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@davidtwco davidtwco commented Jun 19, 2025

Depends on #144064

Default sizedness bounds were not being added to explicit_super_predicates_of and explicit_implied_predicates_of which meant that a trait bound added to a associated type projection would be missing the implied predicate of the default sizedness supertrait of that trait.

An unexpected consequence of this change was that the check for multiple principals was now finding an additional MetaSized principal when eagerly expanding trait aliases - which is fixed by skipping MetaSized when elaborating trait aliases in lowering dyn TraitAlias.

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rustbot commented Jun 19, 2025

r? @petrochenkov

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@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. WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) labels Jun 19, 2025
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rustbot commented Jun 19, 2025

HIR ty lowering was modified

cc @fmease

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Wasn't expecting that failure, looking into it.

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davidtwco commented Jun 19, 2025

A smaller example of one of the failing crates (which happens with a stage two build):

#![crate_type = "lib"]

use std::marker::PhantomData;

pub trait ZeroMapKV<'a> {
    type Container;
}

pub trait ZeroFrom<'zf, C: ?Sized> {}

pub struct ZeroMap<'a, K: ZeroMapKV<'a>>(PhantomData<&'a K>);

impl<'zf, 's, K> ZeroFrom<'zf, ZeroMap<'s, K>> for ZeroMap<'zf, K>
where
    K: for<'b> ZeroMapKV<'b>,
    <K as ZeroMapKV<'zf>>::Container: ZeroFrom<'zf, <K as ZeroMapKV<'s>>::Container>,
{
}
error[E0308]: mismatched types
  --> f.rs:16:39
   |
16 |     <K as ZeroMapKV<'zf>>::Container: ZeroFrom<'zf, <K as ZeroMapKV<'s>>::Container>,
   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected trait `<<K as ZeroMapKV<'s>>::Container as MetaSized>`
              found trait `<<K as ZeroMapKV<'zf>>::Container as MetaSized>`
note: the lifetime `'zf` as defined here...
  --> f.rs:13:6
   |
13 | impl<'zf, 's, K> ZeroFrom<'zf, ZeroMap<'s, K>> for ZeroMap<'zf, K>
   |      ^^^
note: ...does not necessarily outlive the lifetime `'s` as defined here
  --> f.rs:13:11
   |
13 | impl<'zf, 's, K> ZeroFrom<'zf, ZeroMap<'s, K>> for ZeroMap<'zf, K>
   |           ^^

error[E0308]: mismatched types
  --> f.rs:16:39
   |
16 |     <K as ZeroMapKV<'zf>>::Container: ZeroFrom<'zf, <K as ZeroMapKV<'s>>::Container>,
   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected trait `<<K as ZeroMapKV<'s>>::Container as MetaSized>`
              found trait `<<K as ZeroMapKV<'zf>>::Container as MetaSized>`
note: the lifetime `'s` as defined here...
  --> f.rs:13:11
   |
13 | impl<'zf, 's, K> ZeroFrom<'zf, ZeroMap<'s, K>> for ZeroMap<'zf, K>
   |           ^^
note: ...does not necessarily outlive the lifetime `'zf` as defined here
  --> f.rs:13:6
   |
13 | impl<'zf, 's, K> ZeroFrom<'zf, ZeroMap<'s, K>> for ZeroMap<'zf, K>
   |      ^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0308`.

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r? types

@rustbot rustbot added the T-types Relevant to the types team, which will review and decide on the PR/issue. label Jun 19, 2025
@rustbot rustbot assigned jackh726 and unassigned petrochenkov Jun 19, 2025
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I'd like to think about this...

Can you remind me, is trait Foo {} actually trait Foo: MetaSized {}?

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I'd like to think about this...

Can you remind me, is trait Foo {} actually trait Foo: MetaSized {}?

Yes, it is

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compiler-errors commented Jun 27, 2025

I think this highlights a problematic interaction with trait aliases, and I'm tempted to say that trait aliases should have their behavior reworked to stop expanding into their bounds in dyn types. This would, for example, also fix dyn TraitAlias when trait TraitAlias = A + B, but that's a bigger change to advocate for I guess.

FOr now, I think it's inconsistent to not add MetaSized to trait aliases but add them to traits, and I think instead, to preserve/fix the tests, we should probably filter out MetaSized when we elaborate trait aliases when lowering dyn TraitAlias. This behavior should probably be tagged with a FIXME though.

@davidtwco davidtwco force-pushed the sized-hierarchy-missing-default-bounds branch from 60d5044 to 7684842 Compare June 27, 2025 16:09
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I think this highlights a problematic interaction with trait aliases, and I'm tempted to say that trait aliases should have their behavior reworked to stop expanding into their bounds in dyn types. This would, for example, also fix dyn TraitAlias when trait TraitAlias = A + B, but that's a bigger change to advocate for I guess.

FOr now, I think it's inconsistent to not add MetaSized to trait aliases but add them to traits, and I think instead, to preserve/fix the tests, we should probably filter out MetaSized when we elaborate trait aliases when lowering dyn TraitAlias. This behavior should probably be tagged with a FIXME though.

Changed to this, thanks.


Still hitting the stage two failure, though I've added a UI test with my reproduction so that will fail first, not had much progress on working out what's going wrong with it.

