IR verifier (for "well-formedness" conditions, type-checking, etc.).

[eddyb]

Right now some SPIR-T consumers (mostly spv::lift) have assertions here and there but it's nowhere near enough.

We really need a dedicated verifier, which could check:

• IR structural well-formedness (e.g. each entity/entity-list must have exactly one "owner")
• disallow embedding any SPIR-V instructions that have SPIR-T replacements
  • ideally deduplicating such rules with spv::{lower,lift},
    see also Bring many basic types/insts into SPIR-T (instead of leaving them as SPIR-V). #1
• SPIR-T-specific type checks (e.g. a SelectionKind::BoolCond select should take a bool input)
  • especially after Bring many basic types/insts into SPIR-T (instead of leaving them as SPIR-V). #1
• more general SPIR-V checks
  • would want to avoid overlapping too much with spirv-val unless we can do in orders of magnitude less code
  • Rust-GPU's SPIR-V type rules could be a starting point (though they lack a lot of "concrete type" cases due to their limited scope, originally)

Ideally a lot of this could be modular/data-driven, to maximize potential reuse of the relevant rules (e.g. any type-checker can theoretically be turned into a constraint solver for e.g. inference purposes).