Allow using Slice Iterators in const Contexts

[onestacked]

based on #102225 by @fee1-dead

[rustbot]

r? @joshtriplett

(rustbot has picked a reviewer for you, use r? to override)

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[onestacked]

@rustbot label +T-libs-api -T-libs

[onestacked]

@rustbot label +S-experimental

[the8472]

This is the 2nd PR blocked on that PR which in turn is blocked on a bunch of concerns one of which is blocked on another WIP PR. It could take many months for these things to percolate.

[onestacked]

I know, but I made those changes mainly because I wanted to know how much would be required for them.
And since I already had working local version I thought I should do a (Draft) PR so that no one will do unnecessary work re-implementing this in the future.

[fee1-dead]

@chriss0612, you could r? @ghost when opening a PR so that the PR will not be automatically assigned a reviewer.

[rustbot]

Failed to set assignee to ghost: invalid assignee

Note: Only org members, users with write permissions, or people who have commented on the PR may be assigned.

[bors]

☔ The latest upstream changes (presumably #104591) made this pull request unmergeable. Please resolve the merge conflicts.

[onestacked]

Currently includes #104100, but DoubleEndedIterator could be made #[const_trait] here and the one for loop that uses a range could be manually converted to a while.

For Slices of zero sized types this is currently blocked on #92512, since there is currently not way to find the calculate the length of a zero sized slice without UB.

Theoretically const_eval_selectcould be used to panic for slices of ZST's at CTFE, but I don't think that is a good approach.

So I think that #92512 should be figured out before this is considerd for merging.

[bors]

☔ The latest upstream changes (presumably #108538) made this pull request unmergeable. Please resolve the merge conflicts.

[onestacked]

@rustbot label -T-compiler

[bors]

☔ The latest upstream changes (presumably #109346) made this pull request unmergeable. Please resolve the merge conflicts.

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Prepare all required actions
Getting action download info
Download action repository 'actions/checkout@v3' (SHA:24cb9080177205b6e8c946b17badbe402adc938f)
Download action repository 'rust-lang/simpleinfra@master' (SHA:3fb2b44a4eaebb9ed8086446bde46c27199ef5ed)
Complete job name: PR (x86_64-gnu-tools, false, ubuntu-20.04-xl)
git config --global core.autocrlf false
shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
env:
  CI_JOB_NAME: x86_64-gnu-tools
---
   Compiling memchr v2.5.0
   Compiling std v0.0.0 (/checkout/library/std)
   Compiling compiler_builtins v0.1.87
   Compiling unwind v0.0.0 (/checkout/library/unwind)
error[E0636]: the feature `const_pointer_byte_offsets` has already been declared
    |
147 | #![feature(const_pointer_byte_offsets)]
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^


error: unreachable block in `if` or `while` expression
    |
29  | /   macro_rules! iterator {
30  | |       (
30  | |       (
31  | |           struct $name:ident -> $ptr:ty,
32  | |           $elem:ty,
404 | |                       if is_empty!(self) {
    | |  ________________________________________^
405 | | |                         None
406 | | |                     } else {
406 | | |                     } else {
    | | |_____________________^ unreachable block in `if` or `while` expression
665 | |       }
666 | |   }
    | |___- in this expansion of `iterator!`
    |
    |
   ::: library/core/src/panic.rs:57:9
    |
57  |             $crate::panicking::panic_fmt($crate::const_format_args!($($t)+))
    |             ---------------------------------------------------------------- any code following this expression is unreachable
    |
   ::: library/core/src/slice/iter.rs:133:1
    |
133 |   / iterator! {struct Iter -> *const T, &'a T, const, {/* no mut */}, {
134 |   |     // FIXME(const_trait_impl)
135 |   |     // fn is_sorted_by<F>(self, mut compare: F) -> bool
136 |   |     // where
143 |   |     // }
144 |   | }}
    |   |__- in this macro invocation
    |
    |
    = note: `-D unreachable-code` implied by `-D warnings`

error: unreachable block in `if` or `while` expression
    |
29  | /   macro_rules! iterator {
30  | |       (
30  | |       (
31  | |           struct $name:ident -> $ptr:ty,
32  | |           $elem:ty,
620 | |                       if is_empty!(self) {
    | |  ________________________________________^
621 | | |                         None
622 | | |                     } else {
622 | | |                     } else {
    | | |_____________________^ unreachable block in `if` or `while` expression
665 | |       }
666 | |   }
    | |___- in this expansion of `iterator!`
    |
    |
   ::: library/core/src/panic.rs:57:9
    |
57  |             $crate::panicking::panic_fmt($crate::const_format_args!($($t)+))
    |             ---------------------------------------------------------------- any code following this expression is unreachable
    |
   ::: library/core/src/slice/iter.rs:133:1
    |
133 |   / iterator! {struct Iter -> *const T, &'a T, const, {/* no mut */}, {
134 |   |     // FIXME(const_trait_impl)
135 |   |     // fn is_sorted_by<F>(self, mut compare: F) -> bool
136 |   |     // where
143 |   |     // }
144 |   | }}
    |   |__- in this macro invocation


error: unreachable block in `if` or `while` expression
   ::: library/core/src/panic.rs:57:9
    |
57  |            $crate::panicking::panic_fmt($crate::const_format_args!($($t)+))
    |            ---------------------------------------------------------------- any code following this expression is unreachable
    |            ---------------------------------------------------------------- any code following this expression is unreachable
    |
   ::: library/core/src/slice/iter.rs:366:1
    |
366 |    iterator! {struct IterMut -> *mut T, &'a mut T, mut, {mut}, {}}
   --> library/core/src/slice/iter/macros.rs:404:40
    |
29  | /  macro_rules! iterator {
30  | |      (
30  | |      (
31  | |          struct $name:ident -> $ptr:ty,
32  | |          $elem:ty,
404 | |                      if is_empty!(self) {
    | | ________________________________________^
405 | ||                         None
406 | ||                     } else {
406 | ||                     } else {
    | ||_____________________^ unreachable block in `if` or `while` expression
665 | |      }
666 | |  }
    | |__- in this expansion of `iterator!`


error: unreachable block in `if` or `while` expression
   ::: library/core/src/panic.rs:57:9
    |
57  |            $crate::panicking::panic_fmt($crate::const_format_args!($($t)+))
    |            ---------------------------------------------------------------- any code following this expression is unreachable
    |            ---------------------------------------------------------------- any code following this expression is unreachable
    |
   ::: library/core/src/slice/iter.rs:366:1
    |
366 |    iterator! {struct IterMut -> *mut T, &'a mut T, mut, {mut}, {}}
   --> library/core/src/slice/iter/macros.rs:620:40
    |
29  | /  macro_rules! iterator {
30  | |      (
30  | |      (
31  | |          struct $name:ident -> $ptr:ty,
32  | |          $elem:ty,
620 | |                      if is_empty!(self) {
    | | ________________________________________^
621 | ||                         None
622 | ||                     } else {
622 | ||                     } else {
    | ||_____________________^ unreachable block in `if` or `while` expression
665 | |      }
666 | |  }
    | |__- in this expansion of `iterator!`

[bors]

☔ The latest upstream changes (presumably #109384) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC]

Closing this as it is an experiment and blocked on multiple things