Constify conversion traits (part 1) #145279

clarfonthey · 2025-08-11T23:49:04Z

This is the first part of #144289 being split into smaller pieces. It adds/moves constness of several traits under the const_convert feature:

From
Into
TryFrom
TryInto
FromStr
AsRef
AsMut
Borrow
BorrowMut
Deref
DerefMut

There are a few methods that are intrinsically tied to these traits which I've included in the feature. Particularly, those which are wrappers over AsRef:

ByteStr::new (unstable under bstr feature)
OsStr::new
Path::new

Those which directly use Into:

Result::into_ok
Result::into_err

And those which use Deref and DerefMut:

Pin::as_ref
Pin::as_mut
Pin::as_deref_mut
Option::as_deref
Option::as_deref_mut
Result::as_deref
Result::as_deref_mut

(note: the Option and Result methods were suggested by @npmccallum initially as #146101)

The parts which are missing from this PR are:

Anything that involves heap-allocated types
Making any method const than the ones listed above
Anything that could rely on the above, or could rely on system-specific code for OsStr or Path (note: this mostly makes these methods useless since str doesn't implement AsRef<OsStr> yet, but it's better to track the method for now and add impls later, IMHO)

r? @tgross35 (who mostly already reviewed this)

clarfonthey · 2025-08-11T23:49:41Z

library/alloc/src/lib.rs

 #![feature(const_default)]
 #![feature(const_eval_select)]
 #![feature(const_heap)]
-#![feature(const_trait_impl)]


Was listed under library features, and is actually a language feature. Since the diff doesn't make that clear.

clarfonthey · 2025-08-11T23:50:59Z

library/core/src/char/convert.rs

+        // FIXME(const-hack): this should use map_err instead
+        match u8::try_from(u32::from(c)) {
+            Ok(b) => Ok(b),
+            Err(_) => Err(TryFromCharError(())),
+        }


You mentioned not wanting to add any more const-hacks, but I figured this one (+ for u16 below) was okay because we're already going to have to fix a lot of const code once things are stable, and I think that simple char <-> integer conversions are useful enough to get on nightly sooner. Especially since const closures seem pretty far off at the moment.

bors · 2025-08-13T11:57:46Z

☔ The latest upstream changes (presumably #145334) made this pull request unmergeable. Please resolve the merge conflicts.

clarfonthey · 2025-08-17T18:56:41Z

Rebasing over #144847 since that one conflicts with this one, which is why I was going to do this one before I got to that one, but, it's in the queue now, so, 🤷🏻.

Only changes with the rebase are eliminating more additions of const_from which is renamed to const_convert.

rustbot · 2025-08-17T18:58:13Z

This PR was rebased onto a different master commit! Check out the changes with our range-diff.

bors · 2025-08-20T09:39:35Z

☔ The latest upstream changes (presumably #145644) made this pull request unmergeable. Please resolve the merge conflicts.

clarfonthey · 2025-08-20T19:23:35Z

Will rebase once that PR gets merged. I can mark it as blocked, but it still can be reviewed before to verify you're comfortable with the changes.

clarfonthey · 2025-08-21T00:48:05Z

(Un-rebased over that PR since it's no longer in the queue.)

…oshtriplett `const`ify (the unstable) `str::as_str` Tracking issue: rust-lang#130366 The method was not initially marked `const` presumably because it is only useful with `Deref`. But now that const traits seem to be a thing that can actually become real, why not make it `const`? PR `const`ifying `Deref`: rust-lang#145279

Rollup merge of #145930 - GrigorenkoPV:const_str_as_str, r=joshtriplett `const`ify (the unstable) `str::as_str` Tracking issue: #130366 The method was not initially marked `const` presumably because it is only useful with `Deref`. But now that const traits seem to be a thing that can actually become real, why not make it `const`? PR `const`ifying `Deref`: #145279

rustbot · 2025-08-29T12:52:06Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

clarfonthey · 2025-09-02T01:40:52Z

Incorporated the methods from #146101 and listed them in the issue description.

clarfonthey · 2025-09-02T19:45:21Z

All good! I didn't want to bug you since I figured you were busy. Thanks for getting to this.

bors · 2025-09-03T01:34:15Z

⌛ Testing commit 1c64d3e with merge 72bf547...

