[META] Frontend completeness

[W95Psp]

We want the frontend of hax to be complete, that is:

• all Rust constructs should be handled;
• the frontend shall not crash.

Currently the main pain with frontend is traits: we implement our own logic for finding implementations for traits. This part is the only "smart" part of the frontend, I would say.
This trait implementation resolver is the only part of the frontend where we (1) go deep down rustc's AST (2) reconstruct some complicated data and ask Rust something. Here "reconstruct some complicated data" involves binders: we have a hard time reconstructing binders correctly, thus we break invariants and rustc crashes.

List of bugs:

• Stack overflow for deep expressions in bincode serialization/deserialization #875
• Panic: type parameter out of range when instantiating #747
• aes crate: the frontend loops indefinitely #686
• Frontend crash: ImplExprPredNotFound on bounds on opaque types #684
• Frontend: panic on default traits parameters #310
• I suspect this is fixed by Support impl Trait in trait resolution #1008:
  Unreachable place in Rust AST on tvix/nix-compat - ImplExprPredNotFound #604

Ccing @R1kM @Nadrieril @sonmarcho who are probably interested

Action items

• Catch those failures and print nice warnings
• Add item-wise catches? to improve UX
• Frontend: tracing-tree: improve usability #944
• How long would that take?

[W95Psp]

Relevant:

• More improvements to trait resolution #977

[W95Psp]

We fixed all the issues here, the only one left is #1046, which is related to GATs
The plan is for the engine to move to @Nadrieril's great new enum FullDef, which gets rid of HIR construct, which is a bit of a technical debt. #1072 tracks that.