Do not promote &mut of a non-ZST ever #75585

RalfJung · 2020-08-16T09:11:59Z

Since ~~pre-1.0~~ 1.36, we have accepted code like this:

static mut TEST: &'static mut [i32] = {
    let x = &mut [1,2,3];
    x
};

I tracked it back to #21744, but unfortunately could not find any discussion or RFC that would explain why we thought this was a good idea. And it's not, it breaks all sorts of things -- see #75556.

To fix #75556, we have to stop promoting non-ZST mutable references no matter the context, which is what this PR does. It's a breaking change.

Notice that this still works, since it does not rely on promotion:

static mut TEST: &'static mut [i32] = &mut [0,1,2];

Cc @rust-lang/wg-const-eval

rust-highfive · 2020-08-16T09:12:04Z

r? @davidtwco

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

src/librustc_mir/transform/mod.rs

davidtwco · 2020-08-16T10:59:00Z

r? @oli-obk

RalfJung · 2020-08-16T11:28:02Z

@bors try (for crater)

bors · 2020-08-16T11:28:14Z

⌛ Trying commit 300900ec7474859b619ae161eb847f5a4f5928bd with merge ffa4a0215752773248fb0f0a8092f1c3e7907e9e...

bors · 2020-08-16T12:14:32Z

☀️ Try build successful - checks-actions, checks-azure
Build commit: ffa4a0215752773248fb0f0a8092f1c3e7907e9e (ffa4a0215752773248fb0f0a8092f1c3e7907e9e)

RalfJung · 2020-08-16T12:25:00Z

@craterbot check

craterbot · 2020-08-16T12:25:03Z

👌 Experiment pr-75585 created and queued.
🤖 Automatically detected try build ffa4a0215752773248fb0f0a8092f1c3e7907e9e
🔍 You can check out the queue and this experiment's details.

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

eddyb · 2020-08-16T15:24:16Z

Left a comment on the issue (#75556 (comment)):

Ugh, that's an ancient rule (believe it or not, it's a workaround for not being able to write static mut Foo: [T; _] = ...; and have the length be inferred!) and it was never meant to get this far, I guess I didn't think it through properly when adding the "outermost scope" rule, which is the only position where allowing it makes sense.

In the current implementation, it should've never been promoted, only allowed, my bad!

Also, you write:

Since pre-1.0, we have accepted code like this:

But aren't let in CTFE a modern addition?

eddyb

Implementation seems fine, but can you add a test that compiled before and now doesn't?
E.g.: static mut FOO: Option<&mut [i32]> = Some(&mut [1, 2, 3]);

RalfJung · 2020-08-16T15:27:27Z

But aren't let in CTFE a modern addition?

Oh, that's a good point. I was just going by this already existing in #21744, but it seems that same analysis was doing something else back then.

Interestingly, in Rust 1.35 this actually fails, even though the analysis already carried this exception:

    #1 with rustc 1.35.0
error[E0597]: borrowed value does not live long enough
 --> <source>:2:18
  |
2 |     let x = &mut [1,2,3];
  |                  ^^^^^^^ temporary value does not live long enough
3 |     x
4 | };
  | - temporary value only lives until here

Rust 1.36 seems to be the first version where this is accepted.

eddyb · 2020-08-16T15:31:03Z

I was just going by this already existing in #21744, but it seems that same analysis was doing something else back then.

Keep in mind that only recently (last year?), did we actually uncouple promotion from const-checking.
So most of the time, the logic was there to prevent static mut X: &mut [i32] = &mut [0]; from erroring.

Also, promotion was in the compiler for a long time (predating MIR itself) but it wasn't exposed to the user (i.e. borrow-checking didn't know about it) until the RFC was accepted, much later.

RalfJung · 2020-08-16T15:31:06Z

This one is even more interesting:

pub static mut FOO: Option<&mut [i32]> = Some(&mut [1, 2, 3]);

It works in Rust 1.21, fails with 1.22 - 1.35, and then works again since 1.36. Wtf.

So most of the time, the logic was there to prevent static mut X: &mut [i32] = &mut [0]; from erroring.

Well it did also accept something, until 1.21, it seems?

eddyb · 2020-08-16T15:39:22Z

It works in Rust 1.21, fails with 1.22 - 1.35, and then works again since 1.36. Wtf.

Hmm, if it broke in between, that means we weren't promoting it, and it should've stayed that way (hopefully that means crater won't find anything. What did 1.36 do to start promoting it again? Sounds like a bug.

