Add root_path_env property to shaders macro (vulkano-rs#2180)

* shaders: minor fix that some Vecs were always allocated with capacity of 0

* shaders: add root_path_env property to allow loading shaders generated by a build script

* shaders: Apply root_path_env suggestions from code review

Co-authored-by: marc0246 <40955683+marc0246@users.noreply.github.com>

* shaders: added root_path_env docs about env vars specified by a build script

---------

Co-authored-by: Firestar99 <4696087-firestar99@users.noreply.gitlab.com>
Co-authored-by: marc0246 <40955683+marc0246@users.noreply.github.com>

vulkano-shaders/src/lib.rs

@@ -126,6 +126,20 @@
 //! in conjunction with the `src` or `path` field. This allows using shaders compiled through a
 //! separate build system.
 //!
+//! ## `root_path_env: "..."`
+//!
+//! Instead of searching relative to your `Cargo.toml`, search relative to some other folder
+//! specified by this env variable. The intended use case is using `OUT_DIR` to be able to load
+//! shaders generated by your build script. Defaults to `CARGO_MANIFEST_DIR` corresponding to the
+//! folder of your `Cargo.toml`.
+//!
+//! See [`cargo-env-vars`] for a full set of env variables set by cargo. It is also possible to
+//! specify env variables from within the build script using the following:
+//! ```rust
+//! # let shader_out_dir = "";
+//! println!("cargo:rustc-env=SHADER_OUT_DIR={shader_out_dir}");
+//! ```
+//!
 //! ## `shaders: { first: { src: "...", ty: "..." }, ... }`
 //!
 //! With these options the user can compile several shaders in a single macro invocation. Each
@@ -200,6 +214,7 @@
 //! | `shaderc-build-from-source` | Build the `shaderc` library from source when compiling. |
 //! | `shaderc-debug`             | Compile shaders with debug information included.        |
 //!
+//! [`cargo-env-vars`]: https://doc.rust-lang.org/cargo/reference/environment-variables.html
 //! [cargo-expand]: https://github.com/dtolnay/cargo-expand
 //! [`ShaderModule::from_words_with_data`]: vulkano::shader::ShaderModule::from_words_with_data
 //! [`SpecializationConstants`]: vulkano::shader::SpecializationConstants
@@ -247,12 +262,34 @@ pub fn shader(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
 }
 
 fn shader_inner(mut input: MacroInput) -> Result<TokenStream> {
-    let root = env::var("CARGO_MANIFEST_DIR").unwrap_or_else(|_| ".".into());
+    let (root, relative_path_error_msg) = match input.root_path_env.as_ref() {
+        None => (
+            env::var("CARGO_MANIFEST_DIR").unwrap_or_else(|_| ".".into()),
+            "to your Cargo.toml".to_owned(),
+        ),
+        Some(root_path_env) => {
+            let root = match env::var(root_path_env.value()) {
+                Ok(e) => e,
+                Err(e) => {
+                    bail!(
+                        root_path_env,
+                        "failed to fetch environment variable: {e}; typical parameters are \
+                        `OUT_DIR` to gather results from your build script, or left default to \
+                        search relative to your Cargo.toml",
+                    )
+                }
+            };
+            let env = root_path_env.value();
+            let error = format!("to the path `{root}` specified by the env variable `{env:?}`");
+            (root, error)
+        }
+    };
+
     let root_path = Path::new(&root);
     let shaders = mem::take(&mut input.shaders); // yoink
 
-    let mut shaders_code = Vec::with_capacity(input.shaders.len());
-    let mut types_code = Vec::with_capacity(input.shaders.len());
+    let mut shaders_code = Vec::with_capacity(shaders.len());
+    let mut types_code = Vec::with_capacity(shaders.len());
     let mut type_registry = TypeRegistry::default();
 
     for (name, (shader_kind, source_kind)) in shaders {
@@ -273,7 +310,7 @@ fn shader_inner(mut input: MacroInput) -> Result<TokenStream> {
                     bail!(
                         path,
                         "file `{full_path:?}` was not found, note that the path must be relative \
-                        to your Cargo.toml",
+                        {relative_path_error_msg}",
                     );
                 }
 
@@ -302,7 +339,7 @@ fn shader_inner(mut input: MacroInput) -> Result<TokenStream> {
                     bail!(
                         path,
                         "file `{full_path:?}` was not found, note that the path must be relative \
-                        to your Cargo.toml",
+                        {relative_path_error_msg}",
                     );
                 }
 
@@ -350,6 +387,7 @@ enum SourceKind {
 }
 
 struct MacroInput {
+    root_path_env: Option<LitStr>,
     include_directories: Vec<PathBuf>,
     macro_defines: Vec<(String, String)>,
     shared_constants: bool,
@@ -365,6 +403,7 @@ impl MacroInput {
     #[cfg(test)]
     fn empty() -> Self {
         MacroInput {
+            root_path_env: None,
             include_directories: Vec::new(),
             macro_defines: Vec::new(),
             shared_constants: false,
@@ -382,6 +421,7 @@ impl Parse for MacroInput {
     fn parse(input: ParseStream<'_>) -> Result<Self> {
         let root = env::var("CARGO_MANIFEST_DIR").unwrap_or_else(|_| ".".into());
 
+        let mut root_path_env = None;
         let mut include_directories = Vec::new();
         let mut macro_defines = Vec::new();
         let mut shared_constants = None;
@@ -583,6 +623,13 @@ impl Parse for MacroInput {
                         }
                     }
                 }
+                "root_path_env" => {
+                    let lit = input.parse::<LitStr>()?;
+                    if root_path_env.is_some() {
+                        bail!(lit, "field `root_path_env` is already defined");
+                    }
+                    root_path_env = Some(lit);
+                }
                 "include" => {
                     let in_brackets;
                     bracketed!(in_brackets in input);
@@ -709,6 +756,7 @@ impl Parse for MacroInput {
         }
 
         Ok(MacroInput {
+            root_path_env,
             include_directories,
             macro_defines,
             shared_constants: shared_constants.unwrap_or(false),