Constify conversion traits (part 1) This is the first part of #144289 being split into smaller pieces. It adds/moves constness of several traits under the `const_convert` feature: * `From` * `Into` * `TryFrom` * `TryInto` * `FromStr` * `AsRef` * `AsMut` * `Borrow` * `BorrowMut` * `Deref` * `DerefMut` There are a few methods that are intrinsically tied to these traits which I've included in the feature. Particularly, those which are wrappers over `AsRef`: * `ByteStr::new` (unstable under `bstr` feature) * `OsStr::new` * `Path::new` Those which directly use `Into`: * `Result::into_ok` * `Result::into_err` And those which use `Deref` and `DerefMut`: * `Pin::as_ref` * `Pin::as_mut` * `Pin::as_deref_mut` * `Option::as_deref` * `Option::as_deref_mut` * `Result::as_deref` * `Result::as_deref_mut` (note: the `Option` and `Result` methods were suggested by `@npmccallum` initially as #146101) The parts which are missing from this PR are: * Anything that involves heap-allocated types * Making any method const than the ones listed above * Anything that could rely on the above, *or* could rely on system-specific code for `OsStr` or `Path` (note: this mostly makes these methods useless since `str` doesn't implement `AsRef<OsStr>` yet, but it's better to track the method for now and add impls later, IMHO) r? `@tgross35` (who mostly already reviewed this)

… r=tgross35 Constify conversion traits (part 1) This is the first part of rust-lang#144289 being split into smaller pieces. It adds/moves constness of several traits under the `const_convert` feature: * `From` * `Into` * `TryFrom` * `TryInto` * `FromStr` * `AsRef` * `AsMut` * `Borrow` * `BorrowMut` * `Deref` * `DerefMut` There are a few methods that are intrinsically tied to these traits which I've included in the feature. Particularly, those which are wrappers over `AsRef`: * `ByteStr::new` (unstable under `bstr` feature) * `OsStr::new` * `Path::new` Those which directly use `Into`: * `Result::into_ok` * `Result::into_err` And those which use `Deref` and `DerefMut`: * `Pin::as_ref` * `Pin::as_mut` * `Pin::as_deref_mut` * `Option::as_deref` * `Option::as_deref_mut` * `Result::as_deref` * `Result::as_deref_mut` (note: the `Option` and `Result` methods were suggested by `@npmccallum` initially as rust-lang#146101) The parts which are missing from this PR are: * Anything that involves heap-allocated types * Making any method const than the ones listed above * Anything that could rely on the above, *or* could rely on system-specific code for `OsStr` or `Path` (note: this mostly makes these methods useless since `str` doesn't implement `AsRef<OsStr>` yet, but it's better to track the method for now and add impls later, IMHO) r? `@tgross35` (who mostly already reviewed this)

tgross35 · 2025-09-03T01:56:45Z

@bors retry
Put in a rollup

Rollup of 8 pull requests Successful merges: - #139113 (unstable book: in a sanitizer example, check the code) - #145279 (Constify conversion traits (part 1)) - #145414 (unicode-table-generator refactors) - #145823 (editorconfig: don't use nonexistent syntax) - #145944 (std: Start supporting WASIp2 natively ) - #145961 (resolve: Avoid a regression from splitting prelude into two scopes) - #146032 (Explicity disable LSX feature for `loongarch64-unknown-none` target) - #146106 (fix(lexer): Only allow horizontal whitespace in frontmatter ) r? `@ghost` `@rustbot` modify labels: rollup

… r=tgross35 Constify conversion traits (part 1) This is the first part of rust-lang#144289 being split into smaller pieces. It adds/moves constness of several traits under the `const_convert` feature: * `From` * `Into` * `TryFrom` * `TryInto` * `FromStr` * `AsRef` * `AsMut` * `Borrow` * `BorrowMut` * `Deref` * `DerefMut` There are a few methods that are intrinsically tied to these traits which I've included in the feature. Particularly, those which are wrappers over `AsRef`: * `ByteStr::new` (unstable under `bstr` feature) * `OsStr::new` * `Path::new` Those which directly use `Into`: * `Result::into_ok` * `Result::into_err` And those which use `Deref` and `DerefMut`: * `Pin::as_ref` * `Pin::as_mut` * `Pin::as_deref_mut` * `Option::as_deref` * `Option::as_deref_mut` * `Result::as_deref` * `Result::as_deref_mut` (note: the `Option` and `Result` methods were suggested by `@npmccallum` initially as rust-lang#146101) The parts which are missing from this PR are: * Anything that involves heap-allocated types * Making any method const than the ones listed above * Anything that could rely on the above, *or* could rely on system-specific code for `OsStr` or `Path` (note: this mostly makes these methods useless since `str` doesn't implement `AsRef<OsStr>` yet, but it's better to track the method for now and add impls later, IMHO) r? `@tgross35` (who mostly already reviewed this)

