Stabilize Vec::new and String::new as const fns
#64028
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| /// If you change `RawVec<T>::new` or dependencies, please take care to not | ||
| /// introduce anything that would truly violate `min_const_fn`. | ||
| /// | ||
| /// NOTE: We could avoid this hack and check conformance with some |
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I'm pretty unhappy about this hack since I think carries the risk that we might stabilize things we don't want in const eval inadvertently and I have WIP more principled solution in https://github.com/Centril/rust/commits/stabilize-vec-new-const which I think we should seriously consider instead (but which can change into after this PR if necessary).
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cc @alexcrichton for T-libs FCP |
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@rfcbot fcp merge |
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Team member @alexcrichton has proposed to merge this. The next step is review by the rest of the tagged team members: No concerns currently listed. Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up! See this document for info about what commands tagged team members can give me. |
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🔔 This is now entering its final comment period, as per the review above. 🔔 |
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The final comment period, with a disposition to merge, as per the review above, is now complete. As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed. The RFC will be merged soon. |
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r=me with a rebase |
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r? @alexcrichton @bors r=alexcrichton rollup |
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📌 Commit 9b3e11f has been approved by |
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…excrichton Stabilize `Vec::new` and `String::new` as `const fn`s Closes rust-lang#64022. r? @oli-obk
Rollup of 10 pull requests Successful merges: - #63955 (Make sure interned constants are immutable) - #64028 (Stabilize `Vec::new` and `String::new` as `const fn`s) - #64119 (ci: ensure all tool maintainers are assignable on issues) - #64444 (fix building libstd without backtrace feature) - #64446 (Fix build script sanitizer check.) - #64451 (when Miri tests are not passing, do not add Miri component) - #64467 (Hide diagnostics emitted during --cfg parsing) - #64497 (Don't print the "total" `-Ztime-passes` output if `--prints=...` is also given) - #64499 (Use `Symbol` in two more functions.) - #64504 (use println!() instead of println!("")) Failed merges: r? @ghost
| // FIXME(mark-i-m): use `cap` when ifs are allowed in const | ||
| cap: [0, !0][(mem::size_of::<T>() == 0) as usize], | ||
| a: Global, | ||
| } |
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@ecstatic-morse @oli-obk are you keeping a list somewhere for all the awful hacks we have in libstd to work around const qualif limitations? This would be another one (there's also #63786).
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Maybe we should have a special string for "const related awful hack"? FIXME(awful_const_hack) :D
We can make an issue also...
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We could also just tag PRs like this as const-workaround-if on GitHub, and someone can go through at the tagged PRs once the required operations are allowed.
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If we're going with a label, I'd make it a bit more general: const-hack.
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Presumably there could be a const-hack-loop and const-hack-mut-ref if anyone gets really frisky.
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@ecstatic-morse I was wondering if we couldn't use one label for all of them?
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I don't care so much. I thought it would be easier to go back and remove the labels if we know exactly which limitation is being worked around, but the absence of conditionals is probably the only thing that can be circumvented with these kinds of hacks.
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Added a const-hack label. Please let me know if there are some PRs I missed.
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I have a todo entry for reviewing all const fns after we get loops and conditions, I'm certain we'll miss some by whitelisting what to check
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Does this improve the performance of String and Vec creation? |
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I wouldn't expect it to have any perf impact one way or the other; it's just copying some code. |
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
Closes #64022.
r? @oli-obk