use initial rustc's std on stage 0

On stage 0, rather than compiling std utilize the version from the initial
rustc. Theoretically, stage 0 should represent the snapshot version not the
compiled one. This makes bootstrapping quicker as well.

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

Author: onur-ozkan

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

@@ -137,6 +137,16 @@ impl Step for Std {
         let target = self.target;
         let compiler = self.compiler;
 
+        // We already have std ready to be used for stage 0.
+        if compiler.stage == 0 {
+            builder.ensure(StdLink::from_std(
+                self,
+                builder.compiler(compiler.stage, builder.config.build),
+            ));
+
+            return;
+        }
+
         // When using `download-rustc`, we already have artifacts for the host available. Don't
         // recompile them.
         if builder.download_rustc() && target == builder.build.build
@@ -585,18 +595,16 @@ impl Step for StdLink {
             (libdir, hostdir)
         };
 
-        add_to_sysroot(builder, &libdir, &hostdir, &libstd_stamp(builder, compiler, target));
+        let is_downloaded_beta_stage0 = builder
+            .build
+            .config
+            .initial_rustc
+            .starts_with(builder.out.join(&compiler.host.triple).join("stage0/bin"));
 
         // 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
-            && builder
-                .build
-                .config
-                .initial_rustc
-                .starts_with(builder.out.join(&compiler.host.triple).join("stage0/bin"))
-        {
+        if compiler.stage == 0 && is_downloaded_beta_stage0 {
             // Copy bin files from stage0/bin to stage0-sysroot/bin
             let sysroot = builder.out.join(&compiler.host.triple).join("stage0-sysroot");
 
@@ -608,15 +616,7 @@ 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");
-            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()));
-                    }
-                }
-            }
+            builder.cp_r(&stage0_lib_dir, &sysroot.join("lib"));
 
             // Copy codegen-backends from stage0
             let sysroot_codegen_backends = builder.sysroot_codegen_backends(compiler);
@@ -630,6 +630,11 @@ impl Step for StdLink {
             if stage0_codegen_backends.exists() {
                 builder.cp_r(&stage0_codegen_backends, &sysroot_codegen_backends);
             }
+        } else if compiler.stage == 0 {
+            let sysroot = builder.out.join(&compiler.host.triple).join("stage0-sysroot");
+            builder.cp_r(&builder.initial_sysroot.join("lib"), &sysroot.join("lib"));
+        } else {
+            add_to_sysroot(builder, &libdir, &hostdir, &libstd_stamp(builder, compiler, target));
         }
     }
 }

src/bootstrap/src/core/build_steps/test.rs

@@ -1121,7 +1121,7 @@ impl Step for Tidy {
         if builder.config.channel == "dev" || builder.config.channel == "nightly" {
             builder.info("fmt check");
             if builder.initial_rustfmt().is_none() {
-                let inferred_rustfmt_dir = builder.initial_rustc.parent().unwrap();
+                let inferred_rustfmt_dir = builder.initial_sysroot.join("bin");
                 eprintln!(
                     "\
 ERROR: no `rustfmt` binary found in {PATH}

src/bootstrap/src/core/builder.rs

@@ -1167,7 +1167,7 @@ impl<'a> Builder<'a> {
     }
 
     pub fn cargo_clippy_cmd(&self, run_compiler: Compiler) -> Command {
-        let initial_sysroot_bin = self.initial_rustc.parent().unwrap();
+        let initial_sysroot_bin = self.initial_sysroot.join("bin");
         // Set PATH to include the sysroot bin dir so clippy can find cargo.
         // FIXME: once rust-clippy#11944 lands on beta, set `CARGO` directly instead.
         let path = t!(env::join_paths(