Stabilize `str::len`, `[T]::len` and `str::as_bytes` as const fn #63770

oli-obk · 2019-08-21T04:52:17Z

r? @Centril

cc @RalfJung

This also introduces a scheme for making certain feature gates legal in stabilized const fns

src/libcore/lib.rs

src/libcore/slice/mod.rs

src/libcore/str/mod.rs

src/libcore/slice/mod.rs

src/test/ui/consts/const-eval/strlen.rs

src/librustc_mir/transform/qualify_min_const_fn.rs

src/test/ui/consts/min_const_fn/min_const_fn_unsafe_bad.stderr

Centril · 2019-08-21T11:32:45Z

cc also @eddyb

src/libsyntax/feature_gate.rs

oli-obk · 2019-08-21T15:52:41Z

all review points have been addressed

Centril · 2019-08-22T22:40:58Z

Dear libs and language teams and the community at large...

...I propose that we stabilize the following functions as const fn as it seems clear (to me) that committing to these being const fn is a safe commitment:

str::len, str::is_empty, and str::as_bytes
[T]::len and [T]::is_empty.

The language team element in this FCP consists of accepting the internal use of #[allow_internal_unstable(const_fn_union)] as seen throughout this PR.

We are not stabilizing const_fn_union at this time because it can be used to transmute between types. This in turn would allow you to encode the operation *[const|mut] T -> usize which can be used to observe addresses during execution. However, a const fn must retain deterministic execution even at run-time as laid out by @RalfJung: - "CTFE is correct if, when it loops forever, completes with a result, or panics, that behavior matches the run-time behavior of the same code.". As such, a const fn which behaves non-deterministically at run-time has undefined behavior. At this point in time we lack sufficient educational material to stabilize this for everyone but can still make internal use of it.

Our policy-to-be re. allow_internal_unstable with regards to const fn is documented here. Additionally, we would appreciate that you always ping @oli-obk, @RalfJung, and @Centril if you are adding more allow_internal_unstable attributes to any const fn.

@rfcbot merge

rfcbot · 2019-08-22T22:43:54Z

src/libcore/slice/mod.rs

src/librustc_mir/transform/qualify_min_const_fn.rs

RalfJung · 2019-08-24T13:04:36Z

However, a const fn must retain deterministic execution even at run-time

To elaborate on that, here's a "bad" const fn:

const fn mk_int() -> usize {
  let v = 42;
  &v as *const _ as usize
}

If we allow raw-to-int casts (or union accesses or transmute, both of which let users implement said casts), such a const fn can be written by stable (unsafe) code.

What's bad about that function is that it would be desirable for unsafe code, and maybe even for the compiler, to rely on the fact that a function with signature const fn() -> usize will always return the same thing, even when called at run-time.

Centril · 2019-08-24T18:40:30Z

To elaborate on that, here's a "bad" const fn:

@oli-obk might be a good idea to add to the rustc guide PR also.

src/libcore/str/mod.rs

Centril · 2019-09-01T10:17:55Z

src/libsyntax/attr/mod.rs

@@ -435,6 +435,30 @@ pub fn find_by_name(attrs: &[Attribute], name: Symbol) -> Option<&Attribute> {
    attrs.iter().find(|attr| attr.check_name(name))
 }

+pub fn allow_internal_unstable<'a>(


Suggested change

pub fn allow_internal_unstable<'a>(

pub fn allow_internal_unstable_list<'a>(

(suggests an action performed as currently named)

Centril · 2019-09-01T10:19:11Z

r=me with comment above addressed and #63770 (comment) incorporated into the rustc guide once FCP has completed.

oli-obk · 2019-09-01T11:31:09Z

Ping @Kimundi @joshtriplett @nikomatsakis @pnkfelix @withoutboats the FCP is waiting on an action by you

bors · 2019-09-24T10:41:37Z

⌛ Testing commit 6324d165fbaa0c2d4f4c36143f7fd27a655187da with merge 27c7b4f41f73f366159bb0fa8f571c3c7f962876...

rust-highfive · 2019-09-24T10:50:31Z

The job dist-i686-freebsd of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

