rustbuild: fix save-analysis not being saved for 2-stage builds

src/bootstrap/lib.rs

@@ -521,7 +521,7 @@ fn cargo(&self,
                  .env(format!("CFLAGS_{}", target), self.cflags(target).join(" "));
         }
 
-        if self.config.channel == "nightly" && compiler.stage == 2 {
+        if self.config.channel == "nightly" && compiler.is_final_stage(self) {
             cargo.env("RUSTC_SAVE_ANALYSIS", "api".to_string());
         }
 
@@ -915,9 +915,10 @@ fn is_snapshot(&self, build: &Build) -> bool {
         self.stage == 0 && self.host == build.config.build
     }
 
-    /// Returns if this compiler is to be treated as a final stage one, whether
-    /// we're performing a full bootstrap or not. Don't do that by comparing
-    /// stages with `2`!
+    /// Returns if this compiler should be treated as a final stage one in the
+    /// current build session.
+    /// This takes into account whether we're performing a full bootstrap or
+    /// not; don't directly compare the stage with `2`!
     fn is_final_stage(&self, build: &Build) -> bool {
         let final_stage = if build.config.full_bootstrap { 2 } else { 1 };
         self.stage >= final_stage