view-types: store view types in the AST by scrabsha · Pull Request #156016 · rust-lang/rust

scrabsha · 2026-04-30T20:49:17Z

Tracking issue: #155938.

Instead of discarding view types, we store them in the AST now!

JonathanBrouwer · 2026-05-03T10:24:05Z

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

rust-bors · 2026-05-03T12:34:42Z

☀️ Try build successful (CI)
Build commit: 9d3dea7 (9d3dea74651f758e35177070e510be8f3639a7fd, parent: 6769f690f947f12e36db6b6503bab265b7b2cced)

rust-timer · 2026-05-03T13:17:31Z

Finished benchmarking commit (9d3dea7): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.7%	[-1.4%, -0.3%]	6
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.1%]	2
All ❌✅ (primary)	-0.7%	[-1.4%, -0.3%]	6

Max RSS (memory usage)

Results (primary 0.1%, secondary -2.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.9%, 1.9%]	1
Regressions ❌ (secondary)	1.2%	[1.2%, 1.2%]	1
Improvements ✅ (primary)	-0.8%	[-0.8%, -0.7%]	2
Improvements ✅ (secondary)	-4.4%	[-7.3%, -1.5%]	2
All ❌✅ (primary)	0.1%	[-0.8%, 1.9%]	3

Cycles

Results (secondary 3.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.0%	[0.5%, 9.9%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.3%	[-1.3%, -1.3%]	1
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 503.61s -> 496.571s (-1.40%)
Artifact size: 394.46 MiB -> 394.47 MiB (0.00%)

scrabsha · 2026-05-03T16:11:34Z

^

rebased to main to fix conflicts
fixed view-types: store view types in the AST #156016 (comment)

r? nikomatsakis

scrabsha · 2026-05-17T16:12:19Z

@rustbot reroll

BoxyUwU · 2026-06-11T19:06:49Z

r? compiler

BoxyUwU · 2026-06-11T19:18:33Z

ah apparently oli was talked to privately about this,

r? oli-obk

oli-obk · 2026-06-17T14:09:43Z

            let eself_hi = this.prev_token.span;
-            let eself = if this.eat(exp!(Colon)) {
-                SelfKind::Explicit(this.parse_ty()?, m)
+            let view = this.maybe_parse_view()?;


for pattern types I used a builtin macro instead of starting out with syntax. While the reason for that was that it was a breaking change due to the follow-rules of ty fragments allowing is after it. Is this not an issue for .?

Further advantages were that it's syntax that is "automatically" supported in r-a and syn, and is much easier to edit and fine tune

View changes since the review

for pattern types I used a builtin macro instead of starting out with syntax. While the reason for that was that it was a breaking change due to the follow-rules of ty fragments allowing is after it. Is this not an issue for .?

Oh, nice catch. I'll switch to pattern_types!(Foo.{ fields }) immediately. Thanks for mentioning this!f

(For those at home who are trying to figure out how this can break code, #107606 (comment) is a good repro)

Further advantages were that it's syntax that is "automatically" supported in r-a and syn, and is much easier to edit and fine tune

Agreed. r-a screams at me every time i open my tests hehe

(writing this here so i avoid the pain of having to discover this again)

the same problem arises for self.{ fields } as well, for instance with this macro:

macro_rules! foo { ($it:item) => {}; (impl Foo { fn bar(self.{ meow }) {} }) => {}; } foo!(impl Foo { fn bar(self.{ meow }) {} });

we can't have nice things :(

I don't see how your example is problematic, it already doesn't compile, so it won't be a regression. However, it's true that the addition of the view types syntax did regress code.

E.g., the following snippet compiles on stable but it no longer compiles on beta & nightly:

macro_rules! m { ($ty:ty) => { /*compile_error!("");*/ }; (&().{}) => {}; } m!(&().{});

I don't see how your example is problematic, it already doesn't compile, so it won't be a regression.

is this another instance of #103534?

