Decoupled naga for wgpu testing (#18099)

# Objective
- Allow bevy and wgpu developers to test newer versions of wgpu without
having to update naga_oil.

## Solution

- Currently bevy feeds wgsl through naga_oil to get a naga::Module that
it passes to wgpu.
- Added a way to pass wgsl through naga_oil, and then serialize the
naga::Module back into a wgsl string to feed to wgpu, allowing wgpu to
parse it using it's internal version of naga (and not the version of
naga bevy_render/naga_oil is using).

## Testing
1. Run 3d_scene (it works)
2. Run 3d_scene with `--features bevy_render/decoupled_naga` (it still
works)
3. Add the following patch to bevy/Cargo.toml, run cargo update, and
compile again (it will fail)
```toml
[patch.crates-io]
wgpu = { git = "https://github.com/gfx-rs/wgpu", rev = "2764e7a39920e23928d300e8856a672f1952da63" }
wgpu-core = { git = "https://github.com/gfx-rs/wgpu", rev = "2764e7a39920e23928d300e8856a672f1952da63" }
wgpu-hal = { git = "https://github.com/gfx-rs/wgpu", rev = "2764e7a39920e23928d300e8856a672f1952da63" }
wgpu-types = { git = "https://github.com/gfx-rs/wgpu", rev = "2764e7a39920e23928d300e8856a672f1952da63" }
```
4. Fix errors and compile again (it will work, and you didn't have to
touch naga_oil)

crates/bevy_render/Cargo.toml

@@ -9,6 +9,18 @@ license = "MIT OR Apache-2.0"
 keywords = ["bevy"]
 
 [features]
+# Bevy users should _never_ turn this feature on.
+#
+# Bevy/wgpu developers can turn this feature on to test a newer version of wgpu without needing to also update naga_oil.
+#
+# When turning this feature on, you can add the following to bevy/Cargo.toml (not this file), and then run `cargo update`:
+# [patch.crates-io]
+# wgpu = { git = "https://github.com/gfx-rs/wgpu", rev = "..." }
+# wgpu-core = { git = "https://github.com/gfx-rs/wgpu", rev = "..." }
+# wgpu-hal = { git = "https://github.com/gfx-rs/wgpu", rev = "..." }
+# wgpu-types = { git = "https://github.com/gfx-rs/wgpu", rev = "..." }
+decoupled_naga = []
+
 # Texture formats (require more than just image support)
 basis-universal = ["bevy_image/basis-universal"]
 dds = ["bevy_image/dds"]

crates/bevy_render/src/render_resource/pipeline_cache.rs

@@ -349,7 +349,28 @@ impl ShaderCache {
                             },
                         )?;
 
-                        ShaderSource::Naga(Cow::Owned(naga))
+                        #[cfg(not(feature = "decoupled_naga"))]
+                        {
+                            ShaderSource::Naga(Cow::Owned(naga))
+                        }
+
+                        #[cfg(feature = "decoupled_naga")]
+                        {
+                            let mut validator = naga::valid::Validator::new(
+                                naga::valid::ValidationFlags::all(),
+                                self.composer.capabilities,
+                            );
+                            let module_info = validator.validate(&naga).unwrap();
+                            let wgsl = Cow::Owned(
+                                naga::back::wgsl::write_string(
+                                    &naga,
+                                    &module_info,
+                                    naga::back::wgsl::WriterFlags::empty(),
+                                )
+                                .unwrap(),
+                            );
+                            ShaderSource::Wgsl(wgsl)
+                        }
                     }
                 };