Give a compile error if neither of the use* features are enabled

CHANGELOG.md

@@ -6,6 +6,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 <!-- next-header -->
 ## [Unreleased] - ReleaseDate
+### Added
+- [PR#4](https://github.com/EmbarkStudios/spirv-tools-rs/pull/4) added more clear compile errors if neither of the `use-*-tools` features are enabled for either `spirv-tools` or `spirv-tools-sys`.
+
+### Changed
+- [PR#4](https://github.com/EmbarkStudios/spirv-tools-rs/pull/4) made `use-compiled-tools` the default feature for `spirv-tools-sys`. This would only affect direct consumers of `spirv-tools-sys`.
+
 ## [0.1.0] - 2020-11-13
 ### Added
 - Added initial implementation, which includes the assembler, validator, and most of the optimizer, which meets the current needs of rust-gpu.

spirv-tools-sys/Cargo.toml

@@ -22,6 +22,7 @@ include = [
 ]
 
 [features]
+default = ["use-compiled-tools"]
 # Using this feature disables the compilation in the build script, but
 # preserves the types so that spirv-tools can still work without needing
 # to keep copies of some of the basic enums etc

spirv-tools-sys/build.rs

@@ -219,9 +219,14 @@ fn val(build: &mut Build) {
 }
 
 fn main() {
-    if std::env::var("CARGO_FEATURE_USE_INSTALLED_TOOLS").is_ok()
-        && std::env::var("CARGO_FEATURE_USE_COMPILED_TOOLS").is_err()
-    {
+    let use_installed = std::env::var("CARGO_FEATURE_USE_INSTALLED_TOOLS").is_ok();
+    let use_compiled = std::env::var("CARGO_FEATURE_USE_COMPILED_TOOLS").is_ok();
+
+    if !use_compiled && !use_installed {
+        panic!("Enable at least one of `use-compiled-tools` or `use-installed-tools` features");
+    }
+
+    if use_installed && !use_compiled {
         println!("cargo:warning=use-installed-tools feature on, skipping compilation of C++ code");
         return;
     }

spirv-tools-sys/src/lib.rs

@@ -1,3 +1,6 @@
+#[cfg(not(any(feature = "use-installed-tools", feature = "use-compiled-tools")))]
+compile_error!("Enable at least one of `use-compiled-tools` or `use-installed-tools` features");
+
 pub mod assembler;
 pub mod diagnostics;
 pub mod opt;

src/lib.rs

@@ -1,3 +1,6 @@
+#[cfg(not(any(feature = "use-installed-tools", feature = "use-compiled-tools")))]
+compile_error!("Enable at least one of `use-compiled-tools` or `use-installed-tools` features");
+
 pub mod assembler;
 pub mod binary;
 pub mod opt;