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

nikomatsakis · 2026-05-07T17:58:02Z

    Ptr(MutTy),
    /// A reference (`&'a T` or `&'a mut T`).
-    Ref(#[visitable(extra = LifetimeCtxt::Ref)] Option<Lifetime>, MutTy),
+    Ref(#[visitable(extra = LifetimeCtxt::Ref)] Option<Lifetime>, MutTy, ViewKind),


This is not the right model. The view is not attached to the reference but rather to the path type. You can have a view type without any reference at all (e.g., Foo.{F1, F2}).

View changes since the review

hi! Unless you have an objection, i would like to go with the following:

Have a TyKind::View(Box<Ty>, ThinVec<Ident>) variant (instead of the change you highlight)

Parse any $ty.{ $( $ident ),* }

Check later in the compilation that $ty is a kind of type we expect

Unless I have missed something, this is functionally identical to what you suggest, but leaves more room for nice error messages to be added.

I just pushed a commit that does what I just described, let me know what you think :)

I don't have an objection to this in the AST, anyway.

rustbot · 2026-05-07T17:58:30Z

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

scrabsha · 2026-05-09T14:45:19Z

@rustbot ready

scrabsha · 2026-05-11T19:22:44Z

+                let (kind, view, span) = match self.ty.kind {
+                    ref kind if let Some(view) = kind.as_implicit_self_maybe_wrapped_in_view() => {
+                        (SelfKind::Value(mutbl), view, self.pat.span)
+                    }
+                    TyKind::Ref(lt, MutTy { ref ty, mutbl })
+                        if let Some(view) = ty.kind.as_implicit_self_maybe_wrapped_in_view() =>
+                    {
+                        (SelfKind::Region(lt, mutbl), view, self.pat.span)
                    }
                    TyKind::PinnedRef(lt, MutTy { ref ty, mutbl })
-                        if ty.kind.is_implicit_self() =>
+                        if let Some(view) = ty.kind.as_implicit_self_maybe_wrapped_in_view() =>
                    {
-                        Some(respan(self.pat.span, SelfKind::Pinned(lt, mutbl)))
+                        (SelfKind::Pinned(lt, mutbl), view, self.pat.span)
                    }
-                    _ => Some(respan(
-                        self.pat.span.to(self.ty.span),
-                        SelfKind::Explicit(self.ty.clone(), mutbl),
-                    )),
+                    _ => (SelfKind::Explicit(self.ty.clone(), mutbl), ViewKind::Full, self.ty.span),
                };
+                let param = respan(span, SelfParam { view, kind });
+                return Some(param);


i am not happy at all with this code, but i cannot think of a better way to express things. opinions are welcome.

View changes since the review

scrabsha · 2026-05-11T19:23:09Z

        matches!(self, TyKind::ImplicitSelf)
    }

+    pub fn as_implicit_self_maybe_wrapped_in_view(&self) -> Option<ViewKind> {


similarly, this method name is very very very meh

View changes since the review

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

scrabsha force-pushed the view-types/in-ast branch from 661a680 to 6035af1 Compare May 4, 2026 17:52

nikomatsakis requested changes May 7, 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 May 7, 2026

scrabsha force-pushed the view-types/in-ast branch from 6035af1 to 9bdbdb2 Compare May 7, 2026 19:53

scrabsha commented May 7, 2026

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scrabsha commented May 8, 2026

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Comment thread compiler/rustc_ast/src/ast.rs Outdated

scrabsha force-pushed the view-types/in-ast branch from 9bdbdb2 to 7b29ee5 Compare May 8, 2026 18:08

scrabsha changed the title ~~view-types: store borrows of view types in the AST~~ view-types: store view types in the AST May 8, 2026

scrabsha force-pushed the view-types/in-ast branch 2 times, most recently from 56b58a1 to b103cec Compare May 8, 2026 18:40

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scrabsha force-pushed the view-types/in-ast branch from b103cec to f0d6895 Compare May 8, 2026 18:54

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 May 9, 2026

scrabsha commented May 10, 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 May 10, 2026

view types: add a few tests

b82cba9

scrabsha force-pushed the view-types/in-ast branch from f0d6895 to 9a4f5ec Compare May 11, 2026 17:21

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scrabsha force-pushed the view-types/in-ast branch from 9a4f5ec to 827c78c Compare May 11, 2026 19:20

scrabsha commented May 11, 2026

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

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scrabsha force-pushed the view-types/in-ast branch from 827c78c to 1707ca0 Compare May 11, 2026 19:42

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 May 11, 2026

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rust-timer commented May 3, 2026

Overall result: ✅ improvements - no action needed

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