tree 34ead6df5dfa7e000ab5ff8474fbf53b22b611c4
parent 6213088c4f0f1a0cd38ab6360276d085eb66fdaf
author Daniel Cheng <dcheng@chromium.org> 1723088032 +0000
committer Chromium LUCI CQ <chromium-scoped@luci-project-accounts.iam.gserviceaccount.com> 1723088032 +0000

Implement `span::to_fixed_extent<N>(...)` to replace `make_span<N>(...)`

`span::to_fixed_extent<N>()` is a helper that converts a dynamic-extent
span to a fixed-extent span at runtime. In theory, this isn't needed
since there are already several ways to do this conversion:
- `first<N>(...)`, `last<N>(...)`, and `subspan<0, N>(...)` all extract
  a fixed-extent span from a dynamic-extent span. However, none of these
  express the intent "this span is exactly size N" without the caller
  writing additional checks.
- `make_span<N>(...)` makes a runtime-checked fixed-extent span from a
  container with dynamic length. However:
  - In general, the `make_span<...>(...)` family of functions will
    migrate to CTAD in the future.
  - As a free function, it can be used with any type of spannable
    object even when it's not necessary.
- Using an explicit `span<T, N>` construction. However, this requires
  writing `T` again, which can sometimes be clunky.

The introduction of this helper allows the removal of
`as_byte_span<N>()` and `as_writable_byte_span<N>()`, and will
facilitate the future removal of `make_span<N>()`.

Finally, remove redundant constraints from `as_byte_span()` and
`as_writable_byte_span()` as part of cleaning up the overloads.

Bug: 341907909, 355603418
Change-Id: I6191eb9191371e32db3a618c77888d773a12fa8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5763092
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1338855}