Infallible Vec operations

[robin-nitrokey]

Some operations on a Vec<T, N> are infallible but can only be implemented using fallible methods. What do you think about adding more infallible methods? For example:

• from_array(data: [T; N]): construct a Vec from an array with the same capacity (would also be useful for constants)
• fill(&mut self, value: T): resize to capacity and clone value to empty slots
• fill_default(&mut self): resize to capacity and write T::default() to empty slots
• replace(&mut self, other: &Self): clear this Vec and copy the data from other

[reitermarkus]

from_array is now implemented in https://github.com/rust-embedded/heapless/pull/352.

fill already exists on slice, i.e. via DerefMut, same with fill_with(Default::default) instead of fill_default. These only fill up to the current length, though. What is the use case for the fill methods filling up to capacity? Is it covered now with .spare_capacity_mut().fill(…)?

For replace, I assume this should work only for Vec with the same capacity, otherwise it would still need to be fallible. Again, what is the use case for this method?

[robin-nitrokey]

Good to see from_array being added! Unfortunately, the implementation is not const. It would still be useful to be able to create non-empty constants.

My use case for fill is preparing a buffer for functions that expect a &mut [u8] for writing. Currently, you have to call v.resize_default(v.capacity()) which is fallible. spare_capacity_mut().fill(MaybeUninit::new(...)) should work but is not really intuitive.

For replace, I don’t remember the specific use case. I’ll report back if I find it again.

[reitermarkus]

@robin-nitrokey, can you give a full example on how fill would be used?

Maybe v.extend(core::iter::repeat(...)) works?

[robin-nitrokey]

I don’t think extend + repeat would work as extend “[p]anics if the vec cannot hold all elements of the iterator.”

Here is a real world example:

https://github.com/trussed-dev/ctap-types/blob/084db87a800e1204bcfdc3e25ca46bc4a81650ba/src/ctap2.rs#L148-L175

serialize receives a &mut Vec<u8, N> buffer and wants to write data to the buffer by calling cbor_serialize, which expects a &mut [u8] buffer. So we need to extend the Vec to capacity, write to it and then truncate it to the written data.

Here, buffer.resize_default(buffer.capacity()).ok() could be replaced with one of:

• buffer.fill(Default::default())
• buffer.fill_default()
• buffer.resize_to_capacity()