Assemble the compiler when running `x.py build`

Previously, there was no way to actually get binaries in
`build/$TARGET/stage1/bin` without building the standard library. This
makes it possible to build just the compiler. This can be useful when
the standard library isn't actually necessary for trying out your tests
(e.g. a bug that can be reproduced with only a `no_core` crate).

2 files changed, 10 insertions, 3 deletions

diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 0a6ed2f49b7..b55e7d8d50e 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -370,7 +370,7 @@ impl<'a> Builder<'a> {
         match kind {
             Kind::Build => describe!(
                 compile::Std,
-                compile::Rustc,
+                compile::Assemble,
                 compile::CodegenBackend,
                 compile::StartupObjects,
                 tool::BuildManifest,
diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index ae234fb1dc7..6a4d57a36b7 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -528,7 +528,7 @@ impl Step for Rustc {
     const DEFAULT: bool = false;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        run.path("compiler/rustc")
+        run.never()
     }
 
     fn make_run(run: RunConfig<'_>) {
@@ -1023,9 +1023,16 @@ pub struct Assemble {
 
 impl Step for Assemble {
     type Output = Compiler;
+    const ONLY_HOSTS: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        run.never()
+        run.path("compiler/rustc")
+    }
+
+    fn make_run(run: RunConfig<'_>) {
+        run.builder.ensure(Assemble {
+            target_compiler: run.builder.compiler(run.builder.top_stage + 1, run.target),
+        });
     }
 
     /// Prepare a new compiler from the artifacts in `stage`