Use Unsafe.BitCast for generic span casting

[xtqqczze]

Related:

• Use MemoryMarshal.Cast in a few places #99835
• Optimize MemoryMarshal.Cast for same type #100750

[xtqqczze]

@MihuBot

[huoyaoyuan]

This was problematic for Mono. See #102998 and #103915 . I'm not sure if anything has changed.

[xtqqczze]

This was problematic for Mono. See #102998 and #103915 . I'm not sure if anything has changed.

#105150

cc: @fanyang-mono

[xtqqczze]

@MihuBot -mono

[MihaZupan]

The bot doesn't support mono

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[xtqqczze]

@EgorBo Build analysis is green. I do not believe this is problematic for Mono since #105150.

[EgorBo]

@EgorBo Build analysis is green. I do not believe this is problematic for Mono since #105150.

One of our main focuses currently is to reduce unsafe code in the repo (ideally, with zero performance impact, but we might tolerate small regressions in favor of memory safety), see #94941. This change seems to replace one unsafe pattern with another. Or in case of MemoryMarshal.Cast -> Unsafe.Bitcast it seems to be even more unsafe now? Could you please clarify the motivation behind it?

[xtqqczze]

@MihuBot

[xtqqczze]

One of our main focuses currently is to reduce unsafe code in the repo (ideally, with zero performance impact, but we might tolerate small regressions in favor of memory safety), see #94941. This change seems to replace one unsafe pattern with another. Or in case of MemoryMarshal.Cast -> Unsafe.Bitcast it seems to be even more unsafe now? Could you please clarify the motivation behind it?

Unsafe.BitCast is only being used to cast from a generic type to the actual type, therefore is safe. Additionally, Unsafe.BitCast provides more clarity than MemoryMarshal.Cast or MemoryMarshal.AsBytes.

[stephentoub]

therefore is safe

All of our uses of "unsafe" constructs are safe; otherwise they'd be bugs to be fixed. The desire to reduce the reliance on such constructs is to reduce the possibility that we might make a mistake, allowing normal runtime protections to be in effect.

[MichalPetryka]

I've tried just making MemoryMarshal use BitCast internally but that gives 0 diffs anyway so not sure either of the changes makes sense.

[xtqqczze]

I've tried just making MemoryMarshal use BitCast internally but that gives 0 diffs anyway so not sure either of the changes makes sense.

Something similar has already been tried: #100750