Rollup merge of rust-lang#89759 - jyn514:x-build-assemble, r=Mark-Simulacrum

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).

Closes rust-lang#73519.

Author: the8472

Diff for: 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 for: 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`