Support unstable moves via stable in unstable items

[yaahc]

part of https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/moving.20items.20to.20core.20unstably and a blocker of #90328.

The libs-api team needs the ability to move an already stable item to a new location unstably, in this case for Error in core. Otherwise these changes are insta-stable making them much harder to merge.

This PR attempts to solve the problem by checking the stability of path segments as well as the last item in the path itself, which is currently the only thing checked.

[rust-highfive]

r? @cjgillot

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[yaahc]

This is now also a T-libs-api PR because of the core::unicode stabilization :/

[yaahc]

Also blocked on rust-lang/stdarch#1303

@rustbot blocked

[yaahc]

This should be ready now that rust-lang/stdarch#1303 has merged.

@rustbot ready

[rylev]

@yaahc so this PR led to a performance regression - I see that a perf run was performed (accidentally) before this was merged, but it wasn't looked at. Is there a reason that the performance difference wasn't taken into account? In either case, we should at least address the performance regression with a justification if one is warranted.

The change seems to be localized to incremental compilation scenarios. It seems queries like incr_comp_encode_dep_graph are being hit more often. Is this simply because of the additional pass that's happening or is there some other explanation I don't see?

[yaahc]

@yaahc so this PR led to a performance regression - I see that a perf run was performed (accidentally) before this was merged, but it wasn't looked at. Is there a reason that the performance difference wasn't taken into account?

only because I hadn't intended to run it in the first place, I just didn't pay attention to it.

In either case, we should at least address the performance regression with a justification if one is warranted.

The change seems to be localized to incremental compilation scenarios. It seems queries like incr_comp_encode_dep_graph are being hit more often. Is this simply because of the additional pass that's happening or is there some other explanation I don't see?

I don't have a solid explanation for why that's happening, I don't have much experience with interpreting perf reports for the compiler. Where are you seeing that incr_comp_encode_dep_graph is being hit more often?

From the impl itself I'd expect that the main difference is that we hit the stability check function much more often because of: https://github.com/rust-lang/rust/pull/95956/files#diff-5f57ad10e1bdde3d046b258d390fd2ecc6f1511158aa130cebb72093da16ef29R851, but I don't know how that relates to incremental compilation at all.

[rylev]

You can see differences in query perf on this page. You can see there that incr_comp_encode_dep_graph was executed more often.

I'm also unsure, as I don't see anything that would cause encoding the dep graph more often, but I'm really not familiar enough with this code to say for sure.

The regressions are sort of right on the line between being small enough to shrug off and big enough to really care about investigating. I'll leave it to you on what to do next. Feel free to reach out if you want to chat through things.

[nnethercote]

I ran a Cachegrind diff. The results absolutely match up with what @rylev said about the incremental dep graph.

--------------------------------------------------------------------------------
Ir
--------------------------------------------------------------------------------
72,523,795 (100.0%)  PROGRAM TOTALS

