Lazy type-alias-impl-trait #92007

oli-obk · 2021-12-16T21:21:10Z

Previously opaque types were processed by

replacing all mentions of them with inference variables
memorizing these inference variables in a side-table
at the end of typeck, resolve the inference variables in the side table and use the resolved type as the hidden type of the opaque type

This worked okayish for impl Trait in return position, but required lots of roundabout type inference hacks and processing.

This PR instead stops this process of replacing opaque types with inference variables, and just keeps the opaque types around.
Whenever an opaque type O is compared with another type T, we make the comparison succeed and record T as the hidden type. If O is compared to U while there is a recorded hidden type for it, we grab the recorded type (T) and compare that against U. This makes implementing

Permit impl Trait in type aliases rfcs#2515

much simpler (previous attempts on the inference based scheme were very prone to ICEs and general misbehaviour that was not explainable except by random implementation defined oddities).

r? @nikomatsakis

fixes #93411
fixes #88236

jackh726 · 2021-12-16T23:17:38Z

Woohoo. Nice job. This is a doosy to review though. 😰

oli-obk · 2021-12-17T07:22:02Z

Yea. The problem is that while I can pull some changes into master, they will be dead code and we usually don't accept such code. I think I may be able to restructure into reviewable commits though

jackh726 · 2021-12-17T19:10:08Z

I personally am mostly fine with splitting out changes here into separate PRs, even if it they're mostly dead code, given that there is this PR which clearly defines to "end goal". But, another approach would be to only split this into meaningful commits, but that's still really prone to bitrot.

oli-obk · 2022-01-20T17:33:46Z

@bors try @rust-timer queue

rust-timer · 2022-01-20T17:33:47Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

bors · 2022-01-20T17:34:01Z

⌛ Testing commit 488ef96531f45667626fb808b45037b633f00e76 with merge c41542603add4dcd0e350ffbde4a1c19a29d0bd6...

pietroalbini · 2022-01-20T17:46:04Z

@bors r- retry

Why did this PR start testing :/

oli-obk · 2022-01-20T19:05:21Z

@bors try @rust-timer queue

rust-timer · 2022-01-20T19:05:23Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

bors · 2022-01-20T19:05:39Z

⌛ Trying commit 488ef96531f45667626fb808b45037b633f00e76 with merge ebeceacb2f4cac04463864a910b2490c0ae511c8...

bors · 2022-01-20T20:37:20Z

☀️ Try build successful - checks-actions
Build commit: ebeceacb2f4cac04463864a910b2490c0ae511c8 (ebeceacb2f4cac04463864a910b2490c0ae511c8)

Mark-Simulacrum · 2022-02-08T12:32:35Z

perf is also indicating this PR broke the gll benchmark (as indicated in the results) - I'll look into that this morning.

lqd · 2022-02-08T12:39:32Z

We should add that information to the summary the bot posts

oli-obk · 2022-02-08T13:18:29Z

Fixing the failure first, then looking into perf

WaffleLapkin · 2022-02-08T13:48:07Z

src/test/ui/impl-trait/nested_impl_trait.stderr

+help: consider further restricting this bound
+   |
+LL |     fn bad(x: impl Into<u32> + std::fmt::Debug) -> impl Into<impl Debug> { x }
+   |                              +++++++++++++++++


What happened to these errors? Why x should implement debug? Why are they emitted, if impl Into<impl Debug> is invalid anyway?

Mark-Simulacrum · 2022-02-08T14:17:11Z

We should add that information to the summary the bot posts

Posted rust-lang/rustc-perf#1169 to do this.

oli-obk · 2022-02-08T16:49:44Z

I fixed the gll problem: #93783 it hasn't hit nightly yet, so we could p=1 it if we wanted to.

…ackh726 Fix regression from lazy opaque types The breakage was found in rust-lang#92007 (comment) and has not hit nightly yet.

