liballoc: introduce String, Vec const-slicing #128399

mammothbane · 2024-07-30T14:45:32Z

This change const-qualifies many methods on Vec and String, notably as_slice, as_str, len. These changes are made behind the unstable feature flag const_vec_string_slice.

Motivation

This is to support simultaneous variance over ownership and constness. I have an enum type that may contain either String or &str, and I want to produce a &str from it in a possibly-const context.

enum StrOrString<'s> {
    Str(&'s str),
    String(String),
}

impl<'s> StrOrString<'s> {
    const fn as_str(&self) -> &str {
        match self {
             // In a const-context, I really only expect to see this variant, but I can't switch the implementation
             // in some mode like #[cfg(const)] -- there has to be a single body
             Self::Str(s) => s,

             // so this is a problem, since it's not `const`
             Self::String(s) => s.as_str(), 
        }
    }
}

Currently String and Vec don't support this, but can without functional changes. Similar logic applies for len, capacity, is_empty.

Changes

The essential thing enabling this change is that Unique::as_ptr is const. This lets us convert RawVec::ptr -> Vec::as_ptr -> Vec::as_slice -> String::as_str.

I had to move the Deref implementations into as_{str,slice} because Deref isn't #[const_trait], but I would expect this change to be invisible up to inlining. I moved the DerefMut implementations as well for uniformity.

rustbot · 2024-07-30T14:45:44Z

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tgross35 · 2024-07-30T15:29:02Z

This shouldn't be implemented as new functions, the existing as_ptr, len etc just get a const keyword and a #[rustc_const_unstable(...)] attribute.

I am not sure that the motivation is that strong but it seems harmless to unstably add const to most of the non-&mut methods. This is an API change though so

r? libs-api

edit: wow, this turned out to be a popular comment for some reason 😄

tgross35 · 2024-07-30T21:15:41Z

You may as well change the implementation before libs-api takes a look since it's pretty easy, and much of this PR as-is won't be needed. Please do the updates to rustc_const_unstable as mentioned above. Including len, is_empty, as_bytes, and as_slice seems reasonable too.

@rustbot author

mammothbane · 2024-07-31T17:59:38Z

Thanks for the review! Made those changes and added capacity on the same logic.

@rustbot review

bors · 2024-08-12T04:59:03Z

☔ The latest upstream changes (presumably #126793) made this pull request unmergeable. Please resolve the merge conflicts.

Amanieu · 2024-08-11T13:52:45Z

library/alloc/src/string.rs

@@ -1006,7 +1006,8 @@ impl String {
    #[inline]
    #[must_use = "`self` will be dropped if the result is not used"]
    #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn into_bytes(self) -> Vec<u8> {
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "none")]


This needs a tracking issue so that we can eventually const-stabilize it.

I've created #129041

Can you update all the rustc_const_unstable to point to that tracking issue? Then it should be ready to merge.

I am not the author of the PR, nor am I a member of the org, so I do not have permissions to do so. Ping @mammothbane, as this PR needs a rebase anyways.

GrigorenkoPV · 2024-08-24T22:26:17Z

@rustbot author

mammothbane · 2024-09-04T17:44:47Z

Sorry for the delays here — my dev env broke and it's taken me a while to get around to fixing it and verifying my rebase. Working on it now.

tgross35 · 2024-09-05T17:51:16Z

I know you're probably not done yet, but please make sure you squash at some point

mammothbane · 2024-09-05T18:39:03Z

Squashed -- all changes made afaik.

@rustbot review

bors · 2024-09-14T16:52:56Z

☔ The latest upstream changes (presumably #128299) made this pull request unmergeable. Please resolve the merge conflicts.

Amanieu · 2024-09-15T11:05:02Z

@rust-lang/wg-const-eval I don't think this introduces any const-eval issues, but just double-checking with you.

r=me once conflicts are resolved.

library/alloc/src/string.rs

bors · 2024-09-19T21:41:33Z

☔ The latest upstream changes (presumably #130572) made this pull request unmergeable. Please resolve the merge conflicts.

tgross35 · 2024-09-28T18:24:11Z

library/alloc/src/raw_vec.rs

@@ -498,7 +498,7 @@ impl<A: Allocator> RawVecInner<A> {
    }


Mark fn non_null const, just replace .into() with .as_non_null_ptr() (source). Then fn ptr can be marked const without changing its implementation.

Sorry, not sure why GH put this comment on an empty line but it's meant to go with the ptr and non_null functions.

Anyway, this change plus the safety comments Oli requested and we can merge this.

library/alloc/src/string.rs

library/alloc/src/vec/mod.rs

This change `const`-qualifies many methods on Vec and String, notably `as_slice`, `as_str`, `len`. These changes are made behind the unstable feature flag `const_vec_string_slice` with the following tracking issue: rust-lang#129041

tgross35 · 2024-10-07T01:04:46Z

@bors r=Amanieu,tgross35 rollup

bors · 2024-10-07T01:04:49Z

📌 Commit d793766 has been approved by Amanieu,tgross35

It is now in the queue for this repository.

Rollup of 3 pull requests Successful merges: - rust-lang#128399 (liballoc: introduce String, Vec const-slicing) - rust-lang#131308 (enable f16 and f128 on windows-gnullvm targets) - rust-lang#131325 (coverage: Multiple small tweaks to counter creation) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of rust-lang#128399 - mammothbane:master, r=Amanieu,tgross35 liballoc: introduce String, Vec const-slicing This change `const`-qualifies many methods on `Vec` and `String`, notably `as_slice`, `as_str`, `len`. These changes are made behind the unstable feature flag `const_vec_string_slice`. ## Motivation This is to support simultaneous variance over ownership and constness. I have an enum type that may contain either `String` or `&str`, and I want to produce a `&str` from it in a possibly-`const` context. ```rust enum StrOrString<'s> { Str(&'s str), String(String), } impl<'s> StrOrString<'s> { const fn as_str(&self) -> &str { match self { // In a const-context, I really only expect to see this variant, but I can't switch the implementation // in some mode like #[cfg(const)] -- there has to be a single body Self::Str(s) => s, // so this is a problem, since it's not `const` Self::String(s) => s.as_str(), } } } ``` Currently `String` and `Vec` don't support this, but can without functional changes. Similar logic applies for `len`, `capacity`, `is_empty`. ## Changes The essential thing enabling this change is that `Unique::as_ptr` is `const`. This lets us convert `RawVec::ptr` -> `Vec::as_ptr` -> `Vec::as_slice` -> `String::as_str`. I had to move the `Deref` implementations into `as_{str,slice}` because `Deref` isn't `#[const_trait]`, but I would expect this change to be invisible up to inlining. I moved the `DerefMut` implementations as well for uniformity.

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Tracking Issue for const_vec_string_slice #129041

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liballoc: introduce String, Vec const-slicing

d793766

This change `const`-qualifies many methods on Vec and String, notably `as_slice`, `as_str`, `len`. These changes are made behind the unstable feature flag `const_vec_string_slice` with the following tracking issue: rust-lang#129041

mammothbane force-pushed the master branch from 9a67440 to d793766 Compare October 6, 2024 23:58

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 Oct 7, 2024

Zalathar mentioned this pull request Oct 7, 2024

Rollup of 3 pull requests #131345

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bors merged commit dd4f062 into rust-lang:master Oct 7, 2024
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liballoc: introduce String, Vec const-slicing #128399

liballoc: introduce String, Vec const-slicing #128399

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liballoc: introduce String, Vec const-slicing #128399

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