Add documentation of compile-time width function alternative.

[dcsommer]

While the PrimInt trait itself doesn't have compile time width functions, there are other ways to find the width of the underlying type. It is useful to present the alternative.

[cuviper]

Should we say 8 * size_of() so it counts bits?

[dcsommer]

Yeah, I can qualify counting bytes vs bits in the text. Typically the size of integers is specified by bytes, not bits, however. Do you think all instances of the pattern `T::zero().count_zeros()` should be rewritten in terms of `core::mem::size_of::<T>()` instead? I could modify the text to that effect if you like.

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On Sat, Jan 4, 2020 at 9:51 PM Josh Stone ***@***.***> wrote: ***@***.**** commented on this pull request. ------------------------------ In src/int.rs <#147 (comment)>: > @@ -22,7 +22,7 @@ use {Num, NumCast}; /// /// All `PrimInt` types are expected to be fixed-width binary integers. The width can be queried /// via `T::zero().count_zeros()`. The trait currently lacks a way to query the width at -/// compile-time. +/// compile-time, but `core::mem::size_of::<T>()` can be used in the meantime. Should we say 8 * size_of() so it counts bits? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#147?email_source=notifications&email_token=AAIIKIFWDUD6NOK6B3HXQZDQ4FYNRA5CNFSM4KB5PIX2YY3PNVWWK3TUL52HS4DFWFIHK3DMKJSXC5LFON2FEZLWNFSXPKTDN5WW2ZLOORPWSZGOCQVUZOY#pullrequestreview-338382011>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAIIKIBZLXCYL44UOP5YLMDQ4FYNRANCNFSM4KB5PIXQ> .

[cuviper]

Typically the size of integers is specified by bytes, not bits, however.

You think so? The bit width is right in the name: i32, u8, etc., and C's stdint.h is similar.

Do you think all instances of the pattern T::zero().count_zeros() should be rewritten in terms of core::mem::size_of::<T>() instead?

AFAICS "all" is just this one instance, but sure, I'd be OK with preferring size_of since it's const.

[dcsommer]

I just remembered that in the case of `dyn T` we can't use `core::mem::size_of<T>`, so there is definitely a use case for both runtime and compile-time width calculation. I'll do it in terms of bits too.

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On Tue, Jan 7, 2020 at 12:02 PM Josh Stone ***@***.***> wrote: Typically the size of integers is specified by bytes, not bits, however. You think so? The bit width is right in the name: i32, u8, etc., and C's stdint.h is similar. Do you think all instances of the pattern T::zero().count_zeros() should be rewritten in terms of core::mem::size_of::<T>() instead? AFAICS "all" is just this one instance, but sure, I'd be OK with preferring size_of since it's const. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#147?email_source=notifications&email_token=AAIIKIGSIKUGNGXG2F5GNV3Q4TNVFA5CNFSM4KB5PIX2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEIKC5EQ#issuecomment-571747986>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAIIKIE4ZMMSJQS2A4GOCHDQ4TNVFANCNFSM4KB5PIXQ> .

[cuviper]

How does dyn T apply in this context though? We already require PrimInt: Sized.

[dcsommer]

I mean when dealing with a `PrimInt` trait object, we can't know the size of the type at compile time. For instance, in code dealing with a `Box<dyn PrimInt>`, there isn't a way to use the `const fn` `core::mem::size_of<T>()` to get the concrete type's width at compile time. We need to dynamic dispatch on the trait object to calculate it.

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On Tue, Jan 7, 2020 at 12:19 PM Josh Stone ***@***.***> wrote: How does dyn T apply in this context though? We already require PrimInt: Sized. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#147?email_source=notifications&email_token=AAIIKIDT6L5UK4DUHAEHJVDQ4TPVNA5CNFSM4KB5PIX2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEIKEMEQ#issuecomment-571754002>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAIIKIGXTOPD7J6UUMZJL53Q4TPVNANCNFSM4KB5PIXQ> .

[cuviper]

PrimInt is not trait object safe -- you can't form any sort of dyn PrimInt at all.