Make config vectors optional.

Allows us to differentiate between an empty array and an array that was
not provided when merging config files.

cross-rs#754 (comment)

Required for cross-rs#754.

src/cross_toml.rs

@@ -9,9 +9,9 @@ use std::collections::{BTreeSet, HashMap};
 #[derive(Debug, Deserialize, PartialEq, Eq, Default)]
 pub struct CrossEnvConfig {
     #[serde(default)]
-    volumes: Vec<String>,
+    volumes: Option<Vec<String>>,
     #[serde(default)]
-    passthrough: Vec<String>,
+    passthrough: Option<Vec<String>>,
 }
 
 /// Build configuration
@@ -89,22 +89,22 @@ impl CrossToml {
 
     /// Returns the list of environment variables to pass through for `build`,
     pub fn env_passthrough_build(&self) -> &[String] {
-        &self.build.env.passthrough
+        self.build.env.passthrough.as_deref().unwrap_or(&[])
     }
 
     /// Returns the list of environment variables to pass through for `target`,
     pub fn env_passthrough_target(&self, target: &Target) -> &[String] {
-        self.get_vec(target, |e| &e.passthrough)
+        self.get_vec(target, |e| e.passthrough.as_deref().unwrap_or(&[]))
     }
 
     /// Returns the list of environment variables to pass through for `build`,
     pub fn env_volumes_build(&self) -> &[String] {
-        &self.build.env.volumes
+        self.build.env.volumes.as_deref().unwrap_or(&[])
     }
 
     /// Returns the list of environment variables to pass through for `target`,
     pub fn env_volumes_target(&self, target: &Target) -> &[String] {
-        self.get_vec(target, |e| &e.volumes)
+        self.get_vec(target, |e| e.volumes.as_deref().unwrap_or(&[]))
     }
 
     /// Returns the default target to build,
@@ -169,8 +169,8 @@ mod tests {
             targets: HashMap::new(),
             build: CrossBuildConfig {
                 env: CrossEnvConfig {
-                    volumes: vec!["VOL1_ARG".to_string(), "VOL2_ARG".to_string()],
-                    passthrough: vec!["VAR1".to_string(), "VAR2".to_string()],
+                    volumes: Some(vec!["VOL1_ARG".to_string(), "VOL2_ARG".to_string()]),
+                    passthrough: Some(vec!["VAR1".to_string(), "VAR2".to_string()]),
                 },
                 xargo: Some(true),
                 build_std: None,
@@ -203,8 +203,8 @@ mod tests {
             },
             CrossTargetConfig {
                 env: CrossEnvConfig {
-                    passthrough: vec!["VAR1".to_string(), "VAR2".to_string()],
-                    volumes: vec!["VOL1_ARG".to_string(), "VOL2_ARG".to_string()],
+                    passthrough: Some(vec!["VAR1".to_string(), "VAR2".to_string()]),
+                    volumes: Some(vec!["VOL1_ARG".to_string(), "VOL2_ARG".to_string()]),
                 },
                 xargo: Some(false),
                 build_std: Some(true),
@@ -234,4 +234,57 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    pub fn parse_mixed_toml() -> Result<()> {
+        let mut target_map = HashMap::new();
+        target_map.insert(
+            Target::BuiltIn {
+                triple: "aarch64-unknown-linux-gnu".to_string(),
+            },
+            CrossTargetConfig {
+                env: CrossEnvConfig {
+                    passthrough: None,
+                    volumes: Some(vec!["VOL".to_string()]),
+                },
+                xargo: Some(false),
+                build_std: None,
+                image: None,
+                runner: None,
+            },
+        );
+
+        let cfg = CrossToml {
+            targets: target_map,
+            build: CrossBuildConfig {
+                env: CrossEnvConfig {
+                    volumes: None,
+                    passthrough: Some(vec![]),
+                },
+                xargo: Some(true),
+                build_std: None,
+                default_target: None,
+            },
+        };
+
+        let test_str = r#"
+            [build]
+            xargo = true
+
+            [build.env]
+            passthrough = []
+
+            [target.aarch64-unknown-linux-gnu]
+            xargo = false
+
+            [target.aarch64-unknown-linux-gnu.env]
+            volumes = ["VOL"]
+        "#;
+        let (parsed_cfg, unused) = CrossToml::parse(test_str)?;
+
+        assert_eq!(parsed_cfg, cfg);
+        assert!(unused.is_empty());
+
+        Ok(())
+    }
 }