Updating target json support

[Firestar99]

Requires publishing rustc_backend_spirv-target-specs and updating the commit rev of spirv-builder

rust-gpu: Rust-GPU/rust-gpu#256

This is the first PR to update the target spec jsons, which causes cargo gpu to run into some trouble. The codegen backend verifies that these jsons match exactly what it expects, and there's no flag to skip that check, and adding one now wouldn't help since we want to keep compatible with at least 0.9.0. Cargo gpu was never designed to have multiple target-spec versions, it always just used the latest. [...]

Rust-GPU/rust-gpu#249 (comment)

Note: replace rustc_codegen_spirv-types with the new rustc_codegen_spirv-target-specs crate

Could we move the target jsons from spirv_builder to rustc_codegen_spirv-types? That would allow cargo-gpu to resolve the required target jsons from rustc_codegen_spirv depending on rustc_codegen_spirv-types (and spirv_builder will retain access to it). If we can't find any target jsons in rustc_codegen_spirv-types, we can just fall back on the current target jsons, which we'd have to ship with cargo-gpu for the forseeable future. But I think that's a fine compromise, at least we wouldn't need to ship all future target jsons as well.

Rust-GPU/rust-gpu#249 (comment)

[Firestar99]

I've tested this locally with a bunch of variations: path legacy, path new, git legacy, git new, git 2024 PR (expected fail), path 2024 PR rebased (works)

[tombh]

Niiiice, so much simpler 🚀

[Firestar99]

these are some of the commits I cherry-picked from my library refactor :D

[tombh]

oooh :D

[Firestar99]

Rust-GPU/rust-gpu#256 got merged, so I'm just waiting for LegNeato to publish a version of rustc_codegen_spirv-target-specs on crates, that I can make this PR use that version and we can get this merged

[LegNeato]

@Firestar99 done! I tried to add you as an owner but it said name not found...you might need to log into crates.io or something 🤷

[Firestar99]

@tombh @schell this one is ready for final review and merge.

I've updated shader-crate-template to point to current master, so it should use the other branch that copies the target specs from the new rustc_codegen_spirv-target-specs crate. However, currently those don't differ from the legacy path so you wouldn't notice it failing (yet).

We should consider adding more CI (in a later PR?) to:

• test against a rev that has different target specs than "legacy", like rustup: update to nightly-2025-05-09 (~1.88) and Rust 2024 edition. rust-gpu#249
• test against a rev that requires target specs but needs the "legacy" route (0.8.0 and 0.9.0 build happily without any target specs)
• test with rust-gpu on a local path