--------------------------------------------------------------------------------
Ir                  file:function
--------------------------------------------------------------------------------
1,988,457 ( 2.74%)  compiler/rustc_query_system/src/dep_graph/graph.rs:<rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::try_mark_previous_green::<rustc_query_impl::plumbing::QueryCtxt>
1,759,692 ( 2.43%)  library/core/src/num/uint_macros.rs:<rustc_data_structures::stable_hasher::StableHasher>::finish::<rustc_data_structures::fingerprint::Fingerprint>
1,537,026 ( 2.12%)  compiler/rustc_serialize/src/leb128.rs:<rustc_query_system::dep_graph::serialized::SerializedDepGraph<rustc_middle::dep_graph::dep_node::DepKind> as rustc_serialize::serialize::Decodable<rustc_serialize::opaque::MemDecoder>>::decode
1,455,664 ( 2.01%)  compiler/rustc_serialize/src/leb128.rs:<(rustc_query_system::dep_graph::serialized::SerializedDepNodeIndex, rustc_query_impl::on_disk_cache::AbsoluteBytePos) as rustc_serialize::serialize::Decodable<rustc_serialize::opaque::MemDecoder>>::decode
1,368,740 ( 1.89%)  compiler/rustc_middle/src/dep_graph/mod.rs:<rustc_middle::dep_graph::dep_node::DepKind as rustc_query_system::dep_graph::DepKind>::read_deps::<<rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::read_index::{closure#0}>
1,260,818 ( 1.74%)  compiler/rustc_query_system/src/query/plumbing.rs:rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, core::option::Option<rustc_attr::builtin::Stability>>>
1,225,791 ( 1.69%)  github.com-1ecc6299db9ec823/hashbrown-0.12.0/src/raw/mod.rs:<rustc_middle::dep_graph::dep_node::DepKind as rustc_query_system::dep_graph::DepKind>::read_deps::<<rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::read_index::{closure#0}>
1,155,286 ( 1.59%)  compiler/rustc_data_structures/src/sip128.rs:<rustc_data_structures::stable_hasher::StableHasher>::finish::<rustc_data_structures::fingerprint::Fingerprint>
1,091,722 ( 1.51%)  github.com-1ecc6299db9ec823/hashbrown-0.12.0/src/raw/mod.rs:rustc_query_system::query::plumbing::try_get_cached::<rustc_middle::ty::context::TyCtxt, rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, core::option::Option<rustc_attr::builtin::Stability>>, core::option::Option<rustc_attr::builtin::Stability>, rustc_middle::ty::query::copy<core::option::Option<rustc_attr::builtin::Stability>>>
1,015,419 ( 1.40%)  compiler/rustc_query_system/src/query/plumbing.rs:rustc_query_system::query::plumbing::try_get_cached::<rustc_middle::ty::context::TyCtxt, rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, core::option::Option<rustc_attr::builtin::Stability>>, core::option::Option<rustc_attr::builtin::Stability>, rustc_middle::ty::query::copy<core::option::Option<rustc_attr::builtin::Stability>>>
  995,121 ( 1.37%)  compiler/rustc_serialize/src/leb128.rs:<[rustc_query_system::dep_graph::graph::DepNodeIndex] as rustc_serialize::serialize::Encodable<rustc_serialize::opaque::FileEncoder>>::encode
  969,062 ( 1.34%)  github.com-1ecc6299db9ec823/smallvec-1.8.1/src/lib.rs:<smallvec::SmallVec<[rustc_query_system::dep_graph::graph::DepNodeIndex; 8]> as core::iter::traits::collect::Extend<rustc_query_system::dep_graph::graph::DepNodeIndex>>::extend::<core::iter::adapters::map::Map<core::slice::iter::Iter<rustc_query_system::dep_graph::serialized::SerializedDepNodeIndex>, <rustc_query_system::dep_graph::graph::CurrentDepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::promote_node_ad_deps_to_current::{closure#0}>>
  958,608 ( 1.32%)  compiler/rustc_serialize/src/leb128.rs:<[(rustc_query_system::dep_graph::serialized::SerializedDepNodeIndex, rustc_query_impl::on_disk_cache::AbsoluteBytePos)] as rustc_serialize::serialize::Encodable<rustc_query_impl::on_disk_cache::CacheEncoder>>::encode
  944,089 ( 1.30%)  compiler/rustc_query_system/src/dep_graph/serialized.rs:<rustc_query_system::dep_graph::serialized::GraphEncoder<rustc_middle::dep_graph::dep_node::DepKind>>::send
  923,416 ( 1.27%)  compiler/rustc_query_system/src/query/plumbing.rs:rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::lookup_stability, rustc_query_impl::plumbing::QueryCtxt>
  923,104 ( 1.27%)  compiler/rustc_query_impl/src/on_disk_cache.rs:<rustc_query_impl::on_disk_cache::OnDiskCache>::try_load_query_result::<core::option::Option<rustc_attr::builtin::Stability>>
  826,887 ( 1.14%)  compiler/rustc_middle/src/ty/query.rs:<rustc_passes::stability::Checker as rustc_hir::intravisit::Visitor>::visit_path
  816,784 ( 1.13%)  compiler/rustc_query_system/src/query/plumbing.rs:rustc_query_system::query::plumbing::try_load_from_disk_and_cache_in_memory::<rustc_query_impl::plumbing::QueryCtxt, rustc_span::def_id::DefId, core::option::Option<rustc_attr::builtin::Stability>>
  745,584 ( 1.03%)  github.com-1ecc6299db9ec823/hashbrown-0.12.0/src/raw/mod.rs:<hashbrown::raw::RawTable<(rustc_query_system::dep_graph::serialized::SerializedDepNodeIndex, rustc_query_impl::on_disk_cache::AbsoluteBytePos)>>::insert::<hashbrown::map::make_hasher<rustc_query_system::dep_graph::serialized::SerializedDepNodeIndex, rustc_query_system::dep_graph::serialized::SerializedDepNodeIndex, rustc_query_impl::on_disk_cache::AbsoluteBytePos, core::hash::BuildHasherDefault<rustc_hash::FxHasher>>::{closure#0}>