Skip to content

Remove impossible panic note from Vec::append #95214

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 1 commit into from
May 28, 2022
Merged

Remove impossible panic note from Vec::append #95214

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
2 changes: 1 addition & 1 deletion library/alloc/src/vec/mod.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1767,7 +1767,7 @@ impl<T, A: Allocator> Vec<T, A> {
///
/// # Panics
///
/// Panics if the number of elements in the vector overflows a `usize`.
/// Panics if the new capacity exceeds `isize::MAX` bytes.
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I think the wording here (and in Vec::push) is confusing because it uses the term capacity which in this case is not related to the Vec::capacity method since for a ZST, Vec::capacity can be bigger than isize::MAX but no actual memory will be allocated:

fn main() {
        let mut vec = vec![(); usize::MAX];
        assert_eq!(vec.len(), usize::MAX);
        assert_eq!(vec.capacity(), usize::MAX);
        assert!(vec.capacity() > usize::try_from(isize::MAX).unwrap());
        vec.append(&mut vec![]);
        assert!(vec.capacity() > usize::try_from(isize::MAX).unwrap());
}

https://play.rust-lang.org/?version=stable&mode=release&edition=2021&gist=b75b2d890ed0f530ca925ff424f4246c

I would rather change both panic notes to use a different terminology like allocated memory with something for ZST that says no memory will be allocated.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Note: It's also not directly related to the Vec::capacity method for other types, it's just for 1-byte large types where it corresponds 1-to-1. E.g. a 2-byte type can at most have isize::MAX / 2 capacity.

What replacement would you suggest?

Panics if the memory to be allocated would exceed isize::MAX bytes.

It seems to me that this would introduce another terminology and would thus make the note harder to understand.

///
/// # Examples
///
Expand Down