feat: add extra-args config option to rattler-build backend

crates/pixi-build-rattler-build/src/config.rs

@@ -12,6 +12,18 @@ pub struct RattlerBuildBackendConfig {
     /// Extra input globs to include in addition to the default ones
     #[serde(default)]
     pub extra_input_globs: Vec<String>,
+    /// Extra arguments to pass to rattler-build.
+    /// Supported arguments include:
+    /// - `--experimental`: Enable experimental features (e.g., cache support)
+    #[serde(default)]
+    pub extra_args: Vec<String>,
+}
+
+impl RattlerBuildBackendConfig {
+    /// Returns `true` if `--experimental` is present in `extra_args`.
+    pub fn experimental(&self) -> bool {
+        self.extra_args.iter().any(|arg| arg == "--experimental")
+    }
 }
 
 impl BackendConfig for RattlerBuildBackendConfig {
@@ -23,6 +35,7 @@ impl BackendConfig for RattlerBuildBackendConfig {
     /// Target-specific values override base values using the following rules:
     /// - debug_dir: Not allowed to have target specific value
     /// - extra_input_globs: Platform-specific completely replaces base
+    /// - extra_args: Platform-specific completely replaces base
     fn merge_with_target_config(&self, target_config: &Self) -> miette::Result<Self> {
         if target_config.debug_dir.is_some() {
             miette::bail!("`debug_dir` cannot have a target specific value");
@@ -35,6 +48,11 @@ impl BackendConfig for RattlerBuildBackendConfig {
             } else {
                 target_config.extra_input_globs.clone()
             },
+            extra_args: if target_config.extra_args.is_empty() {
+                self.extra_args.clone()
+            } else {
+                target_config.extra_args.clone()
+            },
         })
     }
 }
@@ -57,11 +75,13 @@ mod tests {
         let base_config = RattlerBuildBackendConfig {
             debug_dir: Some(PathBuf::from("/base/debug")),
             extra_input_globs: vec!["*.base".to_string()],
+            extra_args: vec!["--base-arg".to_string()],
         };
 
         let target_config = RattlerBuildBackendConfig {
             debug_dir: None,
             extra_input_globs: vec!["*.target".to_string()],
+            extra_args: vec!["--target-arg".to_string()],
         };
 
         let merged = base_config
@@ -73,13 +93,17 @@ mod tests {
 
         // extra_input_globs should be completely overridden
         assert_eq!(merged.extra_input_globs, vec!["*.target".to_string()]);
+
+        // extra_args should be completely overridden
+        assert_eq!(merged.extra_args, vec!["--target-arg".to_string()]);
     }
 
     #[test]
     fn test_merge_with_empty_target_config() {
         let base_config = RattlerBuildBackendConfig {
             debug_dir: Some(PathBuf::from("/base/debug")),
             extra_input_globs: vec!["*.base".to_string()],
+            extra_args: vec!["--experimental".to_string()],
         };
 
         let empty_target_config = RattlerBuildBackendConfig::default();
@@ -91,6 +115,7 @@ mod tests {
         // Should keep base values when target is empty
         assert_eq!(merged.debug_dir, Some(PathBuf::from("/base/debug")));
         assert_eq!(merged.extra_input_globs, vec!["*.base".to_string()]);
+        assert_eq!(merged.extra_args, vec!["--experimental".to_string()]);
     }
 
     #[test]
@@ -110,4 +135,36 @@ mod tests {
         let error_msg = result.unwrap_err().to_string();
         assert!(error_msg.contains("`debug_dir` cannot have a target specific value"));
     }
+
+    #[test]
+    fn test_experimental_flag() {
+        // Test that experimental() returns true when --experimental is present
+        let config_with_experimental = RattlerBuildBackendConfig {
+            extra_args: vec!["--experimental".to_string()],
+            ..Default::default()
+        };
+        assert!(config_with_experimental.experimental());
+
+        // Test that experimental() returns true when --experimental is among other args
+        let config_with_multiple_args = RattlerBuildBackendConfig {
+            extra_args: vec![
+                "--some-arg".to_string(),
+                "--experimental".to_string(),
+                "--another-arg".to_string(),
+            ],
+            ..Default::default()
+        };
+        assert!(config_with_multiple_args.experimental());
+
+        // Test that experimental() returns false when --experimental is not present
+        let config_without_experimental = RattlerBuildBackendConfig {
+            extra_args: vec!["--other-flag".to_string()],
+            ..Default::default()
+        };
+        assert!(!config_without_experimental.experimental());
+
+        // Test that experimental() returns false when extra_args is empty
+        let config_empty = RattlerBuildBackendConfig::default();
+        assert!(!config_empty.experimental());
+    }
 }

crates/pixi-build-rattler-build/src/protocol.rs

@@ -69,7 +69,7 @@ impl Protocol for RattlerBuildBackend {
             build_platform,
             hash: None,
             variant: Default::default(),
-            experimental: false,
+            experimental: self.config.experimental(),
             allow_undefined: false,
             recipe_path: Some(self.recipe_source.path.clone()),
         };
@@ -299,7 +299,7 @@ impl Protocol for RattlerBuildBackend {
             build_platform,
             hash: None,
             variant: Default::default(),
-            experimental: false,
+            experimental: self.config.experimental(),
             allow_undefined: false,
             recipe_path: Some(self.recipe_source.path.clone()),
         };

docs/backends/pixi-build-rattler-build.md

@@ -118,6 +118,33 @@ extra-input-globs = ["*.yaml", "*.md", "*.sh", "patches-linux/**/*"]
 # Result for linux-64: ["*.yaml", "*.md", "*.sh", "patches-linux/**/*"]
 ```
 
+### `extra-args`
+
+- **Type**: `Array<String>`
+- **Default**: `[]`
+- **Target Merge Behavior**: `Overwrite` - Platform-specific args completely replace base args
+
+Extra arguments to pass to rattler-build. This allows you to enable experimental features or pass other CLI flags without needing backend updates.
+
+Currently supported arguments:
+
+- `--experimental`: Enable experimental features (e.g., `cache:` support for multi-output recipes)
+
+```toml
+[package.build.config]
+extra-args = ["--experimental"]
+```
+
+For target-specific configuration, platform-specific args completely replace the base:
+
+```toml
+[package.build.config]
+extra-args = ["--experimental"]
+
+[package.build.target.linux-64.config]
+extra-args = []  # Disable experimental for linux-64
+```
+
 ## Build Process
 
 The rattler-build backend follows this build process: