hir_analysis: add missing sizedness bounds #142712

davidtwco · 2025-06-19T09:17:29Z

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.

rustbot · 2025-06-19T09:17:34Z

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rustbot · 2025-06-19T09:17:37Z

HIR ty lowering was modified

cc @fmease

davidtwco · 2025-06-19T09:41:26Z

Wasn't expecting that failure, looking into it.

davidtwco · 2025-06-19T10:01:32Z

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`.

petrochenkov · 2025-06-19T15:05:52Z

r? types

compiler-errors · 2025-06-19T16:43:05Z

I'd like to think about this...

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

tests/ui/sized-hierarchy/bound-on-assoc-type-projection.rs

davidtwco · 2025-06-23T20:38:24Z

I'd like to think about this...

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

Yes, it is

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`.

compiler-errors · 2025-06-27T14:15:40Z

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 · 2025-06-27T16:10:29Z

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.

rust-log-analyzer · 2025-06-27T16:23:04Z

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

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

   Compiling crc32fast v1.4.2
error[E0308]: mismatched types
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:96:39
   |
96 |     <K as ZeroMapKV<'zf>>::Container: ZeroFrom<'zf, <K as ZeroMapKV<'s>>::Container>,
   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected trait `<<K as kv::ZeroMapKV<'s>>::Container as MetaSized>`
              found trait `<<K as kv::ZeroMapKV<'zf>>::Container as MetaSized>`
note: the lifetime `'zf` as defined here...
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:92:6
   |
92 | impl<'zf, 's, K, V> ZeroFrom<'zf, ZeroMap<'s, K, V>> for ZeroMap<'zf, K, V>
   |      ^^^
note: ...does not necessarily outlive the lifetime `'s` as defined here
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:92:11
   |
92 | impl<'zf, 's, K, V> ZeroFrom<'zf, ZeroMap<'s, K, V>> for ZeroMap<'zf, K, V>
   |           ^^

error[E0308]: mismatched types
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:96:39
   |
96 |     <K as ZeroMapKV<'zf>>::Container: ZeroFrom<'zf, <K as ZeroMapKV<'s>>::Container>,
   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected trait `<<K as kv::ZeroMapKV<'s>>::Container as MetaSized>`
              found trait `<<K as kv::ZeroMapKV<'zf>>::Container as MetaSized>`
note: the lifetime `'s` as defined here...
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:92:11
   |
92 | impl<'zf, 's, K, V> ZeroFrom<'zf, ZeroMap<'s, K, V>> for ZeroMap<'zf, K, V>
   |           ^^
note: ...does not necessarily outlive the lifetime `'zf` as defined here
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:92:6
   |
92 | impl<'zf, 's, K, V> ZeroFrom<'zf, ZeroMap<'s, K, V>> for ZeroMap<'zf, K, V>
   |      ^^^

error[E0308]: mismatched types
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:112:40
    |
112 |     <K0 as ZeroMapKV<'zf>>::Container: ZeroFrom<'zf, <K0 as ZeroMapKV<'s>>::Container>,
    |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
    |
    = note: expected trait `<<K0 as kv::ZeroMapKV<'s>>::Container as MetaSized>`
               found trait `<<K0 as kv::ZeroMapKV<'zf>>::Container as MetaSized>`
note: the lifetime `'zf` as defined here...
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:107:6
    |
107 | impl<'zf, 's, K0, K1, V> ZeroFrom<'zf, ZeroMap2d<'s, K0, K1, V>> for ZeroMap2d<'zf, K0, K1, V>
    |      ^^^
note: ...does not necessarily outlive the lifetime `'s` as defined here
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:107:11
    |
107 | impl<'zf, 's, K0, K1, V> ZeroFrom<'zf, ZeroMap2d<'s, K0, K1, V>> for ZeroMap2d<'zf, K0, K1, V>
    |           ^^

error[E0308]: mismatched types
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:112:40
    |
112 |     <K0 as ZeroMapKV<'zf>>::Container: ZeroFrom<'zf, <K0 as ZeroMapKV<'s>>::Container>,
    |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
    |
    = note: expected trait `<<K0 as kv::ZeroMapKV<'s>>::Container as MetaSized>`
               found trait `<<K0 as kv::ZeroMapKV<'zf>>::Container as MetaSized>`
note: the lifetime `'s` as defined here...
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:107:11
    |
107 | impl<'zf, 's, K0, K1, V> ZeroFrom<'zf, ZeroMap2d<'s, K0, K1, V>> for ZeroMap2d<'zf, K0, K1, V>
    |           ^^
note: ...does not necessarily outlive the lifetime `'zf` as defined here
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:107:6
    |
107 | impl<'zf, 's, K0, K1, V> ZeroFrom<'zf, ZeroMap2d<'s, K0, K1, V>> for ZeroMap2d<'zf, K0, K1, V>
    |      ^^^

