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unstably allow constants to refer to statics and read from immutable statics #119614

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merged 5 commits into from
Feb 10, 2024
Merged

unstably allow constants to refer to statics and read from immutable statics #119614

merged 5 commits into from
Feb 10, 2024

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@RalfJung RalfJung commented Jan 5, 2024

I am not aware of any fundamental reason why we cannot allow constants to mention statics. What we really need is that constants do not read from statics that can change their value:

  • This would break the principle that "constants behave as-if their expression was inlined everywhere and executed at runtime". This is enforced by halting const-eval interpretation when a read from a mutable global occurs.
  • When building a valtree we want to be sure that the constant and everything it refers to is truly immutable. This is enforced by aborting valtree construction when a read from a mutable global occurs.

r? @oli-obk -- if you are okay with experimenting with this feature, I will create a tracking issue.
Based on and blocked on #119044; only the last commit is new.

@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. labels Jan 5, 2024
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rustbot commented Jan 5, 2024

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

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fine by me, as long as the tracking issue mentions this new source of monomorphization time errors as an open question to put to T-lang

@RalfJung RalfJung mentioned this pull request Jan 5, 2024
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@RalfJung RalfJung force-pushed the const-refs-to-static branch 2 times, most recently from 768c020 to 7f21ace Compare January 5, 2024 14:34
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@RalfJung RalfJung force-pushed the const-refs-to-static branch 2 times, most recently from 6a0f1bc to 1ad4b85 Compare January 5, 2024 15:13
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@RalfJung RalfJung force-pushed the const-refs-to-static branch from 1ad4b85 to 8db79e5 Compare January 5, 2024 15:34
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@RalfJung RalfJung force-pushed the const-refs-to-static branch from 8db79e5 to 8344f1b Compare January 5, 2024 16:02
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@RalfJung RalfJung force-pushed the const-refs-to-static branch from 8344f1b to 60b12fd Compare January 5, 2024 17:16
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rustbot commented Jan 6, 2024

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

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RalfJung commented Jan 6, 2024

I realized I have to make validation descend into statics, or else valtree construction can encounter unvalidated data. This also makes the feature more conservative: consts that refer to mutable memory behind a shared reference are immediately rejected, not just when they are used as a pattern.

@RalfJung RalfJung force-pushed the const-refs-to-static branch from d9943e9 to 0eb6726 Compare January 6, 2024 12:56
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@RalfJung RalfJung force-pushed the const-refs-to-static branch from 0eb6726 to aee26a8 Compare January 6, 2024 13:32
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@RalfJung RalfJung force-pushed the const-refs-to-static branch from aea0509 to c7cca7c Compare January 6, 2024 14:19
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oli-obk commented Jan 8, 2024

seems fishy to allow valtrees to read from immutable statics. That means we meaningfully lose the static's identity during rountdrip.

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RalfJung commented Jan 8, 2024

Valtrees are about the value. How would the static's identity matter?

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oli-obk commented Feb 10, 2024

@bors r- conflicts

@bors bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Feb 10, 2024
@RalfJung RalfJung force-pushed the const-refs-to-static branch from ea7654f to 4def373 Compare February 10, 2024 15:13
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@bors r=oli-obk

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bors commented Feb 10, 2024

📌 Commit 4def373 has been approved by oli-obk

It is now in the queue for this repository.

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 10, 2024
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bors commented Feb 10, 2024

⌛ Testing commit 4def373 with merge 6cc4843...

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bors commented Feb 10, 2024

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 6cc4843 to master...

@bors bors added the merged-by-bors This PR was explicitly merged by bors. label Feb 10, 2024
@bors bors merged commit 6cc4843 into rust-lang:master Feb 10, 2024
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@rustbot rustbot added this to the 1.78.0 milestone Feb 10, 2024
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Finished benchmarking commit (6cc4843): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

mean range count
Regressions ❌
(primary)		 - - 0
Regressions ❌
(secondary)		 0.2% [0.2%, 0.2%] 1
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) - - 0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

mean range count
Regressions ❌
(primary)		 - - 0
Regressions ❌
(secondary)		 3.3% [2.3%, 4.4%] 2
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) - - 0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

mean range count
Regressions ❌
(primary)		 - - 0
Regressions ❌
(secondary)		 0.1% [0.1%, 0.1%] 1
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) - - 0

Bootstrap: 666.587s -> 667.259s (0.10%)
Artifact size: 308.05 MiB -> 308.02 MiB (-0.01%)

@RalfJung RalfJung deleted the const-refs-to-static branch February 11, 2024 20:51
@RalfJung RalfJung mentioned this pull request Feb 11, 2024
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@DaniPopes DaniPopes mentioned this pull request Feb 11, 2024
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@RalfJung RalfJung mentioned this pull request Feb 12, 2024
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matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request Feb 12, 2024
@matthiaskrgr
Rollup merge of rust-lang#120970 - RalfJung:static-promoted-test, r=o…
338a844 
…li-obk

add another test for promoteds-in-static

rust-lang#119614 led to validation of promoteds recursing into statics. These statics can point to `static mut` and interior mutable `static` and do other things we don't allow in `const`, but promoteds are validated as `const`, so we get strange errors (saying "in `const`" when there is no const) and surprising validation failures.

rust-lang#120960 fixes that; this here adds another test.

r? `@oli-obk`
Fixes rust-lang#120968
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request Feb 12, 2024
@matthiaskrgr
Rollup merge of rust-lang#120970 - RalfJung:static-promoted-test, r=o…
f759c23 
…li-obk

add another test for promoteds-in-static

rust-lang#119614 led to validation of promoteds recursing into statics. These statics can point to `static mut` and interior mutable `static` and do other things we don't allow in `const`, but promoteds are validated as `const`, so we get strange errors (saying "in `const`" when there is no const) and surprising validation failures.

rust-lang#120960 fixes that; this here adds another test.

r? ``@oli-obk``
Fixes rust-lang#120968
rust-timer added a commit to rust-lang-ci/rust that referenced this pull request Feb 13, 2024
@rust-timer
Unrolled build for rust-lang#120970
19f2847 
Rollup merge of rust-lang#120970 - RalfJung:static-promoted-test, r=oli-obk

add another test for promoteds-in-static

rust-lang#119614 led to validation of promoteds recursing into statics. These statics can point to `static mut` and interior mutable `static` and do other things we don't allow in `const`, but promoteds are validated as `const`, so we get strange errors (saying "in `const`" when there is no const) and surprising validation failures.

rust-lang#120960 fixes that; this here adds another test.

r? ``@oli-obk``
Fixes rust-lang#120968
github-actions bot pushed a commit to rust-lang/miri that referenced this pull request Feb 13, 2024
@matthiaskrgr
Rollup merge of #120970 - RalfJung:static-promoted-test, r=oli-obk
4567c19 
add another test for promoteds-in-static

rust-lang/rust#119614 led to validation of promoteds recursing into statics. These statics can point to `static mut` and interior mutable `static` and do other things we don't allow in `const`, but promoteds are validated as `const`, so we get strange errors (saying "in `const`" when there is no const) and surprising validation failures.

rust-lang/rust#120960 fixes that; this here adds another test.

r? ``@oli-obk``
Fixes rust-lang/rust#120968
@ojeda ojeda mentioned this pull request Feb 17, 2024
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@fmease fmease added the F-const_refs_to_static `#![feature(const_refs_to_static)]` label Feb 28, 2024
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