allow building/testing std with stage0 compiler

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

Author: onur-ozkan

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

@@ -138,7 +138,7 @@ impl Step for Std {
         let compiler = self.compiler;
 
         // We already have std ready to be used for stage 0.
-        if compiler.stage == 0 {
+        if compiler.stage == 0 && !builder.config.build_std_on_stage0 {
             builder.ensure(StdLink::from_std(
                 self,
                 builder.compiler(compiler.stage, builder.config.build),
@@ -604,7 +604,7 @@ impl Step for StdLink {
         // Special case for stage0, to make `rustup toolchain link` and `x dist --stage 0`
         // work for stage0-sysroot. We only do this if the stage0 compiler comes from beta,
         // and is not set to a custom path.
-        if compiler.stage == 0 && is_downloaded_beta_stage0 {
+        if compiler.stage == 0 && is_downloaded_beta_stage0 && !builder.config.build_std_on_stage0 {
             // Copy bin files from stage0/bin to stage0-sysroot/bin
             let sysroot = builder.out.join(&compiler.host.triple).join("stage0-sysroot");
 
@@ -616,7 +616,18 @@ impl Step for StdLink {
 
             // Copy all *.so files from stage0/lib to stage0-sysroot/lib
             let stage0_lib_dir = builder.out.join(&host).join("stage0/lib");
-            builder.cp_r(&stage0_lib_dir, &sysroot.join("lib"));
+
+            if !builder.config.build_std_on_stage0 {
+                builder.cp_r(&stage0_lib_dir, &sysroot.join("lib"));
+            } else if let Ok(files) = fs::read_dir(&stage0_lib_dir) {
+                for file in files {
+                    let file = t!(file);
+                    let path = file.path();
+                    if path.is_file() && is_dylib(&file.file_name().into_string().unwrap()) {
+                        builder.copy(&path, &sysroot.join("lib").join(path.file_name().unwrap()));
+                    }
+                }
+            }
 
             // Copy codegen-backends from stage0
             let sysroot_codegen_backends = builder.sysroot_codegen_backends(compiler);
@@ -630,7 +641,7 @@ impl Step for StdLink {
             if stage0_codegen_backends.exists() {
                 builder.cp_r(&stage0_codegen_backends, &sysroot_codegen_backends);
             }
-        } else if compiler.stage == 0 {
+        } else if compiler.stage == 0 && !builder.config.build_std_on_stage0 {
             let sysroot = builder.out.join(&compiler.host.triple).join("stage0-sysroot");
             builder.cp_r(&builder.initial_sysroot.join("lib"), &sysroot.join("lib"));
         } else {

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

@@ -186,6 +186,7 @@ pub struct Config {
 
     pub on_fail: Option<String>,
     pub stage: u32,
+    pub build_std_on_stage0: bool,
     pub keep_stage: Vec<u32>,
     pub keep_stage_std: Vec<u32>,
     pub src: PathBuf,
@@ -1219,6 +1220,7 @@ impl Config {
         config.incremental = flags.incremental;
         config.dry_run = if flags.dry_run { DryRun::UserSelected } else { DryRun::Disabled };
         config.dump_bootstrap_shims = flags.dump_bootstrap_shims;
+        config.build_std_on_stage0 = flags.build_std_on_stage0;
         config.keep_stage = flags.keep_stage;
         config.keep_stage_std = flags.keep_stage_std;
         config.color = flags.color;

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

@@ -92,6 +92,9 @@ pub struct Flags {
     /// stage to build (indicates compiler to use/test, e.g., stage 0 uses the
     /// bootstrap compiler, stage 1 the stage 0 rustc artifacts, etc.)
     pub stage: Option<u32>,
+    /// Useful for library testing with the beta compiler (without building the stage 1 compiler)
+    #[arg(global(true), long)]
+    pub build_std_on_stage0: bool,
 
     #[arg(global(true), value_hint = clap::ValueHint::Other, long, value_name = "N")]
     /// stage(s) to keep without recompiling