2019-09-24T10:48:04.4327844Z    Compiling std v0.0.0 (/checkout/src/libstd)
2019-09-24T10:48:04.7436576Z error[E0133]: access to union field is unsafe and requires unsafe function or block
2019-09-24T10:48:04.7437069Z   --> src/libcore/slice/mod.rs:70:13
2019-09-24T10:48:04.7437752Z    |
2019-09-24T10:48:04.7438101Z 70 |             crate::ptr::Repr { rust: self }.raw.len
2019-09-24T10:48:04.7438712Z    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ access to union field
2019-09-24T10:48:04.7439000Z    |
2019-09-24T10:48:04.7439347Z    = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
2019-09-24T10:48:04.7471984Z error[E0133]: access to union field is unsafe and requires unsafe function or block
2019-09-24T10:48:04.7472326Z     --> src/libcore/str/mod.rs:2180:18
2019-09-24T10:48:04.7472545Z      |
2019-09-24T10:48:04.7472545Z      |
2019-09-24T10:48:04.7472880Z 2180 |         unsafe { Slices { str: self }.slice }
2019-09-24T10:48:04.7473252Z      |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^ access to union field
2019-09-24T10:48:04.7473490Z      |
2019-09-24T10:48:04.7474038Z      = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
2019-09-24T10:48:04.7519607Z error: aborting due to 2 previous errors
2019-09-24T10:48:04.7519742Z 
2019-09-24T10:48:04.7528249Z For more information about this error, try `rustc --explain E0133`.
2019-09-24T10:48:04.8476969Z [RUSTC-TIMING] build_script_build test:false 0.410
---
2019-09-24T10:48:05.3772048Z == clock drift check ==
2019-09-24T10:48:05.3784430Z   local time: Tue Sep 24 10:48:05 UTC 2019
2019-09-24T10:48:05.5310826Z   network time: Tue, 24 Sep 2019 10:48:05 GMT
2019-09-24T10:48:05.5317046Z == end clock drift check ==
2019-09-24T10:48:09.6491482Z ##[error]Bash exited with code '1'.
2019-09-24T10:48:09.6527647Z ##[section]Starting: Upload CPU usage statistics
2019-09-24T10:48:09.6531121Z ==============================================================================
2019-09-24T10:48:09.6531198Z Task         : Bash
2019-09-24T10:48:09.6531281Z Description  : Run a Bash script on macOS, Linux, or Windows

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

bors · 2019-09-24T10:50:44Z

💔 Test failed - checks-azure

…onst fn

Centril · 2019-09-24T10:58:02Z

@bors p=20 r+

bors · 2019-09-24T10:58:03Z

📌 Commit 7767e7f has been approved by Centril

bors · 2019-09-24T10:58:49Z

⌛ Testing commit 7767e7f with merge 6ef275e...

@Centril

Stabilize `str::len`, `[T]::len` and `str::as_bytes` as const fn r? @Centril cc @RalfJung This also introduces a scheme for making certain feature gates legal in stabilized const fns

bors · 2019-09-24T14:50:21Z

☀️ Test successful - checks-azure
Approved by: Centril
Pushing 6ef275e to master...