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@davidtwco davidtwco force-pushed the sized-hierarchy-missing-default-bounds branch from 7684842 to eac7da1 Compare July 3, 2025 14:18
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See discussion on Zulip (#t-types > relaxing associated type bounds @ 💬) for background behind the last commit's changes.

davidtwco added a commit to davidtwco/rust that referenced this pull request Jul 3, 2025
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@bors try

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bors commented Jul 7, 2025

⌛ Trying commit eac7da1 with merge 7df376b...

bors added a commit that referenced this pull request Jul 7, 2025
…nds, r=<try>

hir_analysis: add missing sizedness bounds

Default sizedness bounds were not being added to `explicit_super_predicates_of` and `explicit_implied_predicates_of` which meant that a trait bound added to a associated type projection would be missing the implied predicate of the default sizedness supertrait of that trait.

An unexpected consequence of this change was that the check for multiple principals was now finding an additional `MetaSized` principal when eagerly expanding trait aliases - this required modifying lowering to no longer add default bounds to trait aliases.
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bors commented Jul 7, 2025

☀️ Try build successful - checks-actions
Build commit: 7df376b (7df376b035b3d12ff0386ad8b45b76dbe020257b)

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@craterbot check

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👌 Experiment pr-142712 created and queued.
🤖 Automatically detected try build 7df376b
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

@craterbot craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 7, 2025
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🚧 Experiment pr-142712 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

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🎉 Experiment pr-142712 is completed!
📊 11 regressed and 5 fixed (661105 total)
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

@craterbot craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Jul 8, 2025
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🎉 Experiment pr-142712 is completed! 📊 11 regressed and 5 fixed (661105 total) 📰 Open the summary report.

All these failures appear spurious—fail during linking or build script.

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Jules-Bertholet commented Jul 8, 2025

I think this highlights a problematic interaction with trait aliases, and I'm tempted to say that trait aliases should have their behavior reworked to stop expanding into their bounds in dyn types. This would, for example, also fix dyn TraitAlias when trait TraitAlias = A + B, but that's a bigger change to advocate for I guess.

That would conflict with the goals of my RFC 3437. Crates that use that RFC’s features to backward-compatibly expand their trait hierarchy, would find themselves saddled with a proliferation of equivalent-yet-distinct dyn types, risking all sorts of complications.

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Jules-Bertholet commented Jul 9, 2025

FOr now, I think it's inconsistent to not add MetaSized to trait aliases but add them to traits, and I think instead, to preserve/fix the tests, we should probably filter out MetaSized when we elaborate trait aliases when lowering dyn TraitAlias.

Actually, I think that adding MetaSized to trait aliases is inconsistent with type aliases. E.g. type Ptr<T> = *const T; doesn’t add an implicit T: Sized. I prefer the behavior from the original version of this PR. I suppose it could cause problems if you have a trait alias with no traits (trait Foo =;), but that can probably just be forbidden. (Edit: actually I think it’s fine?)

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fmease commented Jul 9, 2025

adding MetaSized to [trait] aliases is inconsistent with type aliases

Trait & type aliases are nothing alike, I wouldn't point at type aliases which are broken AF atm.

E.g. type Ref<T> = &T; doesn’t add an implicit T: Sized

We do actually (and always have) (you can see that for yourself by applying #[rustc_dump_predicates] to the type alias), it just doesn't get enforced — which is but a bug. CC #112792. This isn't a technicality, I just want to highlight that you truly shouldn't take inspiration from today's type aliases.

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We do actually (and always have) (you can see that for yourself by applying #[rustc_dump_predicates] to the type alias), it just doesn't get enforced — which is but a bug.

I don’t see it as a bug, I think it’s a feature. To me, “alias” implying “the thing on the left means exactly the same as the thing on the right, no more nor less” is the natural and intuitive behavior. (Explicit bounds being unenforced is a bug, of course.)

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Similarly, in this example:

struct Foo<T: Send>(T);
type Alias<T> = Foo<T>;

I think it’s good and useful that the declaration of Alias does not need to repeat the T: Send bound.

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This also fixes #143992 by preferring the alias candidates over param candidates for sizedness. Re-assigning to lcnr as I've more discussions about this PR with them:

r? @lcnr

Default sizedness bounds were not being added to
`explicit_super_predicates_of` and `explicit_implied_predicates_of`
which meant that a trait bound added to a associated type projection
would be missing the implied predicate of the default sizedness
supertrait of that trait.

An unexpected consequence of this change was that the check for multiple
principals was now finding an additional `MetaSized` principal when
eagerly expanding trait aliases. Instead of special-casing trait aliases
as different from traits and not adding a `MetaSized` supertrait to trait
aliases, filter out `MetaSized` when lowering `dyn Trait`.
@davidtwco davidtwco force-pushed the sized-hierarchy-missing-default-bounds branch from eac7da1 to e6f31f1 Compare July 17, 2025 14:07
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  • There are issue links (such as #123) in the commit messages of the following commits.
    Please move them to the PR description, to avoid spamming the issues with references to the commit, and so this bot can automatically canonicalize them to avoid issues with subtree.

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We've split out some of the changes in this PR to #144064, so I've rebased this on top of that and will wait for it to land.

@davidtwco davidtwco marked this pull request as draft July 17, 2025 14:08
@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 Jul 17, 2025
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