Simplify a range usage and add some safety comments

Author: chriskrycho

crates/archive/src/tarball.rs

@@ -115,9 +115,11 @@ impl Archive for Tarball {
 /// more efficient than simply downloading the entire file up front.
 fn fetch_isize(url: &str, len: u64) -> Result<[u8; 4], ArchiveError> {
     let mut header_values = Vec::with_capacity(1);
-    Range::bytes(len - 4..=len - 1)
+    Range::bytes(len - 4..len)
         .unwrap()
         .encode(&mut header_values);
+    // We just pushed a header in with the `.encode` above, so there will always
+    // be a value at `.first()`.
     let range_header = header_values.first().unwrap();
 
     let (status, headers, mut response) = attohttpc::get(url)

crates/volta-core/src/tool/node/resolve.rs

@@ -258,6 +258,7 @@ fn resolve_node_versions(url: &str) -> Fallible<RawNodeIndex> {
 
             let mut header_values = Vec::with_capacity(1);
             expires.encode(&mut header_values);
+            // Since we just `.encode()`d into `header_values
             let encoded_expires = header_values.first().unwrap().to_str().unwrap();
 
             write!(expiry_file, "{}", encoded_expires).with_context(|| {

tests/acceptance/support/sandbox.rs

@@ -52,6 +52,8 @@ impl CacheBuilder {
 
         let mut header_values = Vec::with_capacity(1);
         expiry_date.encode(&mut header_values);
+        // Since we just `.encode()`d into `header_values, it is guaranteed to
+        // have a `.first()`.
         let encoded_expiry_date = header_values.first().unwrap();
 
         let mut expiry_file = File::create(&self.expiry_path).unwrap_or_else(|e| {