(ack for the rest of your message, i will fix it soon)

rustbot · 2026-06-22T20:38:17Z

This PR was rebased onto a different main 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.

scrabsha · 2026-06-22T20:38:27Z

^

rebased over main
added the view_type builtin macro replace the $ty.{ field } syntax with view_type!($ty.{ field }) (i took a lot of inspiration from Implement minimal, internal-only pattern types in the type system #120131 :3)
added a test for the regression reported in view-types: store view types in the AST #156016 (comment)
removed the self.{ field } syntax. i will bring this back in a later PR

@rustbot ready

scrabsha · 2026-06-22T20:43:34Z

this file has been moved to tests/ui/view-types/feature-gate-view-types.rs (which is, AIUI what's done for pattern types)

View changes since the review

ytmimi · 2026-06-23T15:40:23Z

+            ast::TyKind::View(ref ty, ref fields) => {
+                let ty = ty.rewrite_result(context, shape)?;
+                let view = rewrite_view_fields(fields);
+                Ok(format!("{ty}.{view}"))
+            }


@scrabsha If we're going with the macro based approach do we still need rustfmt to format the new syntax? If it's behind a macro then rustfmt will see a macro call since it operates on the AST pre expansion.

We should handle these similar to type_ascribe!, include_bytes!, offset_of! which are all defined as builtin macros. When we see those in the AST we just return Err(RewriteError::Unknown)

View changes since the review

@ytmimi thank you for pointing this out! i just pushed a fix for this, it should be good now :)

while i'm at it, @oli-obk, do you mind if i do the same thing for pattern types? (otherwise i can open a second PR just for this, it's your call)

(pattern types and view types are created using macros, i guess pattern types should go here too hehe)

rust-log-analyzer · 2026-06-23T21:16:21Z

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

...............................F.................. (50/144)
.................................................. (100/144)
............................................       (144/144)

======== tests/rustdoc-gui/go-to-collapsed-elem.goml ========

[ERROR] line 40
    at `tests/rustdoc-gui/go-to-collapsed-elem.goml` line 21: Error: Node is detached from document: for command `click: "//*[@id='search']//a[@href='../test_docs/struct.Foo.html#method.must_use']"`
    at <file:///checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/test_docs/struct.Foo.html?search=t_use>


<= doc-ui tests done: 143 succeeded, 1 failed, 0 filtered out

Error: ()

scrabsha · 2026-06-23T21:32:36Z

i... don't really understand how my changes trigger this CI error? is the test being flaky?

scrabsha · 2026-06-24T14:09:44Z

ok, it's probably linked to #93784

@bors try

rust-bors · 2026-06-24T14:09:47Z

@scrabsha: 🔑 Insufficient privileges: not in try users

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 30, 2026

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scrabsha force-pushed the view-types/in-ast branch from 52c4eee to 2bafb0c Compare May 1, 2026 13:55

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scrabsha force-pushed the view-types/in-ast branch from 2bafb0c to 7f6749b Compare May 1, 2026 15:17

rustbot added T-clippy Relevant to the Clippy team. T-rustfmt Relevant to the rustfmt team, which will review and decide on the PR/issue. labels May 1, 2026

scrabsha force-pushed the view-types/in-ast branch from 7f6749b to 77bdbc2 Compare May 1, 2026 15:21

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scrabsha force-pushed the view-types/in-ast branch 4 times, most recently from d9ef8d8 to ec789ac Compare May 3, 2026 09:31

scrabsha commented May 3, 2026

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label May 3, 2026

rust-bors Bot pushed a commit that referenced this pull request May 3, 2026

Auto merge of #156016 - scrabsha:view-types/in-ast, r=<try>

9d3dea7

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

fmease reviewed May 3, 2026

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

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scrabsha mentioned this pull request May 3, 2026

Tracking Issue for view types #155938

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label May 3, 2026

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scrabsha force-pushed the view-types/in-ast branch from ec789ac to 93208f2 Compare May 3, 2026 16:11

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label May 17, 2026

rustbot assigned TaKO8Ki and unassigned nikomatsakis May 17, 2026

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scrabsha force-pushed the view-types/in-ast branch from 1707ca0 to b65ae52 Compare May 17, 2026 16:34

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scrabsha force-pushed the view-types/in-ast branch from b65ae52 to c916edc Compare May 17, 2026 16:41

rustbot assigned dingxiangfei2009 and unassigned TaKO8Ki Jun 11, 2026

rustbot assigned oli-obk and unassigned dingxiangfei2009 Jun 11, 2026

oli-obk reviewed Jun 17, 2026

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view types: add a few tests

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scrabsha force-pushed the view-types/in-ast branch from c916edc to 1f0b3bf Compare June 22, 2026 20:38

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scrabsha force-pushed the view-types/in-ast branch from 1f0b3bf to 23bf586 Compare June 23, 2026 08:58

ytmimi reviewed Jun 23, 2026

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view-types: store view types in the AST

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scrabsha force-pushed the view-types/in-ast branch from 23bf586 to dcdf133 Compare June 23, 2026 20:05

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Overall result: ✅ improvements - no action needed

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