Well it did also accept something, until 1.21, it seems?

No, I think 1.21 must've been when I introduced the "outermost scope" rule, i.e. these two started being different:

const X: &i32 = &0; (inherently &'static, even without promotion)
const Y: Option<&i32> = Some(&0); (requires promotion)

RalfJung · 2020-08-16T15:42:26Z

Rust 1.36 is when we got "NLL for Rust 2015", not sure if that would have an effect? Maybe this indicates a difference between HIR and MIR promotion / borrowck?

RalfJung · 2020-08-16T15:52:07Z

can you add a test that compiled before and now doesn't?
E.g.: static mut FOO: Option<&mut [i32]> = Some(&mut [1, 2, 3]);

Done.

eddyb · 2020-08-16T16:14:05Z

Rust 1.36 is when we got "NLL for Rust 2015", not sure if that would have an effect? Maybe this indicates a difference between HIR and MIR promotion / borrowck?

Were you testing with --edition=2018 or not? You can also add #![feature(nll)] to force it but if you're trying on stable... at least on godbolt there's no way to set RUSTC_BOOTSTRAP=1 I don't think.

Anyway I was expecting a change to MIR promotion, but maybe MIR promotion was wrong all along?

Okay I went and checked, and indeed it looks like MIR borrowck is the culprit here, which explains why we ended up with this bug: the MIR version of the code was just never 100% correct, so if we were trying to maintain its behavior, we were keeping original bugs around.

RalfJung · 2020-08-16T16:19:48Z

it looks like MIR borrowck is the culprit here, which explains why we ended up with this bug: the MIR version of the code was just never 100% correct, so if we were trying to maintain its behavior, we were keeping original bugs around.

That makes sense.
Now I wonder if there are more such bugs...

craterbot · 2020-09-04T16:38:09Z

🚧 Experiment pr-75585 is now running

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

craterbot · 2020-09-07T04:20:20Z

🎉 Experiment pr-75585 is completed!
📊 11 regressed and 3 fixed (117681 total)
📰 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

RalfJung · 2020-09-07T07:34:11Z

That's great, there wasn't a single actual regression. :)
@oli-obk @eddyb how do we go on here?

oli-obk · 2020-09-07T10:02:12Z

@bors r+

Bugfix, simpler compiler code and no regressions, I see no reason to not merge this.

bors · 2020-09-07T10:02:13Z

📌 Commit 28ddda7 has been approved by oli-obk

Do not promote &mut of a non-ZST ever Since ~pre-1.0~ 1.36, we have accepted code like this: ```rust static mut TEST: &'static mut [i32] = { let x = &mut [1,2,3]; x }; ``` I tracked it back to rust-lang#21744, but unfortunately could not find any discussion or RFC that would explain why we thought this was a good idea. And it's not, it breaks all sorts of things -- see rust-lang#75556. To fix rust-lang#75556, we have to stop promoting non-ZST mutable references no matter the context, which is what this PR does. It's a breaking change. Notice that this still works, since it does not rely on promotion: ```rust static mut TEST: &'static mut [i32] = &mut [0,1,2]; ``` Cc @rust-lang/wg-const-eval

bors · 2020-09-08T05:13:49Z

⌛ Testing commit 28ddda7 with merge e82584a...

bors · 2020-09-08T07:08:44Z

☀️ Test successful - checks-actions, checks-azure
Approved by: oli-obk
Pushing e82584a to master...

RalfJung · 2020-09-08T12:19:21Z

I marked this as relnotes but I am not fully sure -- I'll leave it up to the release team if they think this should be mentioned or not.

…tatic-morse Some promotion cleanup Based on top of both rust-lang#75502 and rust-lang#75585, this does some cleanup of the promotion code. The last 2 commits are new. * Remove the remaining cases where `const fn` is treated different from `fn`. This means no longer promoting ptr-to-int casts, raw ptr operations, and union field accesses in `const fn` -- or anywhere, for that matter. These are all unstable in const-context so this should not break any stable code. Fixes rust-lang#75586. * ~~Promote references to statics even outside statics (i.e., in functions) for consistency.~~ * Promote `&mut []` everywhere, not just in non-`const` functions, for consistency. * Explain why we do not promote deref's of statics outside statics. ~~(This is the only remaining direct user of `const_kind`.)~~ This can only land once the other two PRs land; I am mostly putting this up already because I couldn't wait ;) and to get some feedback from @rust-lang/wg-const-eval .

