Add rustc_index::StableIdx

[susitsm]

StableIdx is for Idx types where iteration is stable between runs. Allows usage of iterator functions for IndexVec and IndexSlice. Otherwise only the unstable_* variants of these functions is available

This is related to #162202 and #162203 and #90317

[rustbot]

Some changes occurred in coverage instrumentation.

cc @Zalathar

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in exhaustiveness checking

cc @Nadrieril

[rustbot]

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[rust-log-analyzer]

The job test-pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

    Checking ui_test v0.30.7
[RUSTC-TIMING] ui_test test:false 0.622
[RUSTC-TIMING] chrono_tz test:false 3.373
    Checking miri v0.1.0 (/checkout/src/tools/miri)
error[E0599]: the method `iter_enumerated` exists for struct `rustc_index::IndexVec<concurrency::thread::ThreadId, concurrency::thread::Thread<'tcx>>`, but its trait bounds were not satisfied
   --> src/tools/miri/src/concurrency/thread.rs:494:22
    |
 47 | pub struct ThreadId(u32);
    | ------------------- doesn't satisfy `concurrency::thread::ThreadId: StableIdx`
...
---
            `concurrency::thread::ThreadId: StableIdx`
note: the trait `StableIdx` must be implemented
   --> compiler/rustc_index/src/idx.rs:35:1
    |
 35 | pub trait StableIdx: Idx {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^

error[E0599]: the method `iter_enumerated` exists for struct `rustc_index::IndexVec<concurrency::thread::ThreadId, concurrency::thread::Thread<'tcx>>`, but its trait bounds were not satisfied
   --> src/tools/miri/src/concurrency/thread.rs:501:14
    |
 47 |   pub struct ThreadId(u32);
    |   ------------------- doesn't satisfy `concurrency::thread::ThreadId: StableIdx`
...
---
            `concurrency::thread::ThreadId: StableIdx`
note: the trait `StableIdx` must be implemented
   --> compiler/rustc_index/src/idx.rs:35:1
    |
 35 | pub trait StableIdx: Idx {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^

error[E0599]: the method `iter_enumerated` exists for struct `std::cell::Ref<'_, rustc_index::IndexVec<VectorIdx, ThreadClockSet>>`, but its trait bounds were not satisfied
    --> src/tools/miri/src/concurrency/data_race.rs:1689:30
     |
1689 |             if vector_clocks.iter_enumerated().all(|(clock_idx, clock)| {
     |                              ^^^^^^^^^^^^^^^
     |
    ::: src/tools/miri/src/concurrency/vector_clock.rs:16:1
     |
  16 | pub(super) struct VectorIdx(u32);
     | --------------------------- doesn't satisfy `VectorIdx: StableIdx`
     |
     = note: the following trait bounds were not satisfied:
             `VectorIdx: StableIdx`
note: the trait `StableIdx` must be implemented
    --> compiler/rustc_index/src/idx.rs:35:1
     |
  35 | pub trait StableIdx: Idx {}
     | ^^^^^^^^^^^^^^^^^^^^^^^^

error[E0599]: the method `iter_enumerated` exists for mutable reference `&mut rustc_index::IndexVec<VectorIdx, ThreadClockSet>`, but its trait bounds were not satisfied
    --> src/tools/miri/src/concurrency/data_race.rs:1808:22
     |
1807 |                   if vector_clocks
     |  ____________________-
1808 | |                     .iter_enumerated()
     | |_____________________-^^^^^^^^^^^^^^^
     |
    ::: src/tools/miri/src/concurrency/vector_clock.rs:16:1
     |
  16 |   pub(super) struct VectorIdx(u32);
     |   --------------------------- doesn't satisfy `VectorIdx: StableIdx`
     |
     = note: the following trait bounds were not satisfied:
             `VectorIdx: StableIdx`
note: the trait `StableIdx` must be implemented
    --> compiler/rustc_index/src/idx.rs:35:1
     |
  35 | pub trait StableIdx: Idx {}
     | ^^^^^^^^^^^^^^^^^^^^^^^^

error[E0277]: `&rustc_index::IndexVec<concurrency::thread::ThreadId, concurrency::thread::Thread<'_>>` is not an iterator
   --> src/tools/miri/src/concurrency/thread.rs:413:23
    |
413 |         for thread in threads {
    |                       ^^^^^^^ `&rustc_index::IndexVec<concurrency::thread::ThreadId, concurrency::thread::Thread<'_>>` is not an iterator
    |
    = help: the trait `IntoIterator` is not implemented for `&rustc_index::IndexVec<concurrency::thread::ThreadId, concurrency::thread::Thread<'_>>`
help: the following other types implement trait `IntoIterator`
   --> compiler/rustc_index/src/vec.rs:353:1
    |
353 | impl<I: StableIdx, T> IntoIterator for IndexVec<I, T> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `rustc_index::IndexVec<I, T>`
...
363 | impl<'a, I: StableIdx, T> IntoIterator for &'a IndexVec<I, T> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `&rustc_index::IndexVec<I, T>`
...
373 | impl<'a, I: StableIdx, T> IntoIterator for &'a mut IndexVec<I, T> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `&mut rustc_index::IndexVec<I, T>`

