Allow setting default target via Cross.toml

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

src/config.rs

@@ -188,7 +188,7 @@ impl Config {
         if let Some(env_value) = self.env.target() {
             return Ok(Some(Target::from(&env_value, target_list)));
         }
-        Ok(None)
+        self.toml.as_ref().map_or(Ok(None), |t| t.target(target_list))
     }
 
     fn sum_of_env_toml_values<'a>(
@@ -402,6 +402,20 @@ 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
@@ -421,7 +435,7 @@ mod tests {
 
         static TOML_DEFAULT_TARGET: &str = r#"
     [build]
-    default = "aarch64-unknown-linux-gnu"
+    target = "aarch64-unknown-linux-gnu"
     "#;
     }
 }

src/main.rs

@@ -485,6 +485,18 @@ impl Toml {
         self.target_env(target, "volumes")
     }
 
+    /// Returns the default target to build,
+    pub fn target(&self, target_list: &TargetList) -> Result<Option<Target>> {
+        if let Some(value) = self.table.get("build").and_then(|t| t.get("target")) {
+            let value = value
+                .as_str()
+                .ok_or_else(|| "build.target must be a string".to_string())?;
+            Ok(Some(Target::from(&value, target_list)))
+        } else {
+            Ok(None)
+        }
+    }
+
     fn target_env(&self, target: &Target, key: &str) -> Result<Vec<&str>> {
         let triple = target.triple();
 

src/rustc.rs

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