Add feature gate for view_types experiment by scrabsha · Pull Request #155939 · rust-lang/rust

scrabsha · 2026-04-28T18:16:11Z

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

nikomatsakis · 2026-04-28T21:31:31Z

@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

nikomatsakis

One nit

View changes since this review

rustbot · 2026-04-28T21:44:17Z

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

rustbot · 2026-04-29T16:26:07Z

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

cc @fmease

nikomatsakis · 2026-04-29T16:44:19Z

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

nikomatsakis

One nit

View changes since this review

nikomatsakis · 2026-04-29T16:49:02Z

+            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);
+        }


ok, so, right now we are effectively discarding the .{...} syntax. Seems ok for the first PR.

nikomatsakis · 2026-04-29T23:11:14Z

@bors r+

rust-bors · 2026-04-29T23:11:17Z

📌 Commit 77fb8d1 has been approved by nikomatsakis

It is now in the queue for this repository.

…r=nikomatsakis Add feature gate for view_types experiment

Rollup of 6 pull requests Successful merges: - #155853 (Use `_mcount` as the mcount symbol name on RISC-V Linux GNU targets) - #155939 (Add feature gate for view_types experiment) - #155974 (add `c_variadic_experimental_arch` feature) - #155523 (Reorganize `tests/ui/issues/` - 02) - #155980 (Move `feature*` methods from `parse` mod to `errors` mod.) - #155987 (Make lifting infallible) Failed merges: - #155237 (Disentangle AST crates and error crates)

…r=nikomatsakis Add feature gate for view_types experiment

…uwer Rollup of 13 pull requests Successful merges: - #155249 (Fix: On wasm targets, call `panic_in_cleanup` if panic occurs in cleanup) - #155853 (Use `_mcount` as the mcount symbol name on RISC-V Linux GNU targets) - #155919 (simplify `ast_fragments!`) - #155939 (Add feature gate for view_types experiment) - #155974 (add `c_variadic_experimental_arch` feature) - #155991 (Catch unwinds from the global ctxt callback to complete queries profiling data in more cases) - #156003 (Pass Session to optimize_and_codegen_fat_lto) - #153566 (Add suggestion for E0401 on inner const items) - #154610 (Suggest public re-exports when a private module makes an import path inaccessible) - #155523 (Reorganize `tests/ui/issues/` - 02) - #155980 (Move `feature*` methods from `parse` mod to `errors` mod.) - #155987 (Make lifting infallible) - #155988 (tests/run-make/print-cfg: add Android target_env case)

Rollup of 19 pull requests Successful merges: - #155237 (Disentangle AST crates and error crates) - #155249 (Fix: On wasm targets, call `panic_in_cleanup` if panic occurs in cleanup) - #155853 (Use `_mcount` as the mcount symbol name on RISC-V Linux GNU targets) - #155919 (simplify `ast_fragments!`) - #155939 (Add feature gate for view_types experiment) - #155954 (rustdoc: preserve parent doc cfg for `macro_export` macros) - #155974 (add `c_variadic_experimental_arch` feature) - #155991 (Catch unwinds from the global ctxt callback to complete queries profiling data in more cases) - #156003 (Pass Session to optimize_and_codegen_fat_lto) - #153566 (Add suggestion for E0401 on inner const items) - #154610 (Suggest public re-exports when a private module makes an import path inaccessible) - #155523 (Reorganize `tests/ui/issues/` - 02) - #155821 (c-variadic: document `Clone` and `Drop` instances and require `VaArgSafe: Copy`) - #155980 (Move `feature*` methods from `parse` mod to `errors` mod.) - #155987 (Make lifting infallible) - #155988 (tests/run-make/print-cfg: add Android target_env case) - #156000 (Fix ICE when using -Zinstrument-mcount and -Clinker-flavor=lld) - #156002 (Allow to use `Diagnostic` directly in `SharedContext::emit_lint`) - #156015 (rustc-dev-guide subtree update)

Rollup merge of #155939 - scrabsha:view-types/feature-gate, r=nikomatsakis Add feature gate for view_types experiment

