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 https://github.com/rust-lang/compiler-team/issues/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 https://github.com/rust-lang/rust/pull/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*`

2 files changed, 14 insertions, 2 deletions

diff --git a/compiler/rustc_span/src/analyze_source_file.rs b/compiler/rustc_span/src/analyze_source_file.rs
index c32593a6d95..55d899c9ada 100644
--- a/compiler/rustc_span/src/analyze_source_file.rs
+++ b/compiler/rustc_span/src/analyze_source_file.rs
@@ -29,7 +29,18 @@ pub(crate) fn analyze_source_file(src: &str) -> (Vec<RelativeBytePos>, Vec<Multi
     (lines, multi_byte_chars)
 }
 
-cfg_select! {
+// cfg(bootstrap)
+macro_rules! cfg_select_dispatch {
+    ($($tokens:tt)*) => {
+        #[cfg(bootstrap)]
+        cfg_match! { $($tokens)* }
+
+        #[cfg(not(bootstrap))]
+        cfg_select! { $($tokens)* }
+    };
+}
+
+cfg_select_dispatch! {
     any(target_arch = "x86", target_arch = "x86_64") => {
         fn analyze_source_file_dispatch(
             src: &str,
diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index 906462a0d22..e950493f135 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -17,10 +17,11 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
+#![cfg_attr(bootstrap, feature(cfg_match))]
+#![cfg_attr(not(bootstrap), feature(cfg_select))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(array_windows)]
-#![feature(cfg_select)]
 #![feature(core_io_borrowed_buf)]
 #![feature(hash_set_entry)]
 #![feature(if_let_guard)]