khronos_api: Resolve WebGL extension files relative to CARGO_MANIFEST_DIR

Fixes #536
Fixes #549
Fixes #554

There are not one, not two but three PRs attempting to fix Bazel build
support, which seems to containerize or sandbox the build script such
that the paths it uses to find the crate source (e.g. `current_dir()`)
and XML files within is different than when the generated source is
being compiled, leading to files no longer being found.

The most simple solution of course is to remove all assumptions and
references to the working directory or absolute location of the crate
when the build script is being ran.  This can only be used to find the
files, after which their relative paths (**within** the crate source)
will be emitted to the output file with a `concat!()` directive that
prepends it with `env!("CARGO_MANIFEST_DIR")` whenever the source is
finally being compiled (in a different sandbox).

Author: MarijnS95

khronos_api/build.rs

@@ -25,11 +25,8 @@ fn main() {
     let dest = env::var("OUT_DIR").unwrap();
     let mut file = File::create(Path::new(&dest).join("webgl_exts.rs")).unwrap();
 
-    // Find the absolute path to the folder containing the WebGL extensions.
-    // The absolute path is needed, because we don't know where the output
-    // directory will be, and `include_bytes!(..)` resolves paths relative to the
-    // containing file.
-    let root = env::current_dir().unwrap().join("api_webgl/extensions");
+    // The relative path within this crate containing the WebGL extensions.
+    let root = Path::new("api_webgl/extensions");
 
     // Generate a slice literal, looking like this:
     // `&[&*include_bytes!(..), &*include_bytes!(..), ..]`
@@ -56,8 +53,17 @@ fn main() {
             // really is an extension.
             let ext_path = path.join("extension.xml");
             if ext_path.is_file() {
-                // Include the XML file, making sure to use an absolute path.
-                writeln!(file, "&*include_bytes!({:?}),", ext_path.to_str().unwrap()).unwrap();
+                // Include the XML file.
+                writeln!(
+                    file,
+                    // The path of the crate must be resolved at compile-time of the generated code
+                    // using this environment variable, not at runtime of this build script using
+                    // current_dir() or canonicalization.  These will differ on at least Bazel
+                    // when/where the build script is containerized.
+                    r#"&*include_bytes!(concat!(env!("CARGO_MANIFEST_DIR"), "/{}")),"#,
+                    ext_path.to_str().unwrap()
+                )
+                .unwrap();
             }
         }
     }