abi: readjust FnAbis to remove unsupported PassModes, via query hooks.

[eddyb]

Background

The rustc "call ABI" computation tries to find efficient ways to pass arguments (into functions) and return values (out of functions), and this can result in situations where e.g. PassMode::Cast is used to indicate that a [u8; 4] is actually being passed using a single 32-bit (CPU) register.

While useful at the machine ABI level, SPIR-V is too high-level and doesn't support the necessary raw manipulation to implement that kind of trick, so for a while now we've had to work around the situations in which rustc would decide for us on a PassMode that is incompatible with SPIR-V, with no room for the backend to override this behavior at all.

To a lesser extent, PassMode::Indirect is also not great for SPIR-V, as MIR allows more by-ref argument/return value passing than SPIR-V considers valid, and extra work was required to remove that after the fact (or bugs like #731 crept in because extra copies would've had to be used to remain correct).

Solution

We can now hook FnAbi computation, thanks to these upstream PRs:

• Morph layout_raw query into layout_of. rust-lang/rust#88308
• rustc_target: TyAndLayout::field should never error. rust-lang/rust#88337
• Provide layout_of automatically (given tcx + param_env + error handling). rust-lang/rust#88499
• Querify FnAbi::of_{fn_ptr,instance} as fn_abi_of_{fn_ptr,instance}. rust-lang/rust#88575
  • this is what enables the hooking, but the new fn_abi_of_* queries wouldn't have been consistent with layout_of, without the other refactors

The Layout/FnAbi interface refactors have already been accounted in rustup PRs (e.g. #759), so the main change in this PR is taking advantage of query hooking, to override fn_abi_of_* queries' behavior.

Both PassMode::Cast and (for good measure) PassMode::Ignore are now impossible, and if we wanted to we could also get rid of PassMode::Pair (like e.g. many C ABIs do), but that could introduce its own problems (so I would leave it for a different PR, if we even want it).

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Fixes #373.

Fixes #731 (unrelated to PassMode::Cast, but because PassMode::Indirect is no longer used).

Note: opened as draft because I would like to add more tests before merging.

[khyperia]

aaah it's so beautiful, this is so good, I've been wanting this for soooo long ❤️

feel free to un-draft and merge whenever, and force-merge past the cargo-deny failure (it might be another day or two until that's fixed. hopefully.)