fail on {dist, install} subcommand if download-rustc is enabled

src/bootstrap/src/core/builder.rs

@@ -414,6 +414,15 @@ impl StepDescription {
             .map(|desc| (desc.should_run)(ShouldRun::new(builder, desc.kind)))
             .collect::<Vec<_>>();
 
+        if builder.download_rustc() && (builder.kind == Kind::Dist || builder.kind == Kind::Install)
+        {
+            eprintln!(
+                "ERROR: '{}' subcommand is incompatible with `rust.download-rustc`.",
+                builder.kind.as_str()
+            );
+            crate::exit!(1);
+        }
+
         // sanity checks on rules
         for (desc, should_run) in v.iter().zip(&should_runs) {
             assert!(