Rollup of 19 pull requests Successful merges: - rust-lang/rust#155237 (Disentangle AST crates and error crates) - rust-lang/rust#155249 (Fix: On wasm targets, call `panic_in_cleanup` if panic occurs in cleanup) - rust-lang/rust#155853 (Use `_mcount` as the mcount symbol name on RISC-V Linux GNU targets) - rust-lang/rust#155919 (simplify `ast_fragments!`) - rust-lang/rust#155939 (Add feature gate for view_types experiment) - rust-lang/rust#155954 (rustdoc: preserve parent doc cfg for `macro_export` macros) - rust-lang/rust#155974 (add `c_variadic_experimental_arch` feature) - rust-lang/rust#155991 (Catch unwinds from the global ctxt callback to complete queries profiling data in more cases) - rust-lang/rust#156003 (Pass Session to optimize_and_codegen_fat_lto) - rust-lang/rust#153566 (Add suggestion for E0401 on inner const items) - rust-lang/rust#154610 (Suggest public re-exports when a private module makes an import path inaccessible) - rust-lang/rust#155523 (Reorganize `tests/ui/issues/` - 02) - rust-lang/rust#155821 (c-variadic: document `Clone` and `Drop` instances and require `VaArgSafe: Copy`) - rust-lang/rust#155980 (Move `feature*` methods from `parse` mod to `errors` mod.) - rust-lang/rust#155987 (Make lifting infallible) - rust-lang/rust#155988 (tests/run-make/print-cfg: add Android target_env case) - rust-lang/rust#156000 (Fix ICE when using -Zinstrument-mcount and -Clinker-flavor=lld) - rust-lang/rust#156002 (Allow to use `Diagnostic` directly in `SharedContext::emit_lint`) - rust-lang/rust#156015 (rustc-dev-guide subtree update)

rustbot assigned nikomatsakis Apr 28, 2026

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 mentioned this pull request Apr 28, 2026

Tracking Issue for view types #155938

Open

8 tasks

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

nikomatsakis requested changes Apr 28, 2026

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Comment thread tests/ui/feature-gates/feature-gate-view-types.rs

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

Add feature gate for view_types

d3992fa

scrabsha force-pushed the view-types/feature-gate branch from 711ae3e to d3992fa Compare April 28, 2026 21:57

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 reviewed Apr 28, 2026

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Comment thread tests/ui/feature-gates/feature-gate-view-types.rs Outdated

nikomatsakis requested changes Apr 29, 2026

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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 reviewed Apr 29, 2026

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Comment thread compiler/rustc_parse/src/parser/ty.rs Outdated

Parse view type syntax, mark it as unstable

77fb8d1

scrabsha force-pushed the view-types/feature-gate branch from e433590 to 77fb8d1 Compare April 29, 2026 18:15

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

nikomatsakis approved these changes Apr 29, 2026

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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

jhpratt added a commit to jhpratt/rust that referenced this pull request Apr 30, 2026

Rollup merge of rust-lang#155939 - scrabsha:view-types/feature-gate, …

cbdb909

…r=nikomatsakis Add feature gate for view_types experiment

jhpratt mentioned this pull request Apr 30, 2026

Rollup of 4 pull requests #155990

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jhpratt added a commit to jhpratt/rust that referenced this pull request Apr 30, 2026

Rollup merge of rust-lang#155939 - scrabsha:view-types/feature-gate, …

e4ea2a4

…r=nikomatsakis Add feature gate for view_types experiment

jhpratt mentioned this pull request Apr 30, 2026

Rollup of 6 pull requests #155994

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JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request Apr 30, 2026

Rollup merge of rust-lang#155939 - scrabsha:view-types/feature-gate, …

71f9f9a

…r=nikomatsakis Add feature gate for view_types experiment

JonathanBrouwer mentioned this pull request Apr 30, 2026

Rollup of 13 pull requests #156010

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scrabsha mentioned this pull request Apr 30, 2026

view-types: store borrows of view types in the AST #156016

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jhpratt mentioned this pull request May 1, 2026

Rollup of 19 pull requests #156023

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rust-bors Bot merged commit f7b1e7d into rust-lang:main May 1, 2026
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rustbot added this to the 1.97.0 milestone May 1, 2026

scrabsha deleted the view-types/feature-gate branch May 1, 2026 07:43

rust-timer added a commit that referenced this pull request May 1, 2026

Unrolled build for #155939

8891d62

Rollup merge of #155939 - scrabsha:view-types/feature-gate, r=nikomatsakis Add feature gate for view_types experiment

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