refactor: clarify how HeaderSource loaded (#140)

This closes #138.

Signed-off-by: tison <wander4096@gmail.com>

fmt/src/license/mod.rs

@@ -12,13 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use snafu::ResultExt;
+use snafu::OptionExt;
 
-use crate::{
-    config::Config,
-    error::{InvalidConfigSnafu, LoadConfigSnafu},
-    Result,
-};
+use crate::{config::Config, error::InvalidConfigSnafu, Result};
 
 #[derive(Debug, Clone)]
 pub struct HeaderSource {
@@ -27,39 +23,42 @@ pub struct HeaderSource {
 
 impl HeaderSource {
     pub fn from_config(config: &Config) -> Result<Self> {
-        if let Some(inline_header) = &config.inline_header {
-            return Ok(HeaderSource {
-                content: inline_header.clone(),
-            });
+        // 1. inline_header takes priority.
+        if let Some(content) = config.inline_header.as_ref().cloned() {
+            return Ok(HeaderSource { content });
         }
 
-        if let Some(header_path) = &config.header_path {
-            if let Some(content) = bundled_headers(header_path) {
-                return Ok(content);
-            }
-
+        // 2. Then, header_path tries to load from base_dir.
+        let header_path = config.header_path.as_ref().context(InvalidConfigSnafu {
+            message: "no header source found (both inline_header and header_path are None)",
+        })?;
+        let path = {
             let mut path = config.base_dir.clone();
             path.push(header_path);
-            let content = std::fs::read_to_string(header_path)
-                .context(LoadConfigSnafu { name: header_path })?;
+            path
+        };
+        if let Ok(content) = std::fs::read_to_string(path) {
             return Ok(HeaderSource { content });
         }
 
-        InvalidConfigSnafu {
-            message: "no header source found in config",
-        }
-        .fail()
+        // 3. Finally, fallback to try bundled headers.
+        bundled_headers(header_path).context(InvalidConfigSnafu {
+            message: format!("no header source found (header_path is invalid: {header_path})"),
+        })
     }
 }
 
-pub fn bundled_headers(name: &str) -> Option<HeaderSource> {
-    match name {
-        "Apache-2.0.txt" => Some(HeaderSource {
-            content: include_str!("Apache-2.0.txt").to_string(),
-        }),
-        "Apache-2.0-ASF.txt" => Some(HeaderSource {
-            content: include_str!("Apache-2.0-ASF.txt").to_string(),
-        }),
-        _ => None,
+macro_rules! match_bundled_headers {
+    ($name:expr, $($file:expr),*) => {
+        match $name {
+            $(
+                $file => Some(HeaderSource { content: include_str!($file).to_string() }),
+            )*
+            _ => None,
+        }
     }
 }
+
+fn bundled_headers(name: &str) -> Option<HeaderSource> {
+    match_bundled_headers!(name, "Apache-2.0.txt", "Apache-2.0-ASF.txt")
+}