Alexendoo · 2022-02-09T13:04:14Z

Many more ICEs that are fixed by this 🎉

ice: Encountered error .. selecting .. during codegen: glacier / fixed/72793.rs #89312
ICE: impl-with-unconstrained-param.rs -Zsave-analysis --emit=mir : Err value: UnresolvedTy(_#0t) #87340
thread 'rustc' panicked at 'assertion failed: !erased_self_ty.has_escaping_bound_vars()' #86800
ICE: compiler/rustc_mir/src/borrow_check/universal_regions.rs:533:26: expected defining type for DefId(0:10 ~ mutant[317d]::{impl#0}::foo::{closure#0}): [type error] #86719
ICE on Current Stable #84073
ICE: compiler/rustc_mir/src/borrow_check/universal_regions.rs:533:26: expected defining type for DefId(0:21 ~ testint[7f45]::ice::{impl#0}::get_fut::{closure#0}): [type error] #83919
ICE with unknown variable in closure and impl trait in type alias #82139
type_alias_impl_trait ICE: Non-defining use of DefId #77987
ICE(expected defining type in borrowck) when existential type causes type error #74282
"internal compiler error" on Higher-Ranked Trait Bound for impl Trait in return position #67830
ICE: Generic type alias to invalid type crashes during type check on latest stable #62742
ICE when using anonymization in impl Trait with HRTB #54895

…sakis" This reverts commit e7cc3bd, reversing changes made to 734368a.

Revert lazy TAIT PR Revert rust-lang#92306 (sorry `@Aaron1011,` will include your changes in the fix PR) Revert rust-lang#93783 Revert rust-lang#92007 fixes rust-lang#93788 fixes rust-lang#93794 fixes rust-lang#93821 fixes rust-lang#93831 fixes rust-lang#93841

…t#92007)" This reverts commit 06c8426, reversing changes made to 68630fb. The fix at rustc has been reverted in rust-lang/rust#93893 due to other problems

Revert lazy TAIT PR Revert rust-lang/rust#92306 (sorry `@Aaron1011,` will include your changes in the fix PR) Revert rust-lang/rust#93783 Revert rust-lang/rust#92007 fixes rust-lang/rust#93788 fixes rust-lang/rust#93794 fixes rust-lang/rust#93821 fixes rust-lang/rust#93831 fixes rust-lang/rust#93841

…sakis" This reverts commit e7cc3bd, reversing changes made to 734368a.

…sakis Lazy type-alias-impl-trait take two ### user visible change 1: RPIT inference from recursive call sites Lazy TAIT has an insta-stable change. The following snippet now compiles, because opaque types can now have their hidden type set from wherever the opaque type is mentioned. ```rust fn bar(b: bool) -> impl std::fmt::Debug { if b { return 42 } let x: u32 = bar(false); // this errors on stable 99 } ``` The return type of `bar` stays opaque, you can't do `bar(false) + 42`, you need to actually mention the hidden type. ### user visible change 2: divergence between RPIT and TAIT in return statements Note that `return` statements and the trailing return expression are special with RPIT (but not TAIT). So ```rust #![feature(type_alias_impl_trait)] type Foo = impl std::fmt::Debug; fn foo(b: bool) -> Foo { if b { return vec![42]; } std::iter::empty().collect() //~ ERROR `Foo` cannot be built from an iterator } fn bar(b: bool) -> impl std::fmt::Debug { if b { return vec![42] } std::iter::empty().collect() // Works, magic (accidentally stabilized, not intended) } ``` But when we are working with the return value of a recursive call, the behavior of RPIT and TAIT is the same: ```rust type Foo = impl std::fmt::Debug; fn foo(b: bool) -> Foo { if b { return vec![]; } let mut x = foo(false); x = std::iter::empty().collect(); //~ ERROR `Foo` cannot be built from an iterator vec![] } fn bar(b: bool) -> impl std::fmt::Debug { if b { return vec![]; } let mut x = bar(false); x = std::iter::empty().collect(); //~ ERROR `impl Debug` cannot be built from an iterator vec![] } ``` ### user visible change 3: TAIT does not merge types across branches In contrast to RPIT, TAIT does not merge types across branches, so the following does not compile. ```rust type Foo = impl std::fmt::Debug; fn foo(b: bool) -> Foo { if b { vec![42_i32] } else { std::iter::empty().collect() //~^ ERROR `Foo` cannot be built from an iterator over elements of type `_` } } ``` It is easy to support, but we should make an explicit decision to include the additional complexity in the implementation (it's not much, see a721052457cf513487fb4266e3ade65c29b272d2 which needs to be reverted to enable this). ### PR formalities previous attempt: rust-lang#92007 This PR also includes rust-lang#92306 and rust-lang#93783, as they were reverted along with rust-lang#92007 in rust-lang#93893 fixes rust-lang#93411 fixes rust-lang#88236 fixes rust-lang#89312 fixes rust-lang#87340 fixes rust-lang#86800 fixes rust-lang#86719 fixes rust-lang#84073 fixes rust-lang#83919 fixes rust-lang#82139 fixes rust-lang#77987 fixes rust-lang#74282 fixes rust-lang#67830 fixes rust-lang#62742 fixes rust-lang#54895

