Fix in-place collect not reallocating when necessary

library/alloc/src/vec/in_place_collect.rs

@@ -168,7 +168,7 @@ const fn in_place_collectible<DEST, SRC>(
     step_merge: Option<NonZeroUsize>,
     step_expand: Option<NonZeroUsize>,
 ) -> bool {
-    if DEST::IS_ZST || mem::align_of::<SRC>() < mem::align_of::<DEST>() {
+    if const { SRC::IS_ZST || DEST::IS_ZST || mem::align_of::<SRC>() < mem::align_of::<DEST>() } {
         return false;
     }
 
@@ -186,6 +186,32 @@ const fn in_place_collectible<DEST, SRC>(
     }
 }
 
+const fn needs_realloc<SRC, DEST>(src_cap: usize, dst_cap: usize) -> bool {
+    if const { mem::align_of::<SRC>() != mem::align_of::<DEST>() } {
+        return true;
+    }
+
+    if const {
+        // examples that may require reallocs unless src/dest capacities turn out to be multiples at runtime
+        // Vec<u8> -> Vec<[u8; 4]>
+        let dst_larger = mem::size_of::<SRC>() < mem::size_of::<DEST>();
+
+        // Vec<[u8; 3]> -> Vec<[u8; 2]>
+        // dest_sz can't actually be 0 since in_place_collectible() checks for that but const eval doesn't know that
+        let dest_sz = mem::size_of::<DEST>();
+        let src_not_multiple_of_dest =
+            dest_sz == 0 || mem::size_of::<SRC>() % mem::size_of::<DEST>() != 0;
+
+        dst_larger || src_not_multiple_of_dest
+    } {
+        return src_cap * mem::size_of::<SRC>() != dst_cap * mem::size_of::<DEST>();
+    }
+
+    // Equal size + alignment won't need a realloc.
+    // src size being an integer multiple of the dest size works too
+    return false;
+}
+
 /// This provides a shorthand for the source type since local type aliases aren't a thing.
 #[rustc_specialization_trait]
 trait InPlaceCollect: SourceIter<Source: AsVecIntoIter> + InPlaceIterable {
@@ -259,12 +285,7 @@ where
         // that wasn't a multiple of the destination type size.
         // Since the discrepancy should generally be small this should only result in some
         // bookkeeping updates and no memmove.
-        if (const {
-            let src_sz = mem::size_of::<I::Src>();
-            src_sz > 0 && mem::size_of::<T>() % src_sz != 0
-        } && src_cap * mem::size_of::<I::Src>() != dst_cap * mem::size_of::<T>())
-            || const { mem::align_of::<T>() != mem::align_of::<I::Src>() }
-        {
+        if needs_realloc::<I::Src, T>(src_cap, dst_cap) {
             let alloc = Global;
             unsafe {
                 // The old allocation exists, therefore it must have a valid layout.
@@ -286,6 +307,8 @@ where
                 let Ok(reallocated) = result else { handle_alloc_error(new_layout) };
                 dst_buf = reallocated.as_ptr() as *mut T;
             }
+        } else {
+            debug_assert_eq!(src_cap * mem::size_of::<I::Src>(), dst_cap * mem::size_of::<T>());
         }
 
         mem::forget(dst_guard);

library/alloc/tests/vec.rs

@@ -1213,6 +1213,14 @@ fn test_in_place_specialization_step_up_down() {
     assert_ne!(src_bytes, sink_bytes);
     assert_eq!(sink.len(), 2);
 
+    let mut src: Vec<[u8; 3]> = Vec::with_capacity(17);
+    src.resize( 8, [0; 3]);
+    let iter = src.into_iter().map(|[a, b, _]| [a, b]);
+    assert_in_place_trait(&iter);
+    let sink: Vec<[u8; 2]> = iter.collect();
+    assert_eq!(sink.len(), 8);
+    assert!(sink.capacity() <= 25);
+
     let src = vec![[0u8; 4]; 256];
     let srcptr = src.as_ptr();
     let iter = src