Pkgsrc changes: * Remove patch which no longer applies (but what about RPATH?) * Adapt a few patches to changed files upstream. Upstream changes: Version 1.39.0 (2019-11-07) =========================== Language -------- - [You can now create `async` functions and blocks with `async fn`, `async move {}`, and `async {}` respectively, and you can now call `.await` on async expressions.][63209] - [You can now use certain attributes on function, closure, and function pointer parameters.][64010] These attributes include `cfg`, `cfg_attr`, `allow`, `warn`, `deny`, `forbid` as well as inert helper attributes used by procedural macro attributes applied to items. e.g. ```rust fn len( #[cfg(windows)] slice: &[u16], #[cfg(not(windows))] slice: &[u8], ) -> usize { slice.len() } ``` - [You can now take shared references to bind-by-move patterns in the `if` guards of `match` arms.][63118] e.g. ```rust fn main() { let array: Box<[u8; 4]> = Box::new([1, 2, 3, 4]); match array { nums // ---- `nums` is bound by move. if nums.iter().sum::<u8>() == 10 // ^------ `.iter()` implicitly takes a reference to `nums`. => { drop(nums); // ----------- Legal as `nums` was bound by move and so we have ownership. } _ => unreachable!(), } } ``` Compiler -------- - [Added tier 3\* support for the `i686-unknown-uefi` target.][64334] - [Added tier 3 support for the `sparc64-unknown-openbsd` target.][63595] - [rustc will now trim code snippets in diagnostics to fit in your terminal.] [63402] **Note** Cargo currently doesn't use this feature. Refer to [cargo#7315][cargo/7315] to track this feature's progress. - [You can now pass `--show-output` argument to test binaries to print the output of successful tests.][62600] \* Refer to Rust's [platform support page][forge-platform-support] for more information on Rust's tiered platform support. Libraries --------- - [`Vec::new` and `String::new` are now `const` functions.][64028] - [`LinkedList::new` is now a `const` function.][63684] - [`str::len`, `[T]::len` and `str::as_bytes` are now `const` functions.][63770] - [The `abs`, `wrapping_abs`, and `overflowing_abs` numeric functions are now `const`.][63786] Stabilized APIs --------------- - [`Pin::into_inner`] - [`Instant::checked_duration_since`] - [`Instant::saturating_duration_since`] Cargo ----- - [You can now publish git dependencies if supplied with a `version`.] [cargo/7237] - [The `--all` flag has been renamed to `--workspace`.][cargo/7241] Using `--all` is now deprecated. Misc ---- - [You can now pass `-Clinker` to rustdoc to control the linker used for compiling doctests.][63834] Compatibility Notes ------------------- - [Code that was previously accepted by the old borrow checker, but rejected by the NLL borrow checker is now a hard error in Rust 2018.][63565] This was previously a warning, and will also become a hard error in the Rust 2015 edition in the 1.40.0 release. - [`rustdoc` now requires `rustc` to be installed and in the same directory to run tests.][63827] This should improve performance when running a large amount of doctests. - [The `try!` macro will now issue a deprecation warning.][62672] It is recommended to use the `?` operator instead. - [`asinh(-0.0)` now correctly returns `-0.0`.][63698] Previously this returned `0.0`. [62600]: rust-lang/rust#62600 [62672]: rust-lang/rust#62672 [63118]: rust-lang/rust#63118 [63209]: rust-lang/rust#63209 [63402]: rust-lang/rust#63402 [63565]: rust-lang/rust#63565 [63595]: rust-lang/rust#63595 [63684]: rust-lang/rust#63684 [63698]: rust-lang/rust#63698 [63770]: rust-lang/rust#63770 [63786]: rust-lang/rust#63786 [63827]: rust-lang/rust#63827 [63834]: rust-lang/rust#63834 [63927]: rust-lang/rust#63927 [63933]: rust-lang/rust#63933 [63934]: rust-lang/rust#63934 [63938]: rust-lang/rust#63938 [63940]: rust-lang/rust#63940 [63941]: rust-lang/rust#63941 [63945]: rust-lang/rust#63945 [64010]: rust-lang/rust#64010 [64028]: rust-lang/rust#64028 [64334]: rust-lang/rust#64334 [cargo/7237]: rust-lang/cargo#7237 [cargo/7241]: rust-lang/cargo#7241 [cargo/7315]: rust-lang/cargo#7315 [`Pin::into_inner`]: https://doc.rust-lang.org/std/pin/struct.Pin.html#method.into_inner [`Instant::checked_duration_since`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.checked_duration_since [`Instant::saturating_duration_since`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.saturating_duration_since

@oli-obk

…r=Centril Don't require `allow_internal_unstable` unless `staged_api` is enabled. rust-lang#63770 changed `qualify_min_const_fn` to require `allow_internal_unstable` for *all* crates that used an unstable feature, regardless of whether `staged_api` was enabled or the `fn` that used that feature was stably const. In practice, this meant that every crate in the ecosystem that wanted to use nightly features added `#![feature(const_fn)]`, which skips `qualify_min_const_fn` entirely. After this PR, crates that do not have `#![feature(staged_api)]` will only need to enable the feature they are interested in. For example, `#![feature(const_if_match)]` will be enough to enable `if` and `match` in constants. Crates with `staged_api` (e.g., `libstd`) require `#[allow_internal_unstable]` to be added to a function if it uses nightly features unless that function is also marked `#[rustc_const_unstable]`. This prevents proliferation of `#[allow_internal_unstable]` into functions that are not callable in a `const` context on stable. r? @oli-obk (author of rust-lang#63770) cc @Centril

