Do not allow building anything on stage 0

Author: Kobzol

src/bootstrap/defaults/bootstrap.library.toml

@@ -1,7 +1,6 @@
 # These defaults are meant for contributors to the standard library and documentation.
 [build]
 bench-stage = 1
-build-stage = 1
 check-stage = 1
 test-stage = 1
 

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

@@ -1023,7 +1023,7 @@ impl Config {
             || install_stage.is_some()
             || check_stage.is_some()
             || bench_stage.is_some();
-        // See https://github.com/rust-lang/compiler-team/issues/326
+
         config.stage = match config.cmd {
             Subcommand::Check { .. } => flags_stage.or(check_stage).unwrap_or(0),
             Subcommand::Clippy { .. } | Subcommand::Fix => flags_stage.or(check_stage).unwrap_or(1),
@@ -1051,6 +1051,12 @@ impl Config {
             | Subcommand::Vendor { .. } => flags_stage.unwrap_or(0),
         };
 
+        // Now check that the selected stage makes sense, and if not, print a warning and end
+        if let (0, Subcommand::Build) = (config.stage, &config.cmd) {
+            eprintln!("WARNING: cannot build anything on stage 0. Use at least stage 1.");
+            exit!(1);
+        }
+
         // CI should always run stage 2 builds, unless it specifically states otherwise
         #[cfg(not(test))]
         if flags_stage.is_none() && config.is_running_on_ci {