Use ReadOnlySpan for DefaultExponent

[vcsjones]

Just something I noticed while perusing code.

/cc @bartonjs

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[GrabYourPitchforks]

Something strikes me as weird about using ref byte in a p/invoke signature to represent a buffer. When I look at such a signature I'd expect "oh, this one value is being read from or written to." Contrast this against byte*, which more canonically represents "this is the start of some block of contiguous data."

[vcsjones]

@GrabYourPitchforks hm, yeah I suppose I could see that. There is prior art for both cases. Since "System.Net.Http.Functional.Tests" saved this from getting merged, is the preference that I change it?

[GrabYourPitchforks]

If auto-merge failed, may as well improve it. Didn't want to hold up an already approved PR just for this though. :)

[bartonjs]

All of the byte[] to span P/Invoke changes I've seen have used ref byte.

@stephentoub Do you have thoughts or feelings for byte* vs ref byte in P/Invokes?

[stephentoub]

Do you have thoughts or feelings for byte* vs ref byte in P/Invokes?

byte* is likely tad cheaper / less magic, assuming it's the only thing that would cause a stub to need to be created. But no strong preference.

[vcsjones]

@bartonjs @GrabYourPitchforks So, uh change it?

While I have y'all here, is there a meaningful difference between:

fixed (byte* x = &MemoryMarshal.GetReference(foo)) {
}

and

fixed (byte* x = foo) {
}

Is the former just from the times before GetPinnableReference was a thing?

[stephentoub]

is there a meaningful difference between

Yes. The latter includes length check that returns null if the length is 0:

runtime/src/libraries/System.Private.CoreLib/src/System/Span.cs

Lines 267 to 269 in 4bdf468

	ref T ret = ref Unsafe.NullRef<T>();
	if (_length != 0) ret = ref _pointer.Value;
	return ref ret;

The former does not:

runtime/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs

Line 88 in 4bdf468

public static ref T GetReference<T>(ReadOnlySpan<T> span) => ref span._pointer.Value;