…tic-morse Some promotion cleanup Based on top of both rust-lang#75502 and rust-lang#75585, this does some cleanup of the promotion code. The last 2 commits are new. * Remove the remaining cases where `const fn` is treated different from `fn`. This means no longer promoting ptr-to-int casts, raw ptr operations, and union field accesses in `const fn` -- or anywhere, for that matter. These are all unstable in const-context so this should not break any stable code. Fixes rust-lang#75586. * ~~Promote references to statics even outside statics (i.e., in functions) for consistency.~~ * Promote `&mut []` everywhere, not just in non-`const` functions, for consistency. * Explain why we do not promote deref's of statics outside statics. ~~(This is the only remaining direct user of `const_kind`.)~~ This can only land once the other two PRs land; I am mostly putting this up already because I couldn't wait ;) and to get some feedback from `@rust-lang/wg-const-eval` .

Clean up some of the pkgsrc Makefile, there's still lots in here that should just be deleted though. Switch SunOS to the illumos bootstrap by default. Version 1.48.0 (2020-11-19) ========================== Language -------- - [The `unsafe` keyword is now syntactically permitted on modules.][75857] This is still rejected *semantically*, but can now be parsed by procedural macros. Compiler -------- - [Stabilised the `-C link-self-contained=<yes|no>` compiler flag.][76158] This tells `rustc` whether to link its own C runtime and libraries or to rely on a external linker to find them. (Supported only on `windows-gnu`, `linux-musl`, and `wasi` platforms.) - [You can now use `-C target-feature=+crt-static` on `linux-gnu` targets.][77386] Note: If you're using cargo you must explicitly pass the `--target` flag. - [Added tier 2\* support for `aarch64-unknown-linux-musl`.][76420] \* Refer to Rust's [platform support page][forge-platform-support] for more information on Rust's tiered platform support. Libraries --------- - [`io::Write` is now implemented for `&ChildStdin` `&Sink`, `&Stdout`, and `&Stderr`.][76275] - [All arrays of any length now implement `TryFrom<Vec<T>>`.][76310] - [The `matches!` macro now supports having a trailing comma.][74880] - [`Vec<A>` now implements `PartialEq<[B]>` where `A: PartialEq<B>`.][74194] - [The `RefCell::{replace, replace_with, clone}` methods now all use `#[track_caller]`.][77055] Stabilized APIs --------------- - [`slice::as_ptr_range`] - [`slice::as_mut_ptr_range`] - [`VecDeque::make_contiguous`] - [`future::pending`] - [`future::ready`] The following previously stable methods are now `const fn`'s: - [`Option::is_some`] - [`Option::is_none`] - [`Option::as_ref`] - [`Result::is_ok`] - [`Result::is_err`] - [`Result::as_ref`] - [`Ordering::reverse`] - [`Ordering::then`] Cargo ----- Rustdoc ------- - [You can now link to items in `rustdoc` using the intra-doc link syntax.][74430] E.g. ``/// Uses [`std::future`]`` will automatically generate a link to `std::future`'s documentation. See ["Linking to items by name"][intradoc-links] for more information. - [You can now specify `#[doc(alias = "<alias>")]` on items to add search aliases when searching through `rustdoc`'s UI.][75740] Compatibility Notes ------------------- - [Promotion of references to `'static` lifetime inside `const fn` now follows the same rules as inside a `fn` body.][75502] In particular, `&foo()` will not be promoted to `'static` lifetime any more inside `const fn`s. - [Associated type bindings on trait objects are