can define a value from inside a shader (#7518)

# Objective

- Fixes #7494
- It is now possible to define a ShaderDef from inside a shader. This can be useful to centralise a value, or making sure an import is only interpreted once

## Solution

- Support `#define <SHADERDEF_NAME> <optional value>`

crates/bevy_render/src/render_resource/shader.rs

@@ -316,6 +316,11 @@ pub enum ProcessShaderError {
         expected: String,
         value: String,
     },
+    #[error("Invalid shader def definition for '{shader_def_name}': {value}")]
+    InvalidShaderDefDefinitionValue {
+        shader_def_name: String,
+        value: String,
+    },
 }
 
 pub struct ShaderImportProcessor {
@@ -388,6 +393,7 @@ pub struct ShaderProcessor {
     else_ifdef_regex: Regex,
     else_regex: Regex,
     endif_regex: Regex,
+    define_regex: Regex,
     def_regex: Regex,
     def_regex_delimited: Regex,
 }
@@ -397,10 +403,11 @@ impl Default for ShaderProcessor {
         Self {
             ifdef_regex: Regex::new(r"^\s*#\s*ifdef\s*([\w|\d|_]+)").unwrap(),
             ifndef_regex: Regex::new(r"^\s*#\s*ifndef\s*([\w|\d|_]+)").unwrap(),
-            ifop_regex: Regex::new(r"^\s*#\s*if\s*([\w|\d|_]+)\s*([^\s]*)\s*([\w|\d]+)").unwrap(),
+            ifop_regex: Regex::new(r"^\s*#\s*if\s*([\w|\d|_]+)\s*([^\s]*)\s*([-\w|\d]+)").unwrap(),
             else_ifdef_regex: Regex::new(r"^\s*#\s*else\s+ifdef\s*([\w|\d|_]+)").unwrap(),
             else_regex: Regex::new(r"^\s*#\s*else").unwrap(),
             endif_regex: Regex::new(r"^\s*#\s*endif").unwrap(),
+            define_regex: Regex::new(r"^\s*#\s*define\s*([\w|\d|_]+)\s*([-\w|\d]+)?").unwrap(),
             def_regex: Regex::new(r"#\s*([\w|\d|_]+)").unwrap(),
             def_regex_delimited: Regex::new(r"#\s*\{([\w|\d|_]+)\}").unwrap(),
         }
@@ -449,24 +456,34 @@ impl ShaderProcessor {
         shader_defs: &[ShaderDefVal],
         shaders: &HashMap<Handle<Shader>, Shader>,
         import_handles: &HashMap<ShaderImport, Handle<Shader>>,
+    ) -> Result<ProcessedShader, ProcessShaderError> {
+        let mut shader_defs_unique =
+            HashMap::<String, ShaderDefVal>::from_iter(shader_defs.iter().map(|v| match v {
+                ShaderDefVal::Bool(k, _) | ShaderDefVal::Int(k, _) | ShaderDefVal::UInt(k, _) => {
+                    (k.clone(), v.clone())
+                }
+            }));
+        self.process_inner(shader, &mut shader_defs_unique, shaders, import_handles)
+    }
+
+    fn process_inner(
+        &self,
+        shader: &Shader,
+        shader_defs_unique: &mut HashMap<String, ShaderDefVal>,
+        shaders: &HashMap<Handle<Shader>, Shader>,
+        import_handles: &HashMap<ShaderImport, Handle<Shader>>,
     ) -> Result<ProcessedShader, ProcessShaderError> {
         let shader_str = match &shader.source {
             Source::Wgsl(source) => source.deref(),
             Source::Glsl(source, _stage) => source.deref(),
             Source::SpirV(source) => {
-                if shader_defs.is_empty() {
+                if shader_defs_unique.is_empty() {
                     return Ok(ProcessedShader::SpirV(source.clone()));
                 }
                 return Err(ProcessShaderError::ShaderFormatDoesNotSupportShaderDefs);
             }
         };
 
-        let shader_defs_unique =
-            HashMap::<String, ShaderDefVal>::from_iter(shader_defs.iter().map(|v| match v {
-                ShaderDefVal::Bool(k, _) | ShaderDefVal::Int(k, _) | ShaderDefVal::UInt(k, _) => {
-                    (k.clone(), v.clone())
-                }
-            }));
         let mut scopes = vec![Scope::new(true)];
         let mut final_string = String::new();
         for line in shader_str.lines() {
@@ -544,6 +561,26 @@ impl ShaderProcessor {
                 let current_valid = scopes.last().unwrap().is_accepting_lines();
 
