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Add guarantee that Vec::default() does not alloc #100872

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merged 1 commit into from
Aug 22, 2022
Merged

Add guarantee that Vec::default() does not alloc #100872

merged 1 commit into from
Aug 22, 2022

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JanBeh
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@JanBeh JanBeh commented Aug 22, 2022

Currently Vec::new() is guaranteed to not allocate until elements are pushed onto the Vec, but such a guarantee is missing for Vec's implementation of Default::default.

This adds such a guarantee for Vec::default() to the API reference.

See also this discussion on URLO.

@JanBeh
Add guarantee that Vec::default() does not alloc
0227b71 
Currently `Vec::new()` is guaranteed to not allocate until elements are
pushed onto the `Vec`, but such a guarantee is missing for `Vec`'s
implementation of `Default::default`. This adds such a guarantee for
`Vec::default()` to the API reference.
@rustbot rustbot added the T-libs Relevant to the library team, which will review and decide on the PR/issue. label Aug 22, 2022
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Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @joshtriplett (or someone else) soon.

Please see the contribution instructions for more information.

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rustbot commented Aug 22, 2022

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

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  • Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
  • Changing public documentation in ways that create new stability guarantees
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@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 22, 2022
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JanBeh commented Aug 22, 2022

@rustbot label +T-libs-api -T-libs

@rustbot rustbot added T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. and removed T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Aug 22, 2022
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fee1-dead commented Aug 22, 2022 •
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LGTM! Thanks for the PR. I am going to approve this (even though I am a compiler contributor) because I don't believe that such a guarantee would be unwanted as it would be a performance regression if it actually allocated.

@bors r+ rollup=always

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bors commented Aug 22, 2022

📌 Commit 0227b71 has been approved by fee1-dead

It is now in the queue for this repository.

@bors 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 Aug 22, 2022
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CryZe commented Aug 22, 2022

Wouldn't it maybe make more sense to just guarantee that it always matches Vec::new()?

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JanBeh commented Aug 22, 2022

Wouldn't it maybe make more sense to just guarantee that it always matches Vec::new()?

I considered that too. It's a slightly stronger guarantee and not sure if it's needed. Maybe Vec::default() and Vec::new() could differ in some other way in future (though I can't think of any right now), apart from the latter being a const fn.

Dylan-DPC added a commit to Dylan-DPC/rust that referenced this pull request Aug 22, 2022
@Dylan-DPC
Rollup merge of rust-lang#100872 - JanBeh:PR_vec_default_alloc_doc, r…
cf51785 
…=fee1-dead

Add guarantee that Vec::default() does not alloc

Currently `Vec::new()` is guaranteed to not allocate until elements are pushed onto the `Vec`, but such a guarantee is missing for `Vec`'s implementation of `Default::default`.

This adds such a guarantee for `Vec::default()` to the API reference.

See also [this discussion on URLO](https://users.rust-lang.org/t/guarantee-that-vec-default-does-not-allocate/79903).
@Dylan-DPC Dylan-DPC mentioned this pull request Aug 22, 2022
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@Dylan-DPC Dylan-DPC mentioned this pull request Aug 22, 2022
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bors added a commit to rust-lang-ci/rust that referenced this pull request Aug 22, 2022
@bors
Auto merge of rust-lang#100881 - Dylan-DPC:rollup-q9rr658, r=Dylan-DPC
a785176 
Rollup of 8 pull requests

Successful merges:

 - rust-lang#98200 (Expand potential inner `Or` pattern for THIR)
 - rust-lang#99770 (Make some const prop mir-opt tests `unit-test`s)
 - rust-lang#99957 (Rework Ipv6Addr::is_global to check for global reachability rather than global scope - rebase)
 - rust-lang#100331 (Guarantee `try_reserve` preserves the contents on error)
 - rust-lang#100336 (Fix two const_trait_impl issues)
 - rust-lang#100713 (Convert diagnostics in parser/expr to SessionDiagnostic)
 - rust-lang#100820 (Use pointer `is_aligned*` methods)
 - rust-lang#100872 (Add guarantee that Vec::default() does not alloc)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
@scottmcm
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@rust-lang/libs-api I'm confident this is uncontroversial, but technically it's a new guarantee. Are you fine with this going in via just an r+, or do you want an FCP too?

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dtolnay commented Aug 22, 2022 •
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This is fine without FCP, Vec::new already guarantees the same.

@bors bors merged commit 4ed8fa4 into rust-lang:master Aug 22, 2022
@rustbot rustbot added this to the 1.65.0 milestone Aug 22, 2022
@JanBeh JanBeh deleted the PR_vec_default_alloc_doc branch October 4, 2022 07:37
@JanBeh JanBeh restored the PR_vec_default_alloc_doc branch October 4, 2022 07:51
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