now verified to meet the bounds declared on the trait when checking that they implement the trait.][27675] - [When trait bounds on associated types or opaque types are ambiguous, the compiler no longer makes an arbitrary choice on which bound to use.][54121] - [Fixed recursive nonterminals not being expanded in macros during pretty-print/reparse check.][77153] This may cause errors if your macro wasn't correctly handling recursive nonterminal tokens. - [`&mut` references to non zero-sized types are no longer promoted.][75585] - [`rustc` will now warn if you use attributes like `#[link_name]` or `#[cold]` in places where they have no effect.][73461] - [Updated `_mm256_extract_epi8` and `_mm256_extract_epi16` signatures in `arch::{x86, x86_64}` to return `i32` to match the vendor signatures.][73166] - [`mem::uninitialized` will now panic if any inner types inside a struct or enum disallow zero-initialization.][71274] - [`#[target_feature]` will now error if used in a place where it has no effect.][78143] - [Foreign exceptions are now caught by `catch_unwind` and will cause an abort.][70212] Note: This behaviour is not guaranteed and is still considered undefined behaviour, see the [`catch_unwind`] documentation for further information. Internal Only ------------- These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools. - [Building `rustc` from source now uses `ninja` by default over `make`.][74922] You can continue building with `make` by setting `ninja=false` in your `config.toml`. - [cg_llvm: `fewer_names` in `uncached_llvm_type`][76030] - [Made `ensure_sufficient_stack()` non-generic][76680] [78143]: rust-lang/rust#78143 [76680]: rust-lang/rust#76680 [76030]: rust-lang/rust#76030 [70212]: rust-lang/rust#70212 [27675]: rust-lang/rust#27675 [54121]: rust-lang/rust#54121 [71274]: rust-lang/rust#71274 [77386]: rust-lang/rust#77386 [77153]: rust-lang/rust#77153 [77055]: rust-lang/rust#77055 [76275]: rust-lang/rust#76275 [76310]: rust-lang/rust#76310 [76420]: rust-lang/rust#76420 [76158]: rust-lang/rust#76158 [75857]: rust-lang/rust#75857 [75585]: rust-lang/rust#75585 [75740]: rust-lang/rust#75740 [75502]: rust-lang/rust#75502 [74880]: rust-lang/rust#74880 [74922]: rust-lang/rust#74922 [74430]: rust-lang/rust#74430 [74194]: rust-lang/rust#74194 [73461]: rust-lang/rust#73461 [73166]: rust-lang/rust#73166 [intradoc-links]: https://doc.rust-lang.org/rustdoc/linking-to-items-by-name.html [`catch_unwind`]: https://doc.rust-lang.org/std/panic/fn.catch_unwind.html [`Option::is_some`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_some [`Option::is_none`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_none [`Option::as_ref`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.as_ref [`Result::is_ok`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_ok [`Result::is_err`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_err [`Result::as_ref`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.as_ref [`Ordering::reverse`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.reverse [`Ordering::then`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.then [`slice::as_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_ptr_range [`slice::as_mut_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_mut_ptr_range [`VecDeque::make_contiguous`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.make_contiguous [`future::pending`]: https://doc.rust-lang.org/std/future/fn.pending.html [`future::ready`]: https://doc.rust-lang.org/std/future/fn.ready.html