…lang/rust#92007)"" This reverts commit c34cdca. 89312 is fixed by rust-lang/rust#95471 The rest by lazy TAIT take two rust-lang/rust#94081

…lang/rust#92007)"" This reverts commit c34cdca. 89312 is fixed by rust-lang/rust#95471 The rest by lazy TAIT take two rust-lang/rust#94081 Closes #1189 Closes #1190 Closes #1191 Closes #1192 Closes #1193 Closes #1194 Closes #1195 Closes #1196 Closes #1197 Closes #1198 Closes #1199 Closes #1200 Closes #1201

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Dec 16, 2021

oli-obk mentioned this pull request Dec 17, 2021

Add new opaque type predicate #92046

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oli-obk mentioned this pull request Jan 12, 2022

Actually instantiate the opaque type when checking bounds #90948

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oli-obk force-pushed the lazy_tait2 branch 2 times, most recently from cfb113a to be6eaf7 Compare January 12, 2022 17:01

rustbot added the T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. label Jan 12, 2022

oli-obk force-pushed the lazy_tait2 branch from be6eaf7 to 847da2e Compare January 19, 2022 13:52

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WaffleLapkin reviewed Feb 8, 2022

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oli-obk mentioned this pull request Feb 8, 2022

Fix regression from lazy opaque types #93783

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matthiaskrgr mentioned this pull request Feb 8, 2022

ICE: glacier fixed/62529-07.rs #93788

Closed

compiler-errors mentioned this pull request Feb 9, 2022

Nightly fails to infer the type of iterator collect & impl IntoIterator, and gives confusing error message #93794

Closed

Alexendoo added a commit to rust-lang/glacier that referenced this pull request Feb 9, 2022

Merge ICEs fixed by lazy type-alias-impl-trait (rust-lang/rust#92007)

06c8426

hawkw mentioned this pull request Feb 9, 2022

#92007 appears to break inference for previously compiling code #93821

Closed

compiler-errors mentioned this pull request Feb 9, 2022

Internal Compiler Error when compiling rocket = "0.5.0-rc.1" #93831

Closed

oli-obk added a commit to oli-obk/rust that referenced this pull request Feb 11, 2022

Revert "Auto merge of rust-lang#92007 - oli-obk:lazy_tait2, r=nikomat…

d54195d

…sakis" This reverts commit e7cc3bd, reversing changes made to 734368a.

oli-obk mentioned this pull request Feb 11, 2022

Revert lazy TAIT PR #93893

Merged

djc mentioned this pull request Feb 15, 2022

Tracking issue for RFC 2515, "Permit impl Trait in type aliases" #63063

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8 tasks

oli-obk mentioned this pull request Feb 17, 2022

Lazy type-alias-impl-trait take two #94081

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flip1995 pushed a commit to flip1995/rust that referenced this pull request Feb 24, 2022

Revert "Auto merge of rust-lang#92007 - oli-obk:lazy_tait2, r=nikomat…

8173aba

…sakis" This reverts commit e7cc3bd, reversing changes made to 734368a.

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Lazy type-alias-impl-trait #92007

Lazy type-alias-impl-trait #92007

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Lazy type-alias-impl-trait #92007

Lazy type-alias-impl-trait #92007

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