array::from_fn should be fully inlined

[HadrienG2]

It regularly happens to me that I need to create an array not because that's the end product of my computation, but because that's a useful intermediate object when going from dynamic-sized entities like iterators, slices, or arbitrary generator functions, to static-sized entities like SIMD types or loops that must be unrolled.

In this scenario, I tend to use array::from_fn in the hope that the intermediary array will be optimized out. And if everything gets inlined correctly, that works out as expected. But unfortunately, while array::from_fn itself is marked #[inline], its implementation is based on <[(); N]>::map, which is not marked #[inline]. And since the code of that latter function is mildly complex before optimization, the part of rustc that splits crates into multiple codegen-units has an unpleasant tendency to outline it into a separate codegen unit, resulting in a performance disaster.

To avoid this, either array::map should be marked inline, or the implementation of array::from_fn should be changed not to rely on it but only on other inline functions.

You can find further discussion of this and a simple reproducer of array::from_fn failing to inline at #102202 (comment) .

[gmorenz]

I think this was already fixed in #107634, array::from_fn no longer users array::map and appears to only rely on inline functions. Have you reproduced this on a recent nightly?

[HadrienG2]

Indeed, with current nightlies, the codegen became reasonable. Thanks for the pointer!