shader preprocessor - do not import if scope is not valid (#4012)

# Objective

- fix #4011 
- imports are not limited by the current `ifdef` they are in

## Solution

- process imports only if the current scope is enabled

crates/bevy_render/src/render_resource/shader.rs

@@ -408,7 +408,8 @@ impl ShaderProcessor {
                 if scopes.is_empty() {
                     return Err(ProcessShaderError::TooManyEndIfs);
                 }
-            } else if let Some(cap) = SHADER_IMPORT_PROCESSOR
+            } else if *scopes.last().unwrap() {
+                if let Some(cap) = SHADER_IMPORT_PROCESSOR
                     .import_asset_path_regex
                     .captures(line)
                 {
@@ -439,11 +440,12 @@ impl ShaderProcessor {
                     .is_match(line)
                 {
                     // ignore import path lines
-            } else if *scopes.last().unwrap() {
+                } else {
                     final_string.push_str(line);
                     final_string.push('\n');
                 }
             }
+        }
 
         if scopes.len() != 1 {
             return Err(ProcessShaderError::NotEnoughEndIfs);
@@ -1231,4 +1233,67 @@ fn in_main() { }
             .unwrap();
         assert_eq!(result.get_wgsl_source().unwrap(), EXPECTED);
     }
+
+    #[test]
+    fn process_import_in_ifdef() {
+        #[rustfmt::skip]
+        const BAR: &str = r"
+fn bar() { }
+";
+        #[rustfmt::skip]
+        const BAZ: &str = r"
+fn baz() { }
+";
+        #[rustfmt::skip]
+        const INPUT: &str = r"
+#ifdef FOO
+    #import BAR
+#else
+    #import BAZ
+#endif
+";
+        #[rustfmt::skip]
+        const EXPECTED_FOO: &str = r"
+
+fn bar() { }
+";
+        #[rustfmt::skip]
+        const EXPECTED: &str = r"
+
+fn baz() { }
+";
+        let processor = ShaderProcessor::default();
+        let mut shaders = HashMap::default();
+        let mut import_handles = HashMap::default();
+        {
+            let bar_handle = Handle::<Shader>::default();
+            shaders.insert(bar_handle.clone_weak(), Shader::from_wgsl(BAR));
+            import_handles.insert(
+                ShaderImport::Custom("BAR".to_string()),
+                bar_handle.clone_weak(),
+            );
+        }
+        {
+            let baz_handle = HandleUntyped::weak_from_u64(Shader::TYPE_UUID, 1).typed();
+            shaders.insert(baz_handle.clone_weak(), Shader::from_wgsl(BAZ));
+            import_handles.insert(
+                ShaderImport::Custom("BAZ".to_string()),
+                baz_handle.clone_weak(),
+            );
+        }
+        let result = processor
+            .process(
+                &Shader::from_wgsl(INPUT),
+                &["FOO".to_string()],
+                &shaders,
+                &import_handles,
+            )
+            .unwrap();
+        assert_eq!(result.get_wgsl_source().unwrap(), EXPECTED_FOO);
+
+        let result = processor
+            .process(&Shader::from_wgsl(INPUT), &[], &shaders, &import_handles)
+            .unwrap();
+        assert_eq!(result.get_wgsl_source().unwrap(), EXPECTED);
+    }
 }