                 scopes.push(Scope::new(current_valid && new_scope));
+            } else if let Some(cap) = self.define_regex.captures(line) {
+                let def = cap.get(1).unwrap();
+                let name = def.as_str().to_string();
+
+                if let Some(val) = cap.get(2) {
+                    if let Ok(val) = val.as_str().parse::<u32>() {
+                        shader_defs_unique.insert(name.clone(), ShaderDefVal::UInt(name, val));
+                    } else if let Ok(val) = val.as_str().parse::<i32>() {
+                        shader_defs_unique.insert(name.clone(), ShaderDefVal::Int(name, val));
+                    } else if let Ok(val) = val.as_str().parse::<bool>() {
+                        shader_defs_unique.insert(name.clone(), ShaderDefVal::Bool(name, val));
+                    } else {
+                        return Err(ProcessShaderError::InvalidShaderDefDefinitionValue {
+                            shader_def_name: name,
+                            value: val.as_str().to_string(),
+                        });
+                    }
+                } else {
+                    shader_defs_unique.insert(name.clone(), ShaderDefVal::Bool(name, true));
+                }
             } else if let Some(cap) = self.else_ifdef_regex.captures(line) {
                 // When should we accept the code in an
                 //
@@ -627,7 +664,7 @@ impl ShaderProcessor {
                         shaders,
                         &import,
                         shader,
-                        shader_defs,
+                        shader_defs_unique,
                         &mut final_string,
                     )?;
                 } else if let Some(cap) = SHADER_IMPORT_PROCESSOR
@@ -640,7 +677,7 @@ impl ShaderProcessor {
                         shaders,
                         &import,
                         shader,
-                        shader_defs,
+                        shader_defs_unique,
                         &mut final_string,
                     )?;
                 } else if SHADER_IMPORT_PROCESSOR
@@ -695,7 +732,7 @@ impl ShaderProcessor {
         shaders: &HashMap<Handle<Shader>, Shader>,
         import: &ShaderImport,
         shader: &Shader,
-        shader_defs: &[ShaderDefVal],
+        shader_defs_unique: &mut HashMap<String, ShaderDefVal>,
         final_string: &mut String,
     ) -> Result<(), ProcessShaderError> {
         let imported_shader = import_handles
@@ -703,7 +740,7 @@ impl ShaderProcessor {
             .and_then(|handle| shaders.get(handle))
             .ok_or_else(|| ProcessShaderError::UnresolvedImport(import.clone()))?;
         let imported_processed =
-            self.process(imported_shader, shader_defs, shaders, import_handles)?;
+            self.process_inner(imported_shader, shader_defs_unique, shaders, import_handles)?;
 
         match &shader.source {
             Source::Wgsl(_) => {
@@ -2441,4 +2478,114 @@ fn vertex(
             .unwrap();
         assert_eq!(result.get_wgsl_source().unwrap(), EXPECTED_REPLACED);
     }
+
+    #[test]
+    fn process_shader_define_in_shader() {
+        #[rustfmt::skip]
+        const WGSL: &str = r"
+#ifdef NOW_DEFINED
+defined at start
+#endif
+#define NOW_DEFINED
+#ifdef NOW_DEFINED
+defined at end
+#endif
+";
+
+        #[rustfmt::skip]
+        const EXPECTED: &str = r"
+defined at end
+";
+        let processor = ShaderProcessor::default();
+        let result = processor
+            .process(
+                &Shader::from_wgsl(WGSL),
+                &[],
+                &HashMap::default(),
+                &HashMap::default(),
+            )
+            .unwrap();
+        assert_eq!(result.get_wgsl_source().unwrap(), EXPECTED);
+    }
+
+    #[test]
+    fn process_shader_define_in_shader_with_value() {
+        #[rustfmt::skip]
+        const WGSL: &str = r"
+#define DEFUINT 1
+#define DEFINT -1
+#define DEFBOOL false
+#if DEFUINT == 1
+uint: #DEFUINT
+#endif
+#if DEFINT == -1
+int: #DEFINT
+#endif
+#if DEFBOOL == false
+bool: #DEFBOOL
+#endif
+";
+
+        #[rustfmt::skip]
+        const EXPECTED: &str = r"
+uint: 1
+int: -1
+bool: false
+";
+        let processor = ShaderProcessor::default();
+        let result = processor
+            .process(
+                &Shader::from_wgsl(WGSL),
+                &[],
+                &HashMap::default(),
+                &HashMap::default(),
+            )
+            .unwrap();
+        assert_eq!(result.get_wgsl_source().unwrap(), EXPECTED);
+    }
+
+    #[test]
+    fn process_shader_define_across_imports() {
+        #[rustfmt::skip]
+        const FOO: &str = r"
+#define IMPORTED
+";
+        const BAR: &str = r"
+#IMPORTED
+";
+        #[rustfmt::skip]
+        const INPUT: &str = r"
+#import FOO
+#import BAR
+";
+        #[rustfmt::skip]
+        const EXPECTED: &str = r"
+
+
+true
+";
+        let processor = ShaderProcessor::default();
+        let mut shaders = HashMap::default();
+        let mut import_handles = HashMap::default();
+        {
+            let foo_handle = HandleUntyped::weak_from_u64(Shader::TYPE_UUID, 0).typed();
+            shaders.insert(foo_handle.clone_weak(), Shader::from_wgsl(FOO));
+            import_handles.insert(
+                ShaderImport::Custom("FOO".to_string()),
+                foo_handle.clone_weak(),
+            );
+        }
+        {
+            let bar_handle = HandleUntyped::weak_from_u64(Shader::TYPE_UUID, 1).typed();
+            shaders.insert(bar_handle.clone_weak(), Shader::from_wgsl(BAR));
+            import_handles.insert(
+                ShaderImport::Custom("BAR".to_string()),
+                bar_handle.clone_weak(),
+            );
+        }
+        let result = processor
+            .process(&Shader::from_wgsl(INPUT), &[], &shaders, &import_handles)
+            .unwrap();
+        assert_eq!(result.get_wgsl_source().unwrap(), EXPECTED);
+    }
 }