update to syn 2.0

[Firestar99]

This was the last dependency in my project that still uses syn 1.0 :D

Note that within the repo there remains one usage of syn 1.0 on some targets (I assume windows only):

$ cargo tree -i syn@1 --target all
syn v1.0.109
├── com_macros v0.6.0 (proc-macro)
│   └── com v0.6.0
│       └── hassle-rs v0.11.0
│           └── wgpu-hal v22.0.0
│               ├── wgpu v22.1.0
│               │   └── example-runner-wgpu v0.0.0 (/rust-gpu/examples/runners/wgpu)
│               └── wgpu-core v22.1.0
│                   └── wgpu v22.1.0 (*)
└── com_macros_support v0.6.0
    └── com_macros v0.6.0 (proc-macro) (*)

[Firestar99]

these only exist to unify features across dependency trees

This made absolutely no difference in my testing with syn 2, A being with and B without this dependency. Tests were run by changing the line, cargo clean, cargo update and time cargo compiletest --target-env vulkan1.2 on a 6900HS (8/16 cores) with installed tools:

    native   spirv dep  real    user
A1  43.43s   9.35s      1m7     9m19    
A2  41.16s   8.79s      1m5     8m59    
B1  42.48s   8.84s      1m6     9m1     
B2  42.37s   9.00s      1m6     9m0    

[Firestar99]

There are other similar dependencies below I have not touched.

[eddyb]

Will discuss this more in-depth off this PR, but for historical context, cargo clean with a single Cargo build after it is probably completely unimpacted by what this was fixing.

The issue was switching between things like the wgpu example and compiletest etc. - and the debugging process to track down the rebuild causes was honestly a bit absurd (there might be better tools now, or Cargo functionality, but I don't know off the top of my head).

[eddyb]

Example of the issue at play (after running each of these commands once):

$ cargo build -p compiletests --release --no-default-features --features use-installed-tools
   Compiling rustc_codegen_spirv v0.9.0 (/home/eddy/Projects/rust-gpu/crates/rustc_codegen_spirv)
   Compiling compiletests v0.0.0 (/home/eddy/Projects/rust-gpu/tests)
    Finished `release` profile [optimized] target(s) in 2.92s
$ cargo build -p example-runner-wgpu --release --no-default-features --features use-installed-tools
   Compiling rustc_codegen_spirv v0.9.0 (/home/eddy/Projects/rust-gpu/crates/rustc_codegen_spirv)
   Compiling spirv-builder v0.9.0 (/home/eddy/Projects/rust-gpu/crates/spirv-builder)
   Compiling example-runner-wgpu v0.0.0 (/home/eddy/Projects/rust-gpu/examples/runners/wgpu)
    Finished `release` profile [optimized] target(s) in 3.58s
$ cargo build -p compiletests --release --no-default-features --features use-installed-tools
   Compiling rustc_codegen_spirv v0.9.0 (/home/eddy/Projects/rust-gpu/crates/rustc_codegen_spirv)
   Compiling compiletests v0.0.0 (/home/eddy/Projects/rust-gpu/tests)
    Finished `release` profile [optimized] target(s) in 2.64s
$ cargo build -p example-runner-wgpu --release --no-default-features --features use-installed-tools
   Compiling rustc_codegen_spirv v0.9.0 (/home/eddy/Projects/rust-gpu/crates/rustc_codegen_spirv)
   Compiling spirv-builder v0.9.0 (/home/eddy/Projects/rust-gpu/crates/spirv-builder)
   Compiling example-runner-wgpu v0.0.0 (/home/eddy/Projects/rust-gpu/examples/runners/wgpu)
    Finished `release` profile [optimized] target(s) in 3.44s

I did try re-adding the build-dep you removed (with version 2 instead of 1), and nothing changed, so you didn't make anything worse (I was expecting this to have bitrotten anyway).