spirv-builder: force a single CGU (codegen-unit).

CHANGELOG.md

@@ -29,8 +29,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added ⭐
+- [PR#1031](https://github.com/EmbarkStudios/rust-gpu/pull/1031) added `Components` generic parameter to `Image` type, allowing images to return lower dimensional vectors and even scalars from the sampling API
+
 ### Changed 🛠
-- [PR#1031](https://github.com/EmbarkStudios/rust-gpu/pull/1031) Added `Components` generic parameter to `Image` type, allowing images to return lower dimensional vectors and even scalars from the sampling API.
+- [PR#1035](https://github.com/EmbarkStudios/rust-gpu/pull/1035) reduced the number of CGUs ("codegen units") used by `spirv-builder` to just `1`
 - [PR#1011](https://github.com/EmbarkStudios/rust-gpu/pull/1011) made `NonWritable` all read-only storage buffers (i.e. those typed `&T`, where `T` doesn't have interior mutability)
 - [PR#1029](https://github.com/EmbarkStudios/rust-gpu/pull/1029) fixed SampledImage::sample() fns being unnecessarily marked as unsafe
 

crates/spirv-builder/src/lib.rs

@@ -578,6 +578,16 @@ fn invoke_rustc(builder: &SpirvBuilder) -> Result<PathBuf, SpirvBuilderError> {
         join_checking_for_separators(rustflags, "\x1f"),
     );
 
+    let profile_in_env_var = profile.replace('-', "_").to_ascii_uppercase();
+
+    // NOTE(eddyb) there's no parallelism to take advantage of multiple CGUs,
+    // and inter-CGU duplication can be wasteful, so this forces 1 CGU for now.
+    let num_cgus = 1;
+    cargo.env(
+        format!("CARGO_PROFILE_{profile_in_env_var}_CODEGEN_UNITS"),
+        num_cgus.to_string(),
+    );
+
     let build = cargo
         .stderr(Stdio::inherit())
         .current_dir(&builder.path_to_crate)