ICE: Rust spins when referencing associated types in `where` clause

[andymac-2]

The following code broke the the text editor I was editing it in:

Version

rustc 1.35.0 (3c235d5 2019-05-20)

Minimal test case:

struct MyStruct<'a, 'b, A, B, F>(F, &'a A, &'b B);

trait MyTrait {
    type Input;
    type Output;
}

trait MyOtherTrait<A, B> {};

impl<'a, 'b, A, B, F> MyTrait for MyStruct<'a, 'b, A, B, F> where
    F: MyOtherTrait<Self::Input, Self::Output>
{
    type Input = A;
    type Output = B;
}

fn garbage () {
    This function body can contain garbage.

    We can put anything here as long as we don't write a close brace
    and the program will still try to compile.

    ((((((( [[[[[[[[ {{{{{
}

Output:

rustc will print any errors that occur, but will still try to compile, hanging indefinitely at the build stage and consuming all system memory.

Expected output:

Compilation failure.

[estebank]

There seems to be some kind of loop when evaluating try_print_visible_def_path:

DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: cur_def_key=DefKey { parent: Some(DefIndex(1432)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("Sized"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate2, id=DefIndex(1432))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(1432))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("marker"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: def_id=DefId(2:1432 ~ core[f7fb]::marker[0])
DEBUG 2019-07-06T16:31:24Z: rustc::ty::query::plumbing: ty::query::get_query<visible_parent_map>(key=crate0, span=file6.rs:1:1: 1:1)
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: cur_def_key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("marker"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate1, id=DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate1 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: def_id=DefId(1:0 ~ std[e8ce])
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate1, id=DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate1 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::query::plumbing: ty::query::get_query<extern_crate>(key=DefId(1:0 ~ std[e8ce]), span=file6.rs:1:1: 1:1)
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(11))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("std"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: def_id=DefId(0:11 ~ file6[317d]::std[0])
DEBUG 2019-07-06T16:31:24Z: rustc::ty::query::plumbing: ty::query::get_query<crate_name>(key=crate1, span=file6.rs:1:1: 1:1)
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate1, id=DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate1 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate2, id=DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: visible_parent=DefId(1:0 ~ std[e8ce]) actual_parent=Some(DefId(2:0 ~ core[f7fb]))
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate1, id=DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate1 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate2, id=DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: data=TypeNs("marker") visible_parent=DefId(1:0 ~ std[e8ce]) actual_parent=Some(DefId(2:0 ~ core[f7fb]))
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate1, id=DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate1 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::query::plumbing: ty::query::get_query<item_children>(key=DefId(1:0 ~ std[e8ce]), span=file6.rs:1:1: 1:1)
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: data=TypeNs("marker")
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate2, id=DefIndex(1432))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(1432))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("marker"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate2, id=DefIndex(1432))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(1432))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("marker"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: visible_parent=DefId(2:1432 ~ core[f7fb]::marker[0]) actual_parent=Some(DefId(2:1432 ~ core[f7fb]::marker[0]))
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate2, id=DefIndex(1432))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(1432))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("marker"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc_metadata::decoder: def_path(cnum=crate2, id=DefIndex(1432))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(1432))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("marker"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate2 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: data=TypeNs("Sized") visible_parent=DefId(2:1432 ~ core[f7fb]::marker[0]) actual_parent=Some(DefId(2:1432 ~ core[f7fb]::marker[0]))
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: data=TypeNs("Sized")
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(23))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(22)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("Input"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(22))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("MyTrait"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print: default_print_def_path: def_id=DefId(0:23 ~ file6[317d]::MyTrait[0]::Input[0]), substs=[Self]
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print: default_print_def_path: key=DefKey { parent: Some(DefIndex(22)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("Input"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(23))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(22)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("Input"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(22))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("MyTrait"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::query::plumbing: ty::query::get_query<generics_of>(key=DefId(0:23 ~ file6[317d]::MyTrait[0]::Input[0]), span=file6.rs:1:1: 1:1)
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(22))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("MyTrait"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::query::plumbing: ty::query::get_query<generics_of>(key=DefId(0:22 ~ file6[317d]::MyTrait[0]), span=file6.rs:1:1: 1:1)
DEBUG 2019-07-06T16:31:24Z: rustc::ty::fold: HasTypeFlagsVisitor: t=Self t.flags=HAS_SELF | HAS_FREE_LOCAL_NAMES self.flags=KEEP_IN_LOCAL_TCX
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(22))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("MyTrait"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print: default_print_def_path: def_id=DefId(0:22 ~ file6[317d]::MyTrait[0]), substs=[Self]
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print: default_print_def_path: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("MyTrait"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(22))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("MyTrait"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::query::plumbing: ty::query::get_query<generics_of>(key=DefId(0:22 ~ file6[317d]::MyTrait[0]), span=file6.rs:1:1: 1:1)
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(22))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("MyTrait"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: def_id=DefId(0:22 ~ file6[317d]::MyTrait[0])
DEBUG 2019-07-06T16:31:24Z: rus^CDEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("MyTrait"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print: default_print_def_path: def_id=DefId(0:22 ~ file6[317d]::MyTrait[0]), substs=[]
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print: default_print_def_path: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("MyTrait"), disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: krate=crate0 index=Some(DefIndex(0))
DEBUG 2019-07-06T16:31:24Z: rustc::hir::map::definitions: DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
DEBUG 2019-07-06T16:31:24Z: rustc::ty::print::pretty: try_print_visible_def_path: def_id=DefId(0:0 ~ file6[317d])

