Clear ChunkedBitSet without reallocating

[Zalathar]

There doesn't appear to be any reason to clear a ChunkedBitSet via its constructor (which allocates a new list of chunks), when we could just fill the existing allocation with Chunk::Zeros instead.

For comparison, the insert_all impl added by the same PR (#93984) does the simple thing here and just overwrites every chunk with Chunk::Ones.

(That fill was then made somewhat easier by #145480, which removes the chunk size from all-zero/all-one chunks.)

r? nnethercote (or compiler)

[Zalathar]

This clear method appears to currently be only used in one place (TransferFunction), so I have no idea how hot it is.

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[rust-bors]

⌛ Trying commit 279a5b3 with merge 9d4cb0c…

To cancel the try build, run the command @bors try cancel.

Workflow: https://github.com/rust-lang/rust/actions/runs/18453583260

[nnethercote]

Ha, I wrote almost exactly the same commit earlier today, but I hadn't uploaded it yet:

commit 0418d4a82fd4b675b61afa284a28d0c81196af95
Author: Nicholas Nethercote <n.nethercote@gmail.com>
Date:   Mon Oct 13 08:06:32 2025 +1100

    Improve `ChunkedBitSet::clear`.

    The current code overwrites the existing bitset with a new empty bitset,
    which requires allocation. This PR changes it to just modify the
    existing bitset without needing extra allocations. This makes `clear`
    very similar to `insert_all`, which is nice. The new code is also
    faster, though this doesn't really matter because `clear` isn't hot.

diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
index f7649c5f5c5..7360c6f3fb8 100644
--- a/compiler/rustc_index/src/bit_set.rs
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -584,9 +584,11 @@ pub fn new_filled(domain_size: usize) -> Self {
         ChunkedBitSet::new(domain_size, /* is_empty */ false)
     }

+    /// Sets all bits to false.
     pub fn clear(&mut self) {
-        let domain_size = self.domain_size();
-        *self = ChunkedBitSet::new_empty(domain_size);
+        for chunk in self.chunks.iter_mut() {
+            *chunk = Zeros;
+        }
     }

     #[cfg(test)]

The only difference is I used a for loop.

r=me if you change clear to use a for loop or insert_all to use fill_with -- either way is fine, but it would be nice for them to look the same.

View changes since this review

[Zalathar]

@bors try cancel

[rust-bors]

Try build cancelled. Cancelled workflows:

• https://github.com/rust-lang/rust/actions/runs/18453583260

[nnethercote]

I don't think this needs a perf run.

@bors r+ rollup

[bors]

📌 Commit 9ff52bf has been approved by nnethercote

It is now in the queue for this repository.