Auto merge of #113341 - Kobzol:stage0-sysroot, r=Mark-Simulacrum

Copy stage0 `rustc` binaries to `stage0-sysroot`

This is basically a revival of https://github.com/rust-lang/rust/pull/101711 and https://github.com/rust-lang/rust/pull/107956, with an added check that the full sysroot will only be created if the original rustc comes from `stage0/bin`.

What is/should be tested:

- [x] `rustup toolchain link stage0` (new libstd is used correctly)
- [x]  `python3 x.py fmt dist --stage 0`
- [x] Custom rustc/cargo in `config.toml` (in this case this logic is ignored)
- [x]  Perfbot (try perf run has succeeded)
- [x] Real use case (https://github.com/rust-lang/backtrace-rs/pull/542)

(Hopefully) fixes: https://github.com/rust-lang/rust/issues/101691

This is not the "end all, be all" solution to this problem, but as long as it resolves the basic use-case, and doesn't break perfbot, I say ship it. This code will probably be nuked anyway Soon™ because of the stage redesign.

src/bootstrap/compile.rs

@@ -508,6 +508,49 @@ impl Step for StdLink {
         };
 
         add_to_sysroot(builder, &libdir, &hostdir, &libstd_stamp(builder, compiler, target));
+
+        // 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"))
+        {
+            // Copy bin files from stage0/bin to stage0-sysroot/bin
+            let sysroot = builder.out.join(&compiler.host.triple).join("stage0-sysroot");
+
+            let host = compiler.host.triple;
+            let stage0_bin_dir = builder.out.join(&host).join("stage0/bin");
+            let sysroot_bin_dir = sysroot.join("bin");
+            t!(fs::create_dir_all(&sysroot_bin_dir));
+            builder.cp_r(&stage0_bin_dir, &sysroot_bin_dir);
+
+            // 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()));
+                    }
+                }
+            }
+
+            // Copy codegen-backends from stage0
+            let sysroot_codegen_backends = builder.sysroot_codegen_backends(compiler);
+            t!(fs::create_dir_all(&sysroot_codegen_backends));
+            let stage0_codegen_backends = builder
+                .out
+                .join(&host)
+                .join("stage0/lib/rustlib")
+                .join(&host)
+                .join("codegen-backends");
+            builder.cp_r(&stage0_codegen_backends, &sysroot_codegen_backends);
+        }
     }
 }