Auto merge of #277 - saethlin:fix-aliasing, r=mbrubeck

Fix all problems encounted with Miri -Ztag-raw-pointers

I poked at this crate with `-Zmiri-tag-raw-pointers` before, and I was unable to fix what I found (I just added a test case that ruled out one of my wrong ideas #271). I tried again just now and I guess I just understand better this time.

This PR fixes 3 separate pointer invalidation problems, which are detected by running `MIRIFLAGS=-Zmiri-tag-raw-pointers cargo miri test`.

Depending on how you squint, 2 or 3 of these are rust-lang/unsafe-code-guidelines#133. The last one is _probably_ still present even with late invalidation, because `set_len` does a write through a `&mut`.

It's unclear to me if any of these things that Miri complains about are potentially a miscompilation in rustc due to the use of LLVM `noalias`. But perhaps given how subtle this codebase is overall, it would be best to run the tools on their pickiest settings, even if there are a few things more like a false positive than a real problem.

Author: bors-servo

.github/workflows/main.yml

@@ -61,6 +61,8 @@ jobs:
       - name: miri
         if: matrix.toolchain == 'nightly'
         run: bash ./scripts/run_miri.sh
+        env:
+          MIRIFLAGS: '-Zmiri-tag-raw-pointers'
 
       - name: fuzz
         if: env.DO_FUZZ == '1'

src/lib.rs

@@ -392,8 +392,11 @@ impl<'a, T: 'a + Array> Drop for Drain<'a, T> {
                 let start = source_vec.len();
                 let tail = self.tail_start;
                 if tail != start {
-                    let src = source_vec.as_ptr().add(tail);
-                    let dst = source_vec.as_mut_ptr().add(start);
+                    // as_mut_ptr creates a &mut, invalidating other pointers.
+                    // This pattern avoids calling it with a pointer already present.
+                    let ptr = source_vec.as_mut_ptr();
+                    let src = ptr.add(tail);
+                    let dst = ptr.add(start);
                     ptr::copy(src, dst, self.tail_len);
                 }
                 source_vec.set_len(start + self.tail_len);
@@ -813,13 +816,14 @@ impl<A: Array> SmallVec<A> {
         unsafe {
             self.set_len(start);
 
-            let range_slice = slice::from_raw_parts_mut(self.as_mut_ptr().add(start), end - start);
+            let range_slice = slice::from_raw_parts(self.as_ptr().add(start), end - start);
 
             Drain {
                 tail_start: end,
                 tail_len: len - end,
                 iter: range_slice.iter(),
-                vec: NonNull::from(self),
+                // Since self is a &mut, passing it to a function would invalidate the slice iterator.
+                vec: NonNull::new_unchecked(self as *mut _),
             }
         }
     }
@@ -1112,6 +1116,10 @@ impl<A: Array> SmallVec<A> {
                 len: old_len + lower_size_bound,
             };
 
+            // The set_len above invalidates the previous pointers, so we must re-create them.
+            let start = self.as_mut_ptr();
+            let ptr = start.add(index);
+
             while num_added < lower_size_bound {
                 let element = match iter.next() {
                     Some(x) => x,