Auto merge of #119899 - onur-ozkan:redesign-stage0-std, r=albertlarsan68,jieyouxu,mark-simulacrum,kobzol,jyn514,Noratrieb,WaffleLapkin,RalfJung,bjorn3

redesign stage 0 std

### Summary

**Blog post: https://blog.rust-lang.org/inside-rust/2025/05/29/redesigning-the-initial-bootstrap-sequence/**

This PR changes how bootstrap builds the stage 1 compiler by switching to precompiled stage 0 standard library instead of building the in-tree one. The goal was to update bootstrap to use the beta standard library at stage 0 rather than compiling it from source (see the motivation at rust-lang/compiler-team#619).

Previously, to build a stage 1 compiler bootstrap followed this path:

```
download stage0 compiler -> build in-tree std -> compile stage1 compiler with in-tree std
```

With this PR, the new path is:

```
download stage0 compiler -> compile stage1 compiler with precompiled stage0 std
```

This also means that `cfg(bootstrap)`/`cfg(not(bootstrap))` is no longer needed for library development.

### Building "library"

Since stage0 `std` is no longer in-tree `x build/test/check library --stage 0` is now no-op. The minimum supported stage to build `std` is now 1. For the same reason, default stage values in the library profile is no longer 0.

Because building the in-tree library now requires a stage1 compiler, I highly recommend library developers to enable `download-rustc` to speed up compilation time.

<hr>

**Blog post: https://blog.rust-lang.org/inside-rust/2025/05/29/redesigning-the-initial-bootstrap-sequence/**

If you encounter a bug or unexpected results please open a topic in the [#t-infra/bootstrap](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap) Zulip channel or create a [bootstrap issue](https://github.com/rust-lang/rust/issues/new?template=bootstrap.md).

(Review thread: https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Review.20thread.3A.20stage.200.20redesign.20PR/with/508271433)

~~Blocked on rust-lang/rust#122709

try-job: dist-x86_64-linux
try-job: `x86_64-msvc*`
try-job: `x86_64-apple-*`
try-job: `aarch64-apple`
try-job: x86_64-gnu
try-job: `x86_64-gnu-llvm*`

Author: bors

src/concurrency/mod.rs

@@ -8,8 +8,19 @@ pub mod thread;
 mod vector_clock;
 pub mod weak_memory;
 
+// cfg(bootstrap)
+macro_rules! cfg_select_dispatch {
+    ($($tokens:tt)*) => {
+        #[cfg(bootstrap)]
+        cfg_match! { $($tokens)* }
+
+        #[cfg(not(bootstrap))]
+        cfg_select! { $($tokens)* }
+    };
+}
+
 // Import either the real genmc adapter or a dummy module.
-cfg_select! {
+cfg_select_dispatch! {
     feature = "genmc" => {
         mod genmc;
         pub use self::genmc::{GenmcCtx, GenmcConfig};

src/lib.rs

@@ -1,5 +1,6 @@
+#![cfg_attr(bootstrap, feature(cfg_match))]
+#![cfg_attr(not(bootstrap), feature(cfg_select))]
 #![feature(rustc_private)]
-#![feature(cfg_select)]
 #![feature(float_gamma)]
 #![feature(float_erf)]
 #![feature(map_try_insert)]

src/shims/unix/fs.rs

@@ -89,8 +89,19 @@ impl UnixFileDescription for FileHandle {
         communicate_allowed: bool,
         op: FlockOp,
     ) -> InterpResult<'tcx, io::Result<()>> {
+        // cfg(bootstrap)
+        macro_rules! cfg_select_dispatch {
+            ($($tokens:tt)*) => {
+                #[cfg(bootstrap)]
+                cfg_match! { $($tokens)* }
+
+                #[cfg(not(bootstrap))]
+                cfg_select! { $($tokens)* }
+            };
+        }
+
         assert!(communicate_allowed, "isolation should have prevented even opening a file");
-        cfg_select! {
+        cfg_select_dispatch! {
             all(target_family = "unix", not(target_os = "solaris")) => {
                 use std::os::fd::AsRawFd;