Document a bit more of the API

readme.md

@@ -12,6 +12,7 @@ and probably less useful externally:
 the compose module allows construction of shaders from modules (which are themselves shaders).
 
 it does this by treating shaders as modules, and 
+- preprocessing each module
 - building each module independently to naga IR
 - creating "header" files for each supported language, which are used to build dependent modules/shaders
 - making final shaders by combining the shader IR with the IR for imported modules
@@ -155,7 +156,7 @@ the `#if` directive requires a def name, an operator, and a value for comparison
 - the operator must be one of `==`, `!=`, `>=`, `>`, `<`, `<=`
 - the value must be an integer literal if comparing to a `ShaderDefValue::Int` or `ShaderDefValue::Uint`, or `true` or `false` if comparing to a `ShaderDef::Bool`.
 
-shader defs can also be used in the shader source with `#SHADER_DEF` or `#{SHADER_DEF}`, and will be substituted for their value.
+shader defs can be used in the shader source with `#SHADER_DEF` or `#{SHADER_DEF}`, and will be substituted for their value. unlike `#define` in C, the shader defs will not aggressively replace all instances of the def name in the shader source. one has to explicitly refer to the def name in the source with the `#{SHADER_DEF}` directive.
 
 the preprocessor branching directives (`ifdef`, `ifndef` and `if`) can be prefixed with `#else` to create more complex control flows:
 
@@ -176,6 +177,7 @@ shader defs can be created or overridden at the start of the top-level shader wi
 #define USER_NUMBER 42
 ```
 the created value will default to `true` if not specified.
+unlike `#define` in C, the value of the def is evaluated once at the very start of shader compilation, and acts as a global constant for the rest of the compilation.
 
 ## error reporting
 

src/compose/mod.rs

@@ -202,6 +202,8 @@ impl ShaderDefValue {
 #[derive(Clone, PartialEq, Eq, Hash, Debug, Default)]
 pub struct OwnedShaderDefs(BTreeMap<String, ShaderDefValue>);
 
+/// Modules are compiled differently based on the shader def values provided.
+/// So a single source code can actually correspond to multiple modules.
 #[derive(Clone, PartialEq, Eq, Hash, Debug)]
 struct ModuleKey(OwnedShaderDefs);
 
@@ -298,15 +300,26 @@ impl ComposableModuleDefinition {
     }
 }
 
+/// #import foo::bar::{a, b, c} would be represented as
+/// ```no_run
+/// ImportDefinition {
+///    import: "foo::bar",
+///    items: vec!["a", "b", "c"]
+/// }
+/// ```
 #[derive(Debug, Clone, Default, PartialEq, Eq)]
 pub struct ImportDefinition {
+    /// Module name
     pub import: String,
+    /// All the items that were imported from the module
     pub items: Vec<String>,
 }
 
 #[derive(Debug, Clone)]
 pub struct ImportDefWithOffset {
+    /// Module and items imported
     definition: ImportDefinition,
+    /// Where in the source the import was declared
     offset: usize,
 }
 
@@ -316,6 +329,7 @@ pub struct ImportDefWithOffset {
 #[derive(Debug)]
 pub struct Composer {
     pub validate: bool,
+    /// All modules, where each module can actually correspond to multiple modules that are built with different shader defs
     pub module_sets: HashMap<String, ComposableModuleDefinition>,
     pub module_index: HashMap<usize, String>,
     pub capabilities: naga::valid::Capabilities,
@@ -1354,20 +1368,34 @@ impl Composer {
 
 #[derive(Default)]
 pub struct ComposableModuleDescriptor<'a> {
+    /// Source code of the module
     pub source: &'a str,
+    /// File path of the module, useful for error reporting
     pub file_path: &'a str,
+    /// WGSL or GLSL
     pub language: ShaderLanguage,
+    /// Overriding/setting the name for this module. Useful for modules that either
+    /// - do not specify #define_import_path
+    /// - have an inconvenient name
     pub as_name: Option<String>,
+    /// Extra imports that are implicitly added. Like Bevy globals
     pub additional_imports: &'a [ImportDefinition],
+    /// extra shader def values bound to this module
     pub shader_defs: HashMap<String, ShaderDefValue>,
 }
 
 #[derive(Default)]
 pub struct NagaModuleDescriptor<'a> {
+    /// Source of main module
     pub source: &'a str,
+    /// File path of main module, useful for error reporting
     pub file_path: &'a str,
+    /// Shader language, but split into separate stages (vertex/fragment)
     pub shader_type: ShaderType,
+    /// #define values. Specified at the last moment to allow for runtime values.
+    /// Apply to all modules.
     pub shader_defs: HashMap<String, ShaderDefValue>,
+    /// Extra imports that are implicitly added. Like Bevy globals
     pub additional_imports: &'a [ImportDefinition],
 }
 
@@ -1563,6 +1591,7 @@ impl Composer {
 
         let sanitized_source = self.sanitize_and_set_auto_bindings(source);
 
+        // Get all possible imports
         let PreprocessorMetaData {
             name,
             defines,
@@ -1638,6 +1667,7 @@ impl Composer {
             modules: Default::default(),
         };
 
+        // Get the concrete imports, given the shader defs
         let PreprocessOutput {
             preprocessed_source,
             imports,

src/compose/preprocess.rs

@@ -48,9 +48,13 @@ impl Default for Preprocessor {
 
 #[derive(Debug)]
 pub struct PreprocessorMetaData {
+    /// Name of the module (#define_import_path)
     pub name: Option<String>,
+    /// The imports that were actually used (analyzes the source code during preprocesisng)
     pub imports: Vec<ImportDefWithOffset>,
+    /// #define values in the module
     pub defines: HashMap<String, ShaderDefValue>,
+    /// Defines that are referenced in the module. Either with #{name} or a variation of #ifdef name
     pub effective_defs: HashSet<String>,
 }