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Add feature gate for view_types experiment#155939

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scrabsha wants to merge 2 commits intorust-lang:mainfrom
scrabsha:view-types/feature-gate
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Add feature gate for view_types experiment#155939
scrabsha wants to merge 2 commits intorust-lang:mainfrom
scrabsha:view-types/feature-gate

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@scrabsha scrabsha commented Apr 28, 2026 •
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Tracking issue: #155938

The goal of this feature is to experiment with the concept of view types: allowing the borrow checker to reason about borrows across function calls by having an syntax to express which fields of a struct are used by a function.

This will have the following limitations:

  • Usage of view types on pub or pub(...) functions is a hard error.
  • View groups are not considered at the moment.
  • Interaction with pattern types are not considered at the moment.

r? nikomatsakis

@rustbot 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. labels Apr 28, 2026
@scrabsha scrabsha mentioned this pull request Apr 28, 2026
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nikomatsakis commented Apr 28, 2026

@rustbot label +I-lang-nominated

Hi y'all, I am starting up a lang-team experiment on view types as champion.

Also cc @rust-lang/types and @lqd in particular

@rustbot rustbot added the I-lang-nominated Nominated for discussion during a lang team meeting. label Apr 28, 2026
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One nit

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Comment thread tests/ui/feature-gates/feature-gate-view-types.rs
@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 28, 2026
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rustbot commented Apr 28, 2026

Reminder, once the PR becomes ready for a review, use @rustbot ready.

@scrabsha scrabsha force-pushed the view-types/feature-gate branch from 711ae3e to d3992fa Compare April 28, 2026 21:57
@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 28, 2026
lqd
lqd reviewed Apr 28, 2026
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Comment thread tests/ui/feature-gates/feature-gate-view-types.rs Outdated
@traviscross traviscross added P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang I-lang-radar Items that are on lang's radar and will need eventual work or consideration. and removed I-lang-nominated Nominated for discussion during a lang team meeting. P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang labels Apr 29, 2026
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rustbot commented Apr 29, 2026

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

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nikomatsakis commented Apr 29, 2026

We discussed this in the @rust-lang/lang meeting today. My recollection is generally positive vibes and excitement.

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One nit

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Comment thread tests/ui/feature-gates/feature-gate-view-types.rs Outdated
Comment thread tests/ui/feature-gates/feature-gate-view-types.rs
Comment thread tests/ui/feature-gates/feature-gate-view-types.rs
Comment thread compiler/rustc_parse/src/parser/ty.rs
let view_end_span = self.prev_token.span;
let span = view_start_span.to(view_end_span);
self.psess.gated_spans.gate(sym::view_types, span);
}
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ok, so, right now we are effectively discarding the .{...} syntax. Seems ok for the first PR.

@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 29, 2026
fmease
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Comment thread compiler/rustc_parse/src/parser/ty.rs Outdated
@scrabsha scrabsha force-pushed the view-types/feature-gate branch from e433590 to 77fb8d1 Compare April 29, 2026 18:15
@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 29, 2026
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nikomatsakis commented Apr 29, 2026

@bors r+

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rust-bors Bot commented Apr 29, 2026

📌 Commit 77fb8d1 has been approved by nikomatsakis

It is now in the queue for this repository.

@rust-bors rust-bors Bot 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 Apr 29, 2026
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I-lang-radar Items that are on lang's radar and will need eventual work or consideration. S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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