Rollup of 9 pull requests Successful merges: - #145279 (Constify conversion traits (part 1)) - #145414 (unicode-table-generator refactors) - #145823 (editorconfig: don't use nonexistent syntax) - #145944 (std: Start supporting WASIp2 natively ) - #145961 (resolve: Avoid a regression from splitting prelude into two scopes) - #146032 (Explicity disable LSX feature for `loongarch64-unknown-none` target) - #146106 (fix(lexer): Only allow horizontal whitespace in frontmatter ) - #146112 (don't uppercase error messages) - #146154 (CI: rfl: move job forward to Linux v6.17-rc3 plus 2 commits) r? `@ghost` `@rustbot` modify labels: rollup

Rollup of 8 pull requests Successful merges: - #145279 (Constify conversion traits (part 1)) - #145414 (unicode-table-generator refactors) - #145823 (editorconfig: don't use nonexistent syntax) - #145944 (std: Start supporting WASIp2 natively ) - #145961 (resolve: Avoid a regression from splitting prelude into two scopes) - #146032 (Explicity disable LSX feature for `loongarch64-unknown-none` target) - #146106 (fix(lexer): Only allow horizontal whitespace in frontmatter ) - #146154 (CI: rfl: move job forward to Linux v6.17-rc3 plus 2 commits) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #145279 - clarfonthey:const-convert-initial, r=tgross35 Constify conversion traits (part 1) This is the first part of #144289 being split into smaller pieces. It adds/moves constness of several traits under the `const_convert` feature: * `From` * `Into` * `TryFrom` * `TryInto` * `FromStr` * `AsRef` * `AsMut` * `Borrow` * `BorrowMut` * `Deref` * `DerefMut` There are a few methods that are intrinsically tied to these traits which I've included in the feature. Particularly, those which are wrappers over `AsRef`: * `ByteStr::new` (unstable under `bstr` feature) * `OsStr::new` * `Path::new` Those which directly use `Into`: * `Result::into_ok` * `Result::into_err` And those which use `Deref` and `DerefMut`: * `Pin::as_ref` * `Pin::as_mut` * `Pin::as_deref_mut` * `Option::as_deref` * `Option::as_deref_mut` * `Result::as_deref` * `Result::as_deref_mut` (note: the `Option` and `Result` methods were suggested by ``@npmccallum`` initially as #146101) The parts which are missing from this PR are: * Anything that involves heap-allocated types * Making any method const than the ones listed above * Anything that could rely on the above, *or* could rely on system-specific code for `OsStr` or `Path` (note: this mostly makes these methods useless since `str` doesn't implement `AsRef<OsStr>` yet, but it's better to track the method for now and add impls later, IMHO) r? ``@tgross35`` (who mostly already reviewed this)

…oshtriplett `const`ify (the unstable) `str::as_str` Tracking issue: rust-lang#130366 The method was not initially marked `const` presumably because it is only useful with `Deref`. But now that const traits seem to be a thing that can actually become real, why not make it `const`? PR `const`ifying `Deref`: rust-lang#145279

… r=tgross35 Constify conversion traits (part 1) This is the first part of rust-lang#144289 being split into smaller pieces. It adds/moves constness of several traits under the `const_convert` feature: * `From` * `Into` * `TryFrom` * `TryInto` * `FromStr` * `AsRef` * `AsMut` * `Borrow` * `BorrowMut` * `Deref` * `DerefMut` There are a few methods that are intrinsically tied to these traits which I've included in the feature. Particularly, those which are wrappers over `AsRef`: * `ByteStr::new` (unstable under `bstr` feature) * `OsStr::new` * `Path::new` Those which directly use `Into`: * `Result::into_ok` * `Result::into_err` And those which use `Deref` and `DerefMut`: * `Pin::as_ref` * `Pin::as_mut` * `Pin::as_deref_mut` * `Option::as_deref` * `Option::as_deref_mut` * `Result::as_deref` * `Result::as_deref_mut` (note: the `Option` and `Result` methods were suggested by ``@npmccallum`` initially as rust-lang#146101) The parts which are missing from this PR are: * Anything that involves heap-allocated types * Making any method const than the ones listed above * Anything that could rely on the above, *or* could rely on system-specific code for `OsStr` or `Path` (note: this mostly makes these methods useless since `str` doesn't implement `AsRef<OsStr>` yet, but it's better to track the method for now and add impls later, IMHO) r? ``@tgross35`` (who mostly already reviewed this)

…r=oli-obk Revert constification of `AsRef for Cow` due to inference failure Reported issue: rust-lang#147964 Original PR: rust-lang#145279 `const AsRef` tracking issue: rust-lang#143773 Should have additional crater run to verify this fixes the issue.

Rollup merge of #148011 - clarfonthey:const-convert-revert, r=oli-obk Revert constification of `AsRef for Cow` due to inference failure Reported issue: #147964 Original PR: #145279 `const AsRef` tracking issue: #143773 Should have additional crater run to verify this fixes the issue.

…, r=cuviper Revert constification of `Borrow` and `Deref for Cow` due to inference failure Reported issue: rust-lang#147964 Original PR: rust-lang#145279 Previous revert: rust-lang#148011 `const Borrow`/`Deref` tracking issue: rust-lang#143773 Should have additional crater run to verify this fixes the issue. Since other PR is in the queue, this will need to be rebased after that merges. Also will want a beta nomination.

Rollup merge of #148016 - clarfonthey:const-convert-revert-2, r=cuviper Revert constification of `Borrow` and `Deref for Cow` due to inference failure Reported issue: #147964 Original PR: #145279 Previous revert: #148011 `const Borrow`/`Deref` tracking issue: #143773 Should have additional crater run to verify this fixes the issue. Since other PR is in the queue, this will need to be rebased after that merges. Also will want a beta nomination.

rustbot assigned tgross35 Aug 11, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Aug 11, 2025

clarfonthey commented Aug 11, 2025

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clarfonthey force-pushed the const-convert-initial branch from 365fcde to 97c330f Compare August 13, 2025 16:40

clarfonthey force-pushed the const-convert-initial branch from 97c330f to 4ebb4b7 Compare August 17, 2025 18:58

clarfonthey force-pushed the const-convert-initial branch from 4ebb4b7 to 4bcf7bd Compare August 21, 2025 00:47

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GrigorenkoPV mentioned this pull request Aug 27, 2025

constify (the unstable) str::as_str #145930

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clarfonthey mentioned this pull request Sep 2, 2025

Tracking Issue for const Derefs #88955

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clarfonthey force-pushed the const-convert-initial branch from 66dcd76 to 636cfe4 Compare September 2, 2025 01:34

Constify conversion traits

1c64d3e

clarfonthey force-pushed the const-convert-initial branch from 636cfe4 to 1c64d3e Compare September 2, 2025 01:38

clarfonthey mentioned this pull request Sep 2, 2025

constify as_deref() / as_deref_mut() on Option / Result #146101

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clarfonthey mentioned this pull request Sep 2, 2025

alloc: make Cow From impls const #146067

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tgross35 approved these changes Sep 2, 2025

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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 2, 2025

tgross35 mentioned this pull request Sep 3, 2025

Rollup of 8 pull requests #146141

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Rollup of 9 pull requests #146157

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Zalathar mentioned this pull request Sep 3, 2025

Rollup of 8 pull requests #146160

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bors merged commit f4b946a into rust-lang:master Sep 3, 2025
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Tracking Issue for const_convert #143773

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theemathas mentioned this pull request Oct 22, 2025

regression: the method as_ref exists for reference #147964

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clarfonthey mentioned this pull request Oct 23, 2025

Revert constification of AsRef for Cow due to inference failure #148011

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Revert constification of Borrow and Deref for Cow due to inference failure #148016

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Constify conversion traits (part 1) #145279

Constify conversion traits (part 1) #145279

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