error[E0599]: the method `iter` exists for struct `rustc_index::IndexVec<concurrency::thread::ThreadId, concurrency::thread::Thread<'tcx>>`, but its trait bounds were not satisfied
   --> src/tools/miri/src/concurrency/thread.rs:537:22
    |
 47 | pub struct ThreadId(u32);
    | ------------------- doesn't satisfy `concurrency::thread::ThreadId: StableIdx`
...
537 |         self.threads.iter().filter(|t| !t.state.is_terminated()).count()
    |                      ^^^^ method cannot be called due to unsatisfied trait bounds
    |
    = note: the following trait bounds were not satisfied:
            `concurrency::thread::ThreadId: StableIdx`
note: the trait `StableIdx` must be implemented
   --> compiler/rustc_index/src/idx.rs:35:1
    |
 35 | pub trait StableIdx: Idx {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^

error[E0599]: the method `iter` exists for struct `rustc_index::IndexVec<concurrency::thread::ThreadId, concurrency::thread::Thread<'tcx>>`, but its trait bounds were not satisfied
   --> src/tools/miri/src/concurrency/thread.rs:547:22
    |
 47 | pub struct ThreadId(u32);
    | ------------------- doesn't satisfy `concurrency::thread::ThreadId: StableIdx`
...
547 |         self.threads.iter().all(|thread| thread.state.is_terminated())
    |                      ^^^^ method cannot be called due to unsatisfied trait bounds
    |
    = note: the following trait bounds were not satisfied:
            `concurrency::thread::ThreadId: StableIdx`
note: the trait `StableIdx` must be implemented
   --> compiler/rustc_index/src/idx.rs:35:1
    |
 35 | pub trait StableIdx: Idx {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^

error[E0599]: the method `iter_enumerated` exists for reference `&rustc_index::IndexVec<concurrency::thread::ThreadId, concurrency::thread::Thread<'tcx>>`, but its trait bounds were not satisfied
   --> src/tools/miri/src/concurrency/thread.rs:808:14
    |
 47 |   pub struct ThreadId(u32);
    |   ------------------- doesn't satisfy `concurrency::thread::ThreadId: StableIdx`
...
---
            `concurrency::thread::ThreadId: StableIdx`
note: the trait `StableIdx` must be implemented
   --> compiler/rustc_index/src/idx.rs:35:1
    |
 35 | pub trait StableIdx: Idx {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^

error[E0599]: the method `iter_enumerated_mut` exists for struct `rustc_index::IndexVec<concurrency::thread::ThreadId, concurrency::thread::Thread<'tcx>>`, but its trait bounds were not satisfied
   --> src/tools/miri/src/concurrency/thread.rs:872:58
    |
 47 | pub struct ThreadId(u32);
    | ------------------- doesn't satisfy `concurrency::thread::ThreadId: StableIdx`
...
---
            `concurrency::thread::ThreadId: StableIdx`
note: the trait `StableIdx` must be implemented
   --> compiler/rustc_index/src/idx.rs:35:1
    |
 35 | pub trait StableIdx: Idx {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^

error[E0599]: the method `iter` exists for reference `&rustc_index::IndexVec<concurrency::thread::ThreadId, concurrency::thread::Thread<'tcx>>`, but its trait bounds were not satisfied
    --> src/tools/miri/src/concurrency/thread.rs:1021:18
     |
  47 |   pub struct ThreadId(u32);
     |   ------------------- doesn't satisfy `concurrency::thread::ThreadId: StableIdx`
...
---
             `concurrency::thread::ThreadId: StableIdx`
note: the trait `StableIdx` must be implemented
    --> compiler/rustc_index/src/idx.rs:35:1
     |
  35 | pub trait StableIdx: Idx {}
     | ^^^^^^^^^^^^^^^^^^^^^^^^

Some errors have detailed explanations: E0277, E0599.
For more information about an error, try `rustc --explain E0277`.
[RUSTC-TIMING] miri test:false 4.897

[hanna-kruppe]

Zulip thread for background: #t-compiler/const-eval > DefId reproducibility with the parallel frontend

[Zalathar]

I don't feel good about this approach, as it:

• Effectively forces every other Idx type to make false claims about being “stable” as a euphemism for not-LocalDefId
• Pushes a big chunk of complexity and confusion onto every other usage of a pretty simple and fundamental data type

[susitsm]

I understand where you are coming from. Maybe a better solution could be to simply remove Idx from LocalDefId? Other similar indices, like LocalExpndId and SyntaxContext do not implement it. However the query system heavily relies on it (which might be a bad thing, since it can use a relatively large amount of memory for sparse maps, I have seen this in the metadata).

It is also possible to simply gate iteration behind I: Ord, which is similar to this, but there is no single place to document why that is needed. Any other ideas?

[susitsm]

* Pushes a big chunk of complexity and confusion onto every other usage of a pretty simple and fundamental data type

This might be a good thing? Like it or not, stable iteration IS something you should think about when creating a new index type, when using maps. Not doing so results in bugs. So as fundamental as IndexVec is, it might be more generic than it should be. It is used both as a vec and a map. For FxHashMap/FxHashSet, iterating over them results in warnings by the compiler. However, adding such warnings to each place where IndexVec iteration is used might be overkill. Adding some kind of UnordIndexVec (or UnordDenseMap could be a better name) counterpart could also help with this

[hanna-kruppe]

@rustbot reroll

[Zalathar]

I think the more promising approach is to try to make LocalDefId not implement Idx.

That will probably be more intrusive to code currently relying on the impl, but less intrusive to all the other code relying on integration between newtype_idx!, Idx, IndexVec, and IndexSlice.

[Walnut356]

I'm finishing up GSoC this weekend, so it's probably better if someone else looks into this

@rustbot release-assignment