Allow setting default target via Cross.toml

N.B. `Debug` is needed for `TargetList` as linting rules demand it.

src/config.rs

@@ -189,7 +189,7 @@ impl Config {
         if let Some(env_value) = self.env.target() {
             return Some(Target::from(&env_value, target_list));
         }
-        None
+        self.toml.as_ref().map_or(None, |t| t.target(target_list))
     }
 
     fn sum_of_env_toml_values(
@@ -391,6 +391,20 @@ mod tests {
             Ok(())
         }
 
+        #[test]
+        pub fn no_env_but_toml_default_target_then_use_toml() -> Result<()> {
+            let env = Environment::new(None);
+            let config = Config::new_with(Some(toml(TOML_DEFAULT_TARGET)?), env);
+
+            let config_target = config.target(&target_list()).unwrap();
+            assert!(matches!(
+                config_target.triple(),
+                "aarch64-unknown-linux-gnu"
+            ));
+
+            Ok(())
+        }
+
         static TOML_BUILD_XARGO_FALSE: &str = r#"
     [build]
     xargo = false

src/cross_toml.rs

@@ -1,7 +1,7 @@
 #![doc = include_str!("../docs/cross_toml.md")]
 
 use crate::errors::*;
-use crate::Target;
+use crate::{Target, TargetList};
 use serde::Deserialize;
 use std::collections::HashMap;
 
@@ -91,6 +91,11 @@ impl CrossToml {
             .map_or(Vec::new(), |t| t.volumes.clone())
     }
 
+    /// Returns the default target to build,
+    pub fn target(&self, target_list: &TargetList) -> Option<Target> {
+        self.build.as_ref().and_then(|b| b.target.as_ref()).map(|t| Target::from(t, target_list))
+    }
+
     /// Returns a reference to the [`CrossTargetConfig`] of a specific `target`
     fn get_target(&self, target: &Target) -> Option<&CrossTargetConfig> {
         self.targets.get(target)
@@ -136,7 +141,7 @@ mod tests {
         let test_str = r#"
           [build]
           xargo = true
- 
+
           [build.env]
           volumes = ["VOL1_ARG", "VOL2_ARG"]
           passthrough = ["VAR1", "VAR2"]

src/rustc.rs

@@ -7,6 +7,7 @@ use crate::errors::*;
 use crate::extensions::CommandExt;
 use crate::{Host, Target};
 
+#[derive(Debug)]
 pub struct TargetList {
     pub triples: Vec<String>,
 }