[RUSTC-TIMING] crc32fast test:false 0.513
   Compiling serde v1.0.219
[RUSTC-TIMING] build_script_build test:false 0.358
   Compiling rustc-literal-escaper v0.0.4
[RUSTC-TIMING] rustc_literal_escaper test:false 0.492
error[E0521]: borrowed data escapes outside of associated function
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/yoke_impls.rs:138:20
    |
107 | unsafe impl<'a, K, V> Yokeable<'a> for ZeroMap<'static, K, V>
    |             -- lifetime `'a` defined here
...
136 |     unsafe fn make(from: Self::Output) -> Self {
    |                    ----
    |                    |
    |                    `from` declared here, outside of the associated function body
    |                    `from` is a reference that is only valid in the associated function body
137 |         debug_assert!(mem::size_of::<Self::Output>() == mem::size_of::<Self>());
138 |         let from = mem::ManuallyDrop::new(from);
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                    |
    |                    `from` escapes the associated function body here
    |                    argument requires that `'a` must outlive `'static`
    |
    = note: requirement occurs because of the type `ZeroMap<'_, K, V>`, which makes the generic argument `'_` invariant
    = note: the struct `ZeroMap<'a, K, V>` is invariant over the parameter `'a`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

error[E0521]: borrowed data escapes outside of associated function
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/yoke_impls.rs:232:20
    |
199 | unsafe impl<'a, K0, K1, V> Yokeable<'a> for ZeroMap2d<'static, K0, K1, V>
    |             -- lifetime `'a` defined here
...
230 |     unsafe fn make(from: Self::Output) -> Self {
    |                    ----
    |                    |
    |                    `from` declared here, outside of the associated function body
    |                    `from` is a reference that is only valid in the associated function body
231 |         debug_assert!(mem::size_of::<Self::Output>() == mem::size_of::<Self>());
232 |         let from = mem::ManuallyDrop::new(from);
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                    |
    |                    `from` escapes the associated function body here
    |                    argument requires that `'a` must outlive `'static`
    |
    = note: requirement occurs because of the type `ZeroMap2d<'_, K0, K1, V>`, which makes the generic argument `'_` invariant
    = note: the struct `ZeroMap2d<'a, K0, K1, V>` is invariant over the parameter `'a`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

error: lifetime may not live long enough
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:101:19
    |
92  | impl<'zf, 's, K, V> ZeroFrom<'zf, ZeroMap<'s, K, V>> for ZeroMap<'zf, K, V>
    |      ---  -- lifetime `'s` defined here
    |      |
    |      lifetime `'zf` defined here
...
101 |             keys: K::Container::zero_from(&other.keys),
    |                   ^^^^^^^^^^^^^^^^^^^^^^^ requires that `'zf` must outlive `'s`
    |
    = help: consider adding the following bound: `'zf: 's`

error: lifetime may not live long enough
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerovec-0.10.4/src/zerofrom_impls.rs:118:20
    |
107 | impl<'zf, 's, K0, K1, V> ZeroFrom<'zf, ZeroMap2d<'s, K0, K1, V>> for ZeroMap2d<'zf, K0, K1, V>
    |      ---  -- lifetime `'s` defined here
    |      |
    |      lifetime `'zf` defined here
...
118 |             keys0: K0::Container::zero_from(&other.keys0),
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'zf` must outlive `'s`
    |
    = help: consider adding the following bound: `'zf: 's`

Some errors have detailed explanations: E0308, E0521.
For more information about an error, try `rustc --explain E0308`.
[RUSTC-TIMING] zerovec test:false 2.411
error: could not compile `zerovec` (lib) due to 8 previous errors

rustbot assigned petrochenkov Jun 19, 2025

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 added the T-types Relevant to the types team, which will review and decide on the PR/issue. label Jun 19, 2025

rustbot assigned jackh726 and unassigned petrochenkov Jun 19, 2025

compiler-errors assigned compiler-errors and unassigned jackh726 Jun 19, 2025

fmease reviewed Jun 19, 2025

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tests/ui/sized-hierarchy/bound-on-assoc-type-projection.rs Show resolved Hide resolved

davidtwco added 2 commits June 27, 2025 10:03

tests: add regression test for stage2 ICE

7684842

davidtwco force-pushed the sized-hierarchy-missing-default-bounds branch from 60d5044 to 7684842 Compare June 27, 2025 16:09

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