@oli-obk

…r=Centril Don't require `allow_internal_unstable` unless `staged_api` is enabled. rust-lang#63770 changed `qualify_min_const_fn` to require `allow_internal_unstable` for *all* crates that used an unstable feature, regardless of whether `staged_api` was enabled or the `fn` that used that feature was stably const. In practice, this meant that every crate in the ecosystem that wanted to use nightly features added `#![feature(const_fn)]`, which skips `qualify_min_const_fn` entirely. After this PR, crates that do not have `#![feature(staged_api)]` will only need to enable the feature they are interested in. For example, `#![feature(const_if_match)]` will be enough to enable `if` and `match` in constants. Crates with `staged_api` (e.g., `libstd`) require `#[allow_internal_unstable]` to be added to a function if it uses nightly features unless that function is also marked `#[rustc_const_unstable]`. This prevents proliferation of `#[allow_internal_unstable]` into functions that are not callable in a `const` context on stable. r? @oli-obk (author of rust-lang#63770) cc @Centril

@oli-obk

…r=Centril Don't require `allow_internal_unstable` unless `staged_api` is enabled. rust-lang#63770 changed `qualify_min_const_fn` to require `allow_internal_unstable` for *all* crates that used an unstable feature, regardless of whether `staged_api` was enabled or the `fn` that used that feature was stably const. In practice, this meant that every crate in the ecosystem that wanted to use nightly features added `#![feature(const_fn)]`, which skips `qualify_min_const_fn` entirely. After this PR, crates that do not have `#![feature(staged_api)]` will only need to enable the feature they are interested in. For example, `#![feature(const_if_match)]` will be enough to enable `if` and `match` in constants. Crates with `staged_api` (e.g., `libstd`) require `#[allow_internal_unstable]` to be added to a function if it uses nightly features unless that function is also marked `#[rustc_const_unstable]`. This prevents proliferation of `#[allow_internal_unstable]` into functions that are not callable in a `const` context on stable. r? @oli-obk (author of rust-lang#63770) cc @Centril

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geofft mentioned this pull request Sep 1, 2019

Use stable Rust (tracking ticket for unstable features we use) fishinabarrel/linux-kernel-module-rust#41

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Stabilize str::len, [T]::len, is_empty and str::as_bytes as c…

7767e7f

…onst fn

oli-obk force-pushed the allow_internal_unstable branch from 6324d16 to 7767e7f Compare September 24, 2019 10:56

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-review Status: Awaiting review from the assignee but also interested parties. labels Sep 24, 2019

bors added the merged-by-bors This PR was explicitly merged by bors. label Sep 24, 2019

bors merged commit 7767e7f into rust-lang:master Sep 24, 2019

taiki-e mentioned this pull request Oct 6, 2019

Remove const_str_as_bytes feature gate rust-lang/stdarch#819

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nvzqz mentioned this pull request Nov 3, 2019

Meta tracking issue for const fn #57563

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ebroto mentioned this pull request Jan 2, 2020

Bandwhich does not compile on Ubuntu 18.04 imsnif/bandwhich#62

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ecstatic-morse mentioned this pull request Jan 11, 2020

Don't require allow_internal_unstable unless staged_api is enabled. #68114

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ecstatic-morse mentioned this pull request Feb 24, 2020

#[allow_internal_unstable] does not allow unstable library features when applied on functions #69399

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Centril mentioned this pull request Feb 26, 2020

Stabilize const for integer {to,from}_{be,le,ne}_bytes methods #69373

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tbu- mentioned this pull request Feb 14, 2021

str::as_bytes and friends have incorrect const stabilization version annotation #82085

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oli-obk deleted the allow_internal_unstable branch March 16, 2021 12:13

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Stabilize `str::len`, `[T]::len` and `str::as_bytes` as const fn #63770

Stabilize `str::len`, `[T]::len` and `str::as_bytes` as const fn #63770

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	pub fn allow_internal_unstable<'a>(
	pub fn allow_internal_unstable_list<'a>(

Stabilize str::len, [T]::len and str::as_bytes as const fn #63770

Stabilize str::len, [T]::len and str::as_bytes as const fn #63770

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Stabilize `str::len`, `[T]::len` and `str::as_bytes` as const fn #63770

Stabilize `str::len`, `[T]::len` and `str::as_bytes` as const fn #63770

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