heap: (NonNull<T>, usize),
+}
+
+impl<T, const N: usize> RawSmallVec<T, N> {
+    const fn new() -> Self {
+        Self::new_inline(MaybeUninit::uninit())
+    }
+
+    const fn new_inline(inline: MaybeUninit<[T; N]>) -> Self {
+        Self { inline: ManuallyDrop::new(inline) }
+    }
+
+    const fn as_mut_ptr_inline(&mut self) -> *mut T {
+        (unsafe { &raw mut self.inline }) as *mut T
+    }
+
+    const unsafe fn as_mut_ptr_heap(&mut self) -> *mut T {
+        self.heap.0.as_ptr()
+    }
+}
+
+#[repr(transparent)]
+#[derive(Clone, Copy)]
+struct TaggedLen(usize);
+
+impl TaggedLen {
+    pub const fn new(len: usize, on_heap: bool, is_zst: bool) -> Self {
+        if is_zst {
+            debug_assert!(!on_heap);
+            TaggedLen(len)
+        } else {
+            debug_assert!(len < isize::MAX as usize);
+            TaggedLen((len << 1) | on_heap as usize)
+        }
+    }
+
+    pub const fn on_heap(self, is_zst: bool) -> bool {
+        if is_zst { false } else { (self.0 & 1_usize) == 1 }
+    }
+
+    pub const fn value(self, is_zst: bool) -> usize {
+        if is_zst { self.0 } else { self.0 >> 1 }
+    }
+}
+
+#[repr(C)]
+pub struct SmallVec<T, const N: usize> {
+    len: TaggedLen,
+    raw: RawSmallVec<T, N>,
+    _marker: PhantomData<T>,
+}
+
+impl<T, const N: usize> SmallVec<T, N> {
+    pub const fn new() -> SmallVec<T, N> {
+        Self {
+            len: TaggedLen::new(0, false, Self::is_zst()),
+            raw: RawSmallVec::new(),
+            _marker: PhantomData,
+        }
+    }
+
+    const fn is_zst() -> bool {
+        size_of::<T>() == 0
+    }
+
+    pub const fn as_mut_ptr(&mut self) -> *mut T {
+        if self.len.on_heap(Self::is_zst()) {
+            // SAFETY: see above
+            unsafe { self.raw.as_mut_ptr_heap() }
+        } else {
+            self.raw.as_mut_ptr_inline()
+        }
+    }
+
+    pub const fn len(&self) -> usize {
+        self.len.value(Self::is_zst())
+    }
+}
+
+fn main() {
+    let mut v = SmallVec::<i32, 4>::new();
+    let ptr = v.as_mut_ptr();
+    let _len = v.len(); // this call incurs a reborrow which just barely does not invalidate `ptr`
+    unsafe { ptr.write(0) };
+}