Forbid running tests on stage 0 unless build.compiletest-allow-stage0 is enabled

Author: Kobzol

src/bootstrap/src/core/config/config.rs

@@ -1379,13 +1379,17 @@ impl Config {
                 eprintln!("ERROR: cannot run clippy on stage 0. Use at least stage 1.");
                 exit!(1);
             }
+            (0, Subcommand::Test { .. }) if build_compiletest_allow_stage0 != Some(true) => {
+                eprintln!(
+                    "ERROR: cannot test anything on stage 0. Use at least stage 1. If you want to run compiletest with an external stage0 toolchain, enable `build.compiletest-allow-stage0`."
+                );
+                exit!(1);
+            }
             _ => {}
         }
 
         if config.compile_time_deps && !matches!(config.cmd, Subcommand::Check { .. }) {
-            eprintln!(
-                "WARNING: Can't use --compile-time-deps with any subcommand other than check."
-            );
+            eprintln!("ERROR: Can't use --compile-time-deps with any subcommand other than check.");
             exit!(1);
         }