Regression: overflow evaluating X: MetaSized

[Mark-Simulacrum]

See https://crater-reports.s3.amazonaws.com/beta-1.89-4/beta-2025-07-03/gh/red-lightning123.hwc/log.txt:

[INFO] [stdout] error[E0275]: overflow evaluating the requirement `ElementArray: MetaSized`
[INFO] [stdout]    --> hwc_lang_cbml/src/tags/src/parse/mod.rs:3:48
[INFO] [stdout]     |
[INFO] [stdout] 3   | pub fn parse_file_tokens(tokens : &[Token]) -> Result<File, String> {
[INFO] [stdout]     |                                                ^^^^^^^^^^^^^^^^^^^^
[INFO] [stdout]     |
[INFO] [stdout] note: required for `Box<ElementArray>` to implement `ParseTokens`
[INFO] [stdout]    --> hwc_lang_cbml/src/tags/src/parse/combinators.rs:5:32
[INFO] [stdout]     |
[INFO] [stdout] 5   | impl<T : ParseTokens + ?Sized> ParseTokens for Box<T> {
[INFO] [stdout]     |      -                         ^^^^^^^^^^^     ^^^^^^
[INFO] [stdout]     |      |
[INFO] [stdout]     |      unsatisfied trait bound introduced here
[INFO] [stdout] note: required because it appears within the type `NestElement`
[INFO] [stdout]    --> hwc_lang_cbml/src/tags/src/parse/nest_element.rs:7:5
[INFO] [stdout]     |
[INFO] [stdout] 7   |     NestElement,
[INFO] [stdout]     |     ^^^^^^^^^^^
[INFO] [stdout] note: required for `combinators::Any<(NestElement, TextElement, EmptyElement)>` to implement `ParseTokens`
[INFO] [stdout]    --> hwc_lang_cbml/src/tags/src/parse/combinators.rs:110:38
[INFO] [stdout]     |
[INFO] [stdout] 110 |           impl<$($T : ParseTokens, )*> ParseTokens for Any<($($T, )*)> {
[INFO] [stdout]     |                                        ^^^^^^^^^^^     ^^^^^^^^^^^^^^^
[INFO] [stdout] ...
[INFO] [stdout] 139 | / impl_any_up_to! {
[INFO] [stdout] 140 | |     [AnyVariants8 AnyVariants7 AnyVariants6 AnyVariants5 AnyVariants4 AnyVariants3 AnyVariants2 AnyVariants1]
[INFO] [stdout] 141 | |     [V8 V7 V6 V5 V4 V3 V2 V1]
[INFO] [stdout] 142 | |     [T8 T7 T6 T5 T4 T3 T2 T1]
[INFO] [stdout]     | |                           -- unsatisfied trait bound introduced here
[INFO] [stdout] 143 | | }
[INFO] [stdout]     | |_- in this macro invocation
[INFO] [stdout] note: required because it appears within the type `element::Element`
[INFO] [stdout]    --> hwc_lang_cbml/src/tags/src/parse/element.rs:7:5
[INFO] [stdout]     |
[INFO] [stdout] 7   |     Element,
[INFO] [stdout]     |     ^^^^^^^
[INFO] [stdout] note: required for `Group<(Optional<Whitespace>, element::Element, Optional<Whitespace>)>` to implement `ParseTokens`
[INFO] [stdout]    --> hwc_lang_cbml/src/tags/src/parse/combinators.rs:47:38
[INFO] [stdout]     |
[INFO] [stdout] 47  |         impl<$($T : ParseTokens, )*> ParseTokens for Group<($($T, )*)> {
[INFO] [stdout]     |                                      ^^^^^^^^^^^     ^^^^^^^^^^^^^^^^^
[INFO] [stdout] ...
[INFO] [stdout] 59  | impl_group_up_to! { T8 T7 T6 T5 T4 T3 T2 T1 }
[INFO] [stdout]     | ---------------------------------------------
[INFO] [stdout]     | |                                     |
[INFO] [stdout]     | |                                     unsatisfied trait bound introduced here
[INFO] [stdout]     | in this macro invocation
[INFO] [stdout] note: required because it appears within the type `file::File`
[INFO] [stdout]    --> hwc_lang_cbml/src/tags/src/parse/file.rs:7:5
[INFO] [stdout]     |
[INFO] [stdout] 7   |     File,
[INFO] [stdout]     |     ^^^^
[INFO] [stdout] note: required by an implicit `Sized` bound in `Result`
[INFO] [stdout]    --> /rustc/b5e10d8c000a153cae6a14d70372dc4b39439a3f/library/core/src/result.rs:548:1
[INFO] [stdout]     = note: this error originates in the macro `impl_any_traits_up_to` which comes from the expansion of the macro `impl_group_up_to` (in Nightly builds, run with -Z macro-backtrace for more info)