[estebank]

This used to fail in a graceful manner in 1.30:

error[E0309]: the parameter type `A` may not live long enough
 --> <source>:3:37
  |
3 | struct MyStruct<'a, 'b, A, B, F>(F, &'a A, &'b B);
  |                         -           ^^^^^
  |                         |
  |                         help: consider adding an explicit lifetime bound `A: 'a`...
  |
note: ...so that the reference type `&'a A` does not outlive the data it points at
 --> <source>:3:37
  |
3 | struct MyStruct<'a, 'b, A, B, F>(F, &'a A, &'b B);
  |                                     ^^^^^

error[E0309]: the parameter type `B` may not live long enough
 --> <source>:3:44
  |
3 | struct MyStruct<'a, 'b, A, B, F>(F, &'a A, &'b B);
  |                            -               ^^^^^
  |                            |
  |                            help: consider adding an explicit lifetime bound `B: 'b`...
  |
note: ...so that the reference type `&'b B` does not outlive the data it points at
 --> <source>:3:44
  |
3 | struct MyStruct<'a, 'b, A, B, F>(F, &'a A, &'b B);
  |                                            ^^^^^

error[E0275]: overflow evaluating the requirement `<MyStruct<'_, '_, A, B, F> as MyTrait>::Input`
  --> <source>:12:1
   |
12 | / impl<'a, 'b, A, B, F> MyTrait for MyStruct<'a, 'b, A, B, F> where
13 | |     F: MyOtherTrait<Self::Input, Self::Output>
14 | | {
15 | |     type Input = A;
16 | |     type Output = B;
17 | | }
   | |_^
   |
   = help: consider adding a `#![recursion_limit="128"]` attribute to your crate
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
8<8<8<8<8<8<8<8<8<8<8<
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'a, 'b, A, B, F>`

error: aborting due to 3 previous errors

[estebank]

This is a case of exponential explosion:

With #![recursion_limit="4"]:

