align rustc and chalk folder type

[nikomatsakis]

• chalk's folder is fallible and has a Result, rustc does not
  • MCP: Make TypeFolder::fold_* return Result compiler-team#432
• chalk's folder is generic over the type family

[nikomatsakis]

@rustbot assign @LeSeulArtichaut

[nikomatsakis]

(cc @rust-lang/release -- any tips on how to make the triagebot work :)

[Mark-Simulacrum]

(Triagebot should work now, fixed the permissions).

[eggyal]

I understand from rust-lang/rust#85485 (comment) that @LeSeulArtichaut isn't able to work on these at present. Would it be okay if I have a crack at moving them forward? I've already opened rust-lang/rust#91230 to pick up where rust-lang/rust#85469 left off.

@rustbot claim

[eggyal]

With rust-lang/rust#91230 merged, the first of these actions can be marked complete.

[eggyal]

The second action here had me confused for a while, until I spotted that TypeFamily was renamed Interner in rust-lang/chalk#329.

[eggyal]

With rust-lang/rust#107924 merged, rustc's folder is also now generic over the Interner (formerly Type Family).

And since chalk is no longer to be integrated with rustc, I guess further alignment of the folder types is not required?

[jackh726]

Well, how are they still different? Might give some clues on if the rustc folder traits should be modified for some future solver state