[theemathas]

Minimized:

pub trait Mark {}

impl<T: ?Sized> Mark for *const T {}

pub struct Recursive(pub <Dummy as Trait>::Assoc);
impl Mark for Recursive {}

pub trait Trait {
    type Assoc;
}

pub struct Dummy;
impl Trait for Dummy
where
    *const Recursive: Mark,
{
    type Assoc = *mut Recursive;
}

Error output on beta:

error[E0275]: overflow evaluating the requirement `Recursive: MetaSized`
  --> src/lib.rs:5:26
   |
5  | pub struct Recursive(pub <Dummy as Trait>::Assoc);
   |                          ^^^^^^^^^^^^^^^^^^^^^^^
   |
note: required for `*const Recursive` to implement `Mark`
  --> src/lib.rs:3:17
   |
3  | impl<T: ?Sized> Mark for *const T {}
   |      -          ^^^^     ^^^^^^^^
   |      |
   |      unsatisfied trait bound introduced here
note: required for `Dummy` to implement `Trait`
  --> src/lib.rs:13:6
   |
13 | impl Trait for Dummy
   |      ^^^^^     ^^^^^
14 | where
15 |     *const Recursive: Mark,
   |                       ---- unsatisfied trait bound introduced here

error[E0275]: overflow evaluating the requirement `*const Recursive: Mark`
  --> src/lib.rs:6:15
   |
6  | impl Mark for Recursive {}
   |               ^^^^^^^^^
   |
note: required for `Dummy` to implement `Trait`
  --> src/lib.rs:13:6
   |
13 | impl Trait for Dummy
   |      ^^^^^     ^^^^^
14 | where
15 |     *const Recursive: Mark,
   |                       ---- unsatisfied trait bound introduced here
note: required because it appears within the type `Recursive`
  --> src/lib.rs:5:12
   |
5  | pub struct Recursive(pub <Dummy as Trait>::Assoc);
   |            ^^^^^^^^^
note: required by a bound in `Mark`
  --> src/lib.rs:1:1
   |
1  | pub trait Mark {}
   | ^^^^^^^^^^^^^^^^^ required by this bound in `Mark`

error[E0275]: overflow evaluating the requirement `Recursive: MetaSized`
  --> src/lib.rs:15:5
   |
15 |     *const Recursive: Mark,
   |     ^^^^^^^^^^^^^^^^^^^^^^
   |
note: required for `*const Recursive` to implement `Mark`
  --> src/lib.rs:3:17
   |
3  | impl<T: ?Sized> Mark for *const T {}
   |      -          ^^^^     ^^^^^^^^
   |      |
   |      unsatisfied trait bound introduced here
   = help: see issue #48214

For more information about this error, try `rustc --explain E0275`.
error: could not compile `tags` (lib) due to 3 previous errors

Presumably caused by #137944, cc @davidtwco

@rustbot labels +T-types +F-sized_hierarchy

[workingjubilee]

Should we revert the change to try and buy some time?

[compiler-errors]

This was a breaking change called out in #137944 (comment).

[compiler-errors]

Though it doesn't really seem like it was resolved: #137944 (comment), and so the breakage was landed without a fix AFAICT.

cc @lcnr @davidtwco