$ time rustc +devel file7.rs
error[E0275]: overflow evaluating the requirement `MyStruct<'_, '_, A, B, F>: MyTrait`
  --> file7.rs:14:1
   |
14 | / impl<'a, 'b, A, B, F> MyTrait for MyStruct<'a, 'b, A, B, F> where
15 | |     F: MyOtherTrait<Self::Input, Self::Output>
16 | | {
17 | |     type Input = A;
18 | |     type Output = B;
19 | | }
   | |_^
   |
   = help: consider adding a `#![recursion_limit="8"]` attribute to your crate
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'a, 'b, A, B, F>`

error[E0275]: overflow evaluating the requirement `MyStruct<'_, '_, A, B, F>: MyTrait`
  --> file7.rs:17:5
   |
17 |     type Input = A;
   |     ^^^^^^^^^^^^^^^
   |
   = help: consider adding a `#![recursion_limit="8"]` attribute to your crate
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'a, 'b, A, B, F>`

error[E0275]: overflow evaluating the requirement `MyStruct<'_, '_, A, B, F>: MyTrait`
  --> file7.rs:18:5
   |
18 |     type Output = B;
   |     ^^^^^^^^^^^^^^^^
   |
   = help: consider adding a `#![recursion_limit="8"]` attribute to your crate
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'a, 'b, A, B, F>`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0275`.

real	0m0.128s
user	0m0.042s
sys	0m0.039s

With #![recursion_limit="8"]:

$ time rustc +devel file7.rs
error[E0275]: overflow evaluating the requirement `MyStruct<'_, '_, A, B, F>: MyTrait`
  --> file7.rs:14:1
   |
14 | / impl<'a, 'b, A, B, F> MyTrait for MyStruct<'a, 'b, A, B, F> where
15 | |     F: MyOtherTrait<Self::Input, Self::Output>
16 | | {
17 | |     type Input = A;
18 | |     type Output = B;
19 | | }
   | |_^
   |
   = help: consider adding a `#![recursion_limit="16"]` attribute to your crate
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'a, 'b, A, B, F>`

error[E0275]: overflow evaluating the requirement `MyStruct<'_, '_, A, B, F>: MyTrait`
  --> file7.rs:17:5
   |
17 |     type Input = A;
   |     ^^^^^^^^^^^^^^^
   |
   = help: consider adding a `#![recursion_limit="16"]` attribute to your crate
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'a, 'b, A, B, F>`

error[E0275]: overflow evaluating the requirement `MyStruct<'_, '_, A, B, F>: MyTrait`
  --> file7.rs:18:5
   |
18 |     type Output = B;
   |     ^^^^^^^^^^^^^^^^
   |
   = help: consider adding a `#![recursion_limit="16"]` attribute to your crate
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'a, 'b, A, B, F>`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0275`.

real	0m0.118s
user	0m0.071s
sys	0m0.038s

With #![recursion_limit="16"]:

$ time rustc +devel file7.rs
error[E0275]: overflow evaluating the requirement `MyStruct<'_, '_, A, B, F>: MyTrait`
  --> file7.rs:14:1
   |
14 | / impl<'a, 'b, A, B, F> MyTrait for MyStruct<'a, 'b, A, B, F> where
15 | |     F: MyOtherTrait<Self::Input, Self::Output>
16 | | {
17 | |     type Input = A;
18 | |     type Output = B;
19 | | }
   | |_^
   |
   = help: consider adding a `#![recursion_limit="32"]` attribute to your crate
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'a, 'b, A, B, F>`

error[E0275]: overflow evaluating the requirement `MyStruct<'_, '_, A, B, F>: MyTrait`
  --> file7.rs:17:5
   |
17 |     type Input = A;
   |     ^^^^^^^^^^^^^^^
   |
   = help: consider adding a `#![recursion_limit="32"]` attribute to your crate
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'a, 'b, A, B, F>`

error[E0275]: overflow evaluating the requirement `MyStruct<'_, '_, A, B, F>: MyTrait`
  --> file7.rs:18:5
   |
18 |     type Output = B;
   |     ^^^^^^^^^^^^^^^^
   |
   = help: consider adding a `#![recursion_limit="32"]` attribute to your crate
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'_, '_, A, B, F>`
   = note: required because of the requirements on the impl of `MyTrait` for `MyStruct<'a, 'b, A, B, F>`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0275`.

real	0m16.257s
user	0m14.831s
sys	0m1.363s

[estebank]

CC #60846 #61960

@nikomatsakis I believe this is related to (not introduced in) #61754.

[nagisa]

Assigning to self to get an accurate bisection.