Miri exception improvements

[RalfJung]

The Miri engine uses "exceptions" (InterpResult) to report misbehaving executions and propagate this information outwards to the machine (const-eval/const-prop/miri-the-tool). When an error is raised, we optionally capture a backtrace to ease debugging.

There are two problems with this:

• Currently, checking whether we want to capture a backtrace is rather expensive (we check an env var). @wesleywiser proposed a scheme to use a session variable instead.
• Sometimes, the engine catches these "exceptions". This means we caught the backtrace in vein. Check RUSTC_CTFE_BACKTRACE much less by generating fewer errors #69290 brought some nice speed-up by removing just one case of catching an interpreter error! I think we should never catch these errors. Is there something we can do to prevent accidental catching?

Cc @oli-obk

[oli-obk]

The obvious solution is to go through the engine (we could still use the ? operator and basically not require any changes except to type signatures if we made it an associated constant or at least not an &self function), but that requires infecting all error types.

Didn't we also discuss checking the current query name somewhere?

[wesleywiser]

Didn't we also discuss checking the current query name somewhere?

Yes, it was proposed here but at least for ctfe-stress-4, that didn't pay off because most of the errors were created during const_eval-raw (see here).

[RalfJung]

@oli-obk I take you you are talking about the first point? As I wrote above, I think @wesleywiser's proposed scheme of using a session var is a good next step there -- certainly better than what we do now.

[RalfJung]

I found some more cases of exceptions being caught:

• rust/src/librustc_mir/interpret/validity.rs

  Line 758 in 97b3d81

  let op = self.force_op_ptr(op).unwrap_or(op);

• rust/src/librustc/mir/interpret/allocation.rs

  Line 384 in 97b3d81

  if self.check_defined(ptr, size).is_err() {

  (fixed by avoid catching InterpError #70375)

The first one occurs quite a bit in Miri because we validate all the time (CTFE validates less often so this should be less of an issue, but still). I am not sure how best to fix it, short of implementing rust-lang/miri#841.

[RalfJung]

@wesleywiser do you still plan to submit this approach as a PR or would you like someone else to take over?

[wesleywiser]

@RalfJung Thanks for the reminder! I just posted a slightly more cleaned up version as #69888.