Pkgsrc changes: * Compensate for files being moved around upstream. * Introduce optional, on-by-default semi-static building of cargo, using the internal curl and openssl sources. This reduces the dynamic dependencies of cargo and therefore the rust package itself. Ref. options.mk. * The 1.47.0 bootstrap kits have been re-built with the above option turned on, so no longer depends on curl or openssl from pkgsrc and/or from earlier OS or pkgsrc versions. This should hopefully fix installation of rust with non-default PREFIX, ref. PR#54453. Upstream changes: Version 1.48.0 (2020-11-19) ========================== Language -------- - [The `unsafe` keyword is now syntactically permitted on modules.][75857] This is still rejected *semantically*, but can now be parsed by procedural macros. Compiler -------- - [Stabilised the `-C link-self-contained=<yes|no>` compiler flag.][76158] This tells `rustc` whether to link its own C runtime and libraries or to rely on a external linker to find them. (Supported only on `windows-gnu`, `linux-musl`, and `wasi` platforms.) - [You can now use `-C target-feature=+crt-static` on `linux-gnu` targets.] [77386] Note: If you're using cargo you must explicitly pass the `--target` flag. - [Added tier 2\* support for `aarch64-unknown-linux-musl`.][76420] \* Refer to Rust's [platform support page][forge-platform-support] for more information on Rust's tiered platform support. Libraries --------- - [`io::Write` is now implemented for `&ChildStdin` `&Sink`, `&Stdout`, and `&Stderr`.][76275] - [All arrays of any length now implement `TryFrom<Vec<T>>`.][76310] - [The `matches!` macro now supports having a trailing comma.][74880] - [`Vec<A>` now implements `PartialEq<[B]>` where `A: PartialEq<B>`.][74194] - [The `RefCell::{replace, replace_with, clone}` methods now all use `#[track_caller]`.][77055] Stabilized APIs --------------- - [`slice::as_ptr_range`] - [`slice::as_mut_ptr_range`] - [`VecDeque::make_contiguous`] - [`future::pending`] - [`future::ready`] The following previously stable methods are now `const fn`'s: - [`Option::is_some`] - [`Option::is_none`] - [`Option::as_ref`] - [`Result::is_ok`] - [`Result::is_err`] - [`Result::as_ref`] - [`Ordering::reverse`] - [`Ordering::then`] Cargo ----- Rustdoc ------- - [You can now link to items in `rustdoc` using the intra-doc link syntax.][74430] E.g. ``/// Uses [`std::future`]`` will automatically generate a link to `std::future`'s documentation. See ["Linking to items by name"][intradoc-links] for more information. - [You can now specify `#[doc(alias = "<alias>")]` on items to add search aliases when searching through `rustdoc`'s UI.][75740] Compatibility Notes ------------------- - [Promotion of references to `'static` lifetime inside `const fn` now follows the same rules as inside a `fn` body.][75502] In particular, `&foo()` will not be promoted to `'static` lifetime any more inside `const fn`s. - [Associated type bindings on trait objects are now verified to meet the bounds declared on the trait when checking that they implement the trait.][27675] - [When trait bounds on associated types or opaque types are ambiguous, the compiler no longer makes an arbitrary choice on which bound to use.][54121] - [Fixed recursive nonterminals not being expanded in macros during pretty-print/reparse check.][77153] This may cause errors if your macro wasn't correctly handling recursive nonterminal tokens. - [`&mut` references to non zero-sized types are no longer promoted.][75585] - [`rustc` will now warn if you use attributes like `#[link_name]` or `#[cold]` in places where they have no effect.][73461] - [Updated `_mm256_extract_epi8` and `_mm256_extract_epi16` signatures in `arch::{x86, x86_64}` to return `i32` to match the vendor signatures.][73166] - [`mem::uninitialized` will now panic if any inner types inside a struct or enum disallow zero-initialization.][71274] - [`#[target_feature]` will now error if used in a place where it has no effect.][78143] - [Foreign exceptions are now caught by `catch_unwind` and will cause an abort.][70212] Note: This behaviour is not guaranteed and is still considered undefined behaviour, see the [`catch_unwind`] documentation for further information. Internal Only ------------- These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools. - [Building `rustc` from source now uses `ninja` by default over `make`.][74922] You can continue building with `make` by setting `ninja=false` in your `config.toml`. - [cg_llvm: `fewer_names` in `uncached_llvm_type`][76030] - [Made `ensure_sufficient_stack()` non-generic][76680] [78143]: rust-lang/rust#78143 [76680]: rust-lang/rust#76680 [76030]: rust-lang/rust#76030 [70212]: rust-lang/rust#70212 [27675]: rust-lang/rust#27675 [54121]: rust-lang/rust#54121 [71274]: rust-lang/rust#71274 [77386]: rust-lang/rust#77386 [77153]: rust-lang/rust#77153 [77055]: rust-lang/rust#77055 [76275]: rust-lang/rust#76275 [76310]: rust-lang/rust#76310 [76420]: rust-lang/rust#76420 [76158]: rust-lang/rust#76158 [75857]: rust-lang/rust#75857 [75585]: rust-lang/rust#75585 [75740]: rust-lang/rust#75740 [75502]: rust-lang/rust#75502 [74880]: rust-lang/rust#74880 [74922]: rust-lang/rust#74922 [74430]: rust-lang/rust#74430 [74194]: rust-lang/rust#74194 [73461]: rust-lang/rust#73461 [73166]: rust-lang/rust#73166 [intradoc-links]: https://doc.rust-lang.org/rustdoc/linking-to-items-by-name.html [`catch_unwind`]: https://doc.rust-lang.org/std/panic/fn.catch_unwind.html [`Option::is_some`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_some [`Option::is_none`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_none [`Option::as_ref`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.as_ref [`Result::is_ok`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_ok [`Result::is_err`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_err [`Result::as_ref`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.as_ref [`Ordering::reverse`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.reverse [`Ordering::then`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.then [`slice::as_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_ptr_range [`slice::as_mut_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_mut_ptr_range [`VecDeque::make_contiguous`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.make_contiguous [`future::pending`]: https://doc.rust-lang.org/std/future/fn.pending.html [`future::ready`]: https://doc.rust-lang.org/std/future/fn.ready.html

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add compile-fail test for &mut promotion

28ddda7

RalfJung force-pushed the demotion branch from f78034e to 28ddda7 Compare September 6, 2020 12:14

RalfJung mentioned this pull request Sep 6, 2020

Some promotion cleanup #76411

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Rollup of 10 pull requests #76456

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Permit mutable references in all const contexts #78578

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Do not promote &mut of a non-ZST ever #75585

Do not promote &mut of a non-ZST ever #75585

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