Fix spurious error when running build --stage 2 compiler/rustc

src/bootstrap/builder/tests.rs

@@ -501,7 +501,10 @@ mod dist {
                 std!(A => C, stage = 2),
             ]
         );
-        assert_eq!(builder.cache.all::<compile::Assemble>().len(), 5);
+        // Theoretically this should be 5, the same as `compile::Rustc`.
+        // Unfortunately, the sysroot for the stage 3 compiler is broken; see #90244.
+        // Instead we only generate the sysroot for non-stage2 build compilers.
+        assert_eq!(builder.cache.all::<compile::Assemble>().len(), 3);
         assert_eq!(
             first(builder.cache.all::<compile::Rustc>()),
             &[

src/bootstrap/compile.rs

@@ -1104,9 +1104,30 @@ impl Step for Assemble {
     }
 
     fn make_run(run: RunConfig<'_>) {
-        run.builder.ensure(Assemble {
-            target_compiler: run.builder.compiler(run.builder.top_stage + 1, run.target),
-        });
+        // Trying to actually create the sysroot for stage 2 is tricky, because everywhere in
+        // bootstrap that touches self.rustc or self.cargo_out suddenly needs to know whether those
+        // artifacts actually exist or whether we're reusing the ones from stage 1.  Instead, just
+        // build the artifacts, like we used to do before `compiler/rustc` implied assembling the
+        // compiler.
+        //
+        // When using full-bootstrap, we never reuse the artifacts from stage 1, so we don't have
+        // any issues.
+        //
+        // NOTE: Anywhere in bootstrap that calls `builder.compiler` has the side effect of running
+        // `Assemble`. In those cases, this workaround in `make_run` doesn't help, we'll still hit a
+        // panic (see #90224). In practice, that's probably ok: nothing currently uses the stage 2
+        // sysroot.
+        if run.builder.top_stage >= 2 && !run.builder.config.full_bootstrap {
+            let build_compiler = run.builder.compiler(run.builder.top_stage, run.builder.config.build);
+            run.builder.ensure(Rustc {
+                compiler: build_compiler,
+                target: run.target,
+                crates: Default::default(),
+            });
+            return;
+        }
+        let target_compiler = Compiler { stage: run.builder.top_stage + 1, host: run.target };
+        run.builder.ensure(Assemble { target_compiler });
     }
 
     /// Prepare a new compiler from the artifacts in `stage`