Limit the promotion of const fns to the libstd and the `rustc_promotable` attribute #53851

oli-obk · 2018-08-31T11:54:03Z

There are so many questions around promoting const fn calls... it seems saner to try to limit automatic promotion to const fns which were explicitly opted in for promotion.

I added the attribute to all public stable const fns that were already promotable (e.g. not Cell::new) in order to not cause any breakage

r? @eddyb

cc @nikomatsakis

oli-obk · 2018-08-31T12:32:14Z

@bors try

bors · 2018-08-31T12:32:25Z

⌛ Trying commit dc280eb2bc82f51f865c5bc3dc9cd8997cc3d69c with merge 952c7dbfa2628309f557b367929ffc17af20b332...

bors · 2018-08-31T14:50:52Z

☀️ Test successful - status-travis
State: approved= try=True

pietroalbini · 2018-09-01T11:06:57Z

@craterbot run start=master#1114ab684fbad001c4e580326d8eb4d8c4e917d3 end=try#952c7dbfa2628309f557b367929ffc17af20b332 mode=check-only

craterbot · 2018-09-01T11:06:58Z

👌 Experiment pr-53851 created and queued.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2018-09-01T11:33:31Z

🚧 Experiment pr-53851 is now running on agent crater-1.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2018-09-02T21:59:58Z

🎉 Experiment pr-53851 is completed!
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the blacklist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

oli-obk · 2018-09-03T08:22:29Z

One spurious regression, and one true regression:

Sep 02 20:49:06.306 INFO kablam! error[E0597]: borrowed value does not live long enough
Sep 02 20:49:06.306 INFO kablam!    --> src/apint/constructors.rs:429:11
Sep 02 20:49:06.306 INFO kablam!     |
Sep 02 20:49:06.307 INFO kablam! 429 |               .chain([
Sep 02 20:49:06.307 INFO kablam!     |  ____________________^
Sep 02 20:49:06.307 INFO kablam! 430 | |                 u8::max_value(),
Sep 02 20:49:06.307 INFO kablam! 431 | |                 10,
Sep 02 20:49:06.307 INFO kablam! 432 | |                 42,
Sep 02 20:49:06.307 INFO kablam! 433 | |                 99,
Sep 02 20:49:06.307 INFO kablam! 434 | |                 123
Sep 02 20:49:06.307 INFO kablam! 435 | |             ].into_iter()
Sep 02 20:49:06.307 INFO kablam!     | |_____________^ temporary value does not live long enough
Sep 02 20:49:06.307 INFO kablam! 436 |                .map(|v| *v))
Sep 02 20:49:06.307 INFO kablam! 437 |       }
Sep 02 20:49:06.307 INFO kablam!     |       - temporary value only lives until here
Sep 02 20:49:06.307 INFO kablam!     |
Sep 02 20:49:06.307 INFO kablam!     = note: borrowed value must be valid for the static lifetime...

RalfJung · 2018-09-03T12:50:09Z

... and that could be fixed by using u8::MAX instead. Seems pretty encouraging to me?

oli-obk · 2018-09-03T13:12:53Z

@eddyb said on IRC that we should instead add a whitelist of promotable const fns, and add the currently stable ones to it in order to not have any possibility of breakage even outside crates.io

RalfJung · 2018-09-03T15:15:42Z

Anything, as long as we do it before min_const_fn is stable. :) We could just whitelist libcore for this...

oli-obk · 2018-09-03T15:24:28Z

src/librustc/ty/constness.rs

+use syntax::attr;
+use rustc_target::spec::abi;
+
+impl<'a, 'tcx> TyCtxt<'a, 'tcx, 'tcx> {


I collected all the logic in this one file in order to make our lives easier and have less code duplication which might cause odd inconsistencies

oli-obk · 2018-09-03T15:28:03Z

src/librustc_mir/const_eval.rs

@@ -253,9 +250,6 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeEvaluator {
                ecx.goto_block(ret)?; // fully evaluated and done
                return Ok(None);
            }
-            return Err(


I had to remove this because the is_const_fn above is now overly restrictive (it will forbid calling str::len with the const_str_len feature gate but without the const_slice_len feature gate because that calls [u8]::len internally). We already check all the guarantees in the const_qualif pass, there's no need to check them again at evaluation time.

oli-obk · 2018-09-03T15:29:15Z

src/librustc_passes/rvalue_promotion.rs

+        &mut self,
+        def_id: DefId,
+    ) -> Promotability {
+        if self.tcx.is_promotable_const_fn(def_id) {


This is now just a single call to is_promotable_const_fn which seems a major improvement over the code duplication we were doing here before.

oli-obk · 2018-09-03T15:31:20Z

src/test/run-pass/issue-49955-2.rs

 #[inline(never)]
 fn tuple_field() -> &'static u32 {
    // This test is MIR-borrowck-only because the old borrowck
    // doesn't agree that borrows of "frozen" (i.e. without any
    // interior mutability) fields of non-frozen temporaries,
    // should be promoted, while MIR promotion does promote them.
-    &(Cell::new(5), 42).1
+    &(FIVE, 42).1


calls to Cell::new are not promotable anymore, thus this breaks. The crater run didn't show any cases of people actually writing this kind of code (also needs MIR borrowck)

oli-obk · 2018-09-03T15:32:54Z

src/test/ui/consts/const-eval/issue-52443.stderr

-   |                     calling non-const fn `<I as std::iter::IntoIterator><std::ops::RangeFrom<usize>>::into_iter`
+   |                     inside call to `std::iter::range::<impl std::iter::Iterator for std::ops::RangeFrom<A>><usize>::next`
+   | 
+  ::: $SRC_DIR/libcore/ptr.rs:LL:COL


Diagnostics improvement or regression. Depending on your view. We used to stop at the first non-const fn, now we just stop if/when we fail to actually evaluate. An error is reported above about non const fn calls and other non-const things happening here.

Although I like the specificity, this is not actually actionable, is it? I would not block on improving this message though.

We could probably make it a delay_span_bug, but i fear that we'll overlook some case and then run into that hypothetical ICE at some point.

These kind of errors only happen for repeat/array lengths and enum variant discriminant initializers (or constants used in such). Everything else reports only the other errors above.

Alternatively we can try to figure out a scheme that prevents const eval of constants with const qualification errors.

oli-obk · 2018-09-03T15:34:15Z

r? @eddyb

oli-obk · 2018-10-04T06:35:43Z

@bors r=eddyb

bors · 2018-10-04T06:35:43Z

📌 Commit c793391 has been approved by eddyb

bors · 2018-10-04T06:48:20Z

⌛ Testing commit c793391 with merge 088fc73...

@eddyb

Limit the promotion of const fns to the libstd and the `rustc_promotable` attribute There are so many questions around promoting const fn calls... it seems saner to try to limit automatic promotion to const fns which were explicitly opted in for promotion. I added the attribute to all public stable const fns that were already promotable (e.g. not Cell::new) in order to not cause any breakage r? @eddyb cc @nikomatsakis

bors · 2018-10-04T09:17:42Z

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb
Pushing 088fc73 to master...

shepmaster · 2019-01-21T17:58:08Z

Broader discussion continues in rust-lang/const-eval#19

rust-highfive assigned RalfJung Aug 31, 2018

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 31, 2018

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craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Sep 2, 2018

oli-obk force-pushed the local_promotion branch from dc280eb to 5f4c14d Compare September 3, 2018 15:21

oli-obk changed the title ~~WIP EXPERIMENT don't promote any const fn calls~~ Don't promote calls to const fns with arguments Sep 3, 2018

oli-obk commented Sep 3, 2018

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rust-highfive assigned eddyb and unassigned RalfJung Sep 3, 2018

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bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Oct 2, 2018

kennytm 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-review Status: Awaiting review from the assignee but also interested parties. labels Oct 3, 2018

oli-obk added 5 commits October 3, 2018 10:07

Only promote calls to #[rustc_promotable] const fns

9e46c0b

Update error id to an unused one

1f94384

Fix typo

a9156e9

Poke in the dark and see if bors+windows stops screaming

f5106a2

Move platform dependent output ui tests to compile-fail

c793391

oli-obk force-pushed the local_promotion branch from 6939244 to c793391 Compare October 3, 2018 09:51

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 Oct 4, 2018

bors merged commit c793391 into rust-lang:master Oct 4, 2018

This was referenced Oct 4, 2018

Report const eval error inside the query #53821

Merged

Allow ptr::{null, null_mut} to create pointers to ?Sized types #53954

Closed

Cleanup rustc/ty part 1 #54717

Merged

oli-obk deleted the local_promotion branch October 4, 2018 09:33

SimonSapin mentioned this pull request Oct 5, 2018

Regression: "temporary value does not live long enough" in a static item in Servo #54846

Open

oli-obk mentioned this pull request Oct 26, 2018

Illegal Instruction Promoting const panic! #54305

Closed

This was referenced Nov 9, 2018

apint fails to compile on beta #55806

Closed

Add missing rustc_promotable attribute to unsigned min_value and max_value #55828

Merged

hanna-kruppe mentioned this pull request May 3, 2019

Values created by const fns aren't rvalue static promoted #60502

Open

spastorino mentioned this pull request Jun 10, 2020

Regression: Not well-formed HRTB doesn't compile #55498

Closed

RalfJung mentioned this pull request Aug 12, 2020

Solving the problem with promotion rust-lang/const-eval#53

Closed

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Limit the promotion of const fns to the libstd and the `rustc_promotable` attribute #53851

Limit the promotion of const fns to the libstd and the `rustc_promotable` attribute #53851

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Limit the promotion of const fns to the libstd and the rustc_promotable attribute #53851

Limit the promotion of const fns to the libstd and the rustc_promotable attribute #53851

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Limit the promotion of const fns to the libstd and the `rustc_promotable` attribute #53851

Limit the promotion of const fns to the libstd and the `rustc_promotable` attribute #53851

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