Account for the fact that access tokens are optional now.

src/error.rs

@@ -154,6 +154,8 @@ pub enum Error {
     UserError(String),
     /// A lower level IO error.
     LowLevelError(io::Error),
+    /// We required an access token, but received a response that didn't contain one.
+    MissingAccessToken,
     /// Other errors produced by a storage provider
     OtherError(anyhow::Error),
 }
@@ -206,6 +208,13 @@ impl fmt::Display for Error {
             }
             Error::UserError(ref s) => s.fmt(f),
             Error::LowLevelError(ref e) => e.fmt(f),
+            Error::MissingAccessToken => {
+                write!(
+                    f,
+                    "Expected an access token, but received a response without one"
+                )?;
+                Ok(())
+            }
             Error::OtherError(ref e) => e.fmt(f),
         }
     }

src/service_account_impersonator.rs

@@ -55,7 +55,7 @@ impl From<TokenResponse> for TokenInfo {
         )
         .ok();
         TokenInfo {
-            access_token: resp.access_token,
+            access_token: Some(resp.access_token),
             refresh_token: None,
             expires_at,
             id_token: None,
@@ -99,7 +99,8 @@ impl ServiceAccountImpersonationFlow {
             .inner_flow
             .token(hyper_client, scopes)
             .await?
-            .access_token;
+            .access_token
+            .ok_or(Error::MissingAccessToken)?;
 
         let scopes: Vec<_> = scopes.iter().map(|s| s.as_ref()).collect();
         let req_body = Request {