distribute actual stage of the compiled compiler

By "actual" we refer to the uplifting logic where we may not compile the requested stage;
instead, we uplift it from the previous stages. Which can lead to bootstrap failures in
specific situations where we request stage X from other steps. However we may end up
uplifting it from stage Y, causing the other stage to fail when attempting to link with
stage X which was never actually built.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/core/build_steps/compile.rs

@@ -803,7 +803,14 @@ impl Rustc {
 }
 
 impl Step for Rustc {
- type Output = ();
+ // We return the stage of the "actual" compiler (not the uplifted one).
+ //
+ // By "actual" we refer to the uplifting logic where we may not compile the requested stage;
+ // instead, we uplift it from the previous stages. Which can lead to bootstrap failures in
+ // specific situations where we request stage X from other steps. However the we may end
+ // up uplifting it from stage Y, causing the other stage to fail when attempting to link with
+ // stage X which was never actually built.
+ type Output = u32;
  const ONLY_HOSTS: bool = true;
  const DEFAULT: bool = false;
 
@@ -834,7 +841,7 @@ impl Step for Rustc {
  /// This will build the compiler for a particular stage of the build using
  /// the `compiler` targeting the `target` architecture. The artifacts
  /// created will also be linked into the sysroot directory.
- fn run(self, builder: &Builder<'_>) {
+ fn run(self, builder: &Builder<'_>) -> u32 {
  let compiler = self.compiler;
  let target = self.target;
 
@@ -848,7 +855,7 @@ impl Step for Rustc {
  compiler,
  builder.config.ci_rustc_dev_contents(),
  );
- return;
+ return compiler.stage;
  }
 
  builder.ensure(Std::new(compiler, target));
@@ -857,7 +864,8 @@ impl Step for Rustc {
  builder.info("WARNING: Using a potentially old librustc. This may not behave well.");
  builder.info("WARNING: Use `--keep-stage-std` if you want to rebuild the compiler when it changes");
  builder.ensure(RustcLink::from_rustc(self, compiler));
- return;
+
+ return compiler.stage;
  }
 
  let compiler_to_use = builder.compiler_for(compiler.stage, compiler.host, target);
@@ -880,7 +888,7 @@ impl Step for Rustc {
  };
  builder.info(&msg);
  builder.ensure(RustcLink::from_rustc(self, compiler_to_use));
- return;
+ return compiler_to_use.stage;
  }
 
  // Ensure that build scripts and proc macros have a std / libproc_macro to link against.
@@ -984,6 +992,8 @@ impl Step for Rustc {
  self,
  builder.compiler(compiler.stage, builder.config.build),
  ));
+
+ compiler.stage
  }
 }
 
@@ -1642,21 +1652,6 @@ impl Step for Assemble {
  return target_compiler;
  }
 
- // Get the compiler that we'll use to bootstrap ourselves.
- //
- // Note that this is where the recursive nature of the bootstrap
- // happens, as this will request the previous stage's compiler on
- // downwards to stage 0.
- //
- // Also note that we're building a compiler for the host platform. We
- // only assume that we can run `build` artifacts, which means that to
- // produce some other architecture compiler we need to start from
- // `build` to get there.
- //
- // FIXME: It may be faster if we build just a stage 1 compiler and then
- // use that to bootstrap this compiler forward.
- let build_compiler = builder.compiler(target_compiler.stage - 1, builder.config.build);
-
  // If we're downloading a compiler from CI, we can use the same compiler for all stages other than 0.
  if builder.download_rustc() {
  let sysroot =
@@ -1671,12 +1666,30 @@ impl Step for Assemble {
  return target_compiler;
  }
 
+ // Get the compiler that we'll use to bootstrap ourselves.
+ //
+ // Note that this is where the recursive nature of the bootstrap
+ // happens, as this will request the previous stage's compiler on
+ // downwards to stage 0.
+ //
+ // Also note that we're building a compiler for the host platform. We
+ // only assume that we can run `build` artifacts, which means that to
+ // produce some other architecture compiler we need to start from
+ // `build` to get there.
+ //
+ // FIXME: It may be faster if we build just a stage 1 compiler and then
+ // use that to bootstrap this compiler forward.
+ let mut build_compiler = builder.compiler(target_compiler.stage - 1, builder.config.build);
+
  // Build the libraries for this compiler to link to (i.e., the libraries
  // it uses at runtime). NOTE: Crates the target compiler compiles don't
  // link to these. (FIXME: Is that correct? It seems to be correct most
  // of the time but I think we do link to these for stage2/bin compilers
  // when not performing a full bootstrap).
- builder.ensure(Rustc::new(build_compiler, target_compiler.host));
+ let actual_stage = builder.ensure(Rustc::new(build_compiler, target_compiler.host));
+ // Current build_compiler.stage might be uplifted instead of being built; so update it
+ // to not fail while linking the artifacts.
+ build_compiler.stage = actual_stage;
 
  for &backend in builder.config.rust_codegen_backends.iter() {
  if backend == "llvm" {