Refactor fetch and install to be more self-contained

src/fetchers/gh_release.rs

@@ -0,0 +1,50 @@
+use std::path::Path;
+
+use log::{debug, info};
+use serde::Serialize;
+
+use crate::{download, head, Template};
+use super::Data;
+
+pub struct GhRelease {
+    url: String,
+}
+
+#[async_trait::async_trait]
+impl super::Fetcher for GhRelease {
+    async fn new(data: &Data) -> Result<Self, anyhow::Error> {
+        // Generate context for URL interpolation
+        let ctx = Context { 
+            name: &data.name,
+            repo: data.repo.as_ref().map(|s| &s[..]),
+            target: &data.target, 
+            version: &data.version,
+            format: data.meta.pkg_fmt.to_string(),
+        };
+        debug!("Using context: {:?}", ctx);
+
+        Ok(Self { url: ctx.render(&data.meta.pkg_url)? })
+    }
+
+    async fn check(&self) -> Result<bool, anyhow::Error> {
+        info!("Checking for package at: '{}'", self.url);
+        head(&self.url).await
+    }
+
+    async fn fetch(&self, dst: &Path) -> Result<(), anyhow::Error> {
+        info!("Downloading package from: '{}'", self.url);
+        download(&self.url, dst).await
+    }
+}
+
+/// Template for constructing download paths
+#[derive(Clone, Debug, Serialize)]
+struct Context<'c> {
+    pub name: &'c str,
+    pub repo: Option<&'c str>,
+    pub target: &'c str,
+    pub version: &'c str,
+    pub format: String,
+}
+
+impl<'c> Template for Context<'c> {}