Auto merge of rust-lang#119899 - onur-ozkan:redesign-stage0-std, r=<try>

redesign stage 0 std

### Summary

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>

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#122709

Author: bors

compiler/rustc_lint/src/lib.rs

@@ -21,6 +21,7 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
+#![cfg_attr(bootstrap, feature(extract_if))]
 #![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]

compiler/rustc_metadata/src/lib.rs

@@ -1,5 +1,6 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
+#![cfg_attr(bootstrap, feature(extract_if))]
 #![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]

compiler/rustc_middle/src/lib.rs

@@ -29,6 +29,7 @@
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::potential_query_instability)]
 #![allow(rustc::untranslatable_diagnostic)]
+#![cfg_attr(bootstrap, feature(extract_if))]
 #![cfg_attr(doc, recursion_limit = "256")] // FIXME(nnethercote): will be removed by #124141
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]

compiler/rustc_resolve/src/lib.rs

@@ -10,6 +10,7 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
+#![cfg_attr(bootstrap, feature(extract_if))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]

compiler/rustