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implement const Iterator for Range#156216

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Randl:const-for-impl
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implement const Iterator for Range#156216
Randl wants to merge 4 commits intorust-lang:mainfrom
Randl:const-for-impl

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@Randl Randl commented May 6, 2026

This allows the use of for i in i..n in const.

This I believe is interesting enough to be justified under #155816

r? @oli-obk

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels May 6, 2026
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Randl commented May 6, 2026

Another issue I noticed is that while is const-stable, but for isn't. So in some places, reverting the const-hack would make currently const-stable functions const-unstable. Alternatively, we wait until Iterator is const-stable.

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@Randl Randl force-pushed the const-for-impl branch from d4ee14e to 6115579 Compare May 6, 2026 02:08
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theemathas commented May 6, 2026

Does this run into #148200 for types that implement non-const TrustedStep but implements const Step?

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@Randl Randl force-pushed the const-for-impl branch from 6115579 to 58f0c19 Compare May 6, 2026 04:23
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Randl commented May 6, 2026

@theemathas
Can you give maybe an example you had in mind? I'm not sure I can think of one. If I understand correctly, there won't be any non-const functions involved in this case, since the entire implementation will be in the const Step?

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theemathas commented May 6, 2026

Maybe there's actually not a problem with specialization here? I suppose it's kind of fine to call a function that requires const TrustedStep even though the type only actually implements TrustedStep + const Step, since TrustedStep itself doesn't have any methods.

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theemathas reviewed May 6, 2026
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Comment thread library/core/src/tuple.rs Outdated
@Randl Randl force-pushed the const-for-impl branch from 58f0c19 to ccbb26c Compare May 6, 2026 08:18
oli-obk
oli-obk reviewed May 6, 2026
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Comment thread library/core/src/iter/range.rs
#[stable(feature = "rust1", since = "1.0.0")]
impl<A: Step> DoubleEndedIterator for ops::Range<A> {
#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
impl<A: [const] Step + [const] Destruct> const DoubleEndedIterator for ops::Range<A> {
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@oli-obk oli-obk May 6, 2026 •
edited by rustbot
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Is this change needed for the MVP? should at least be a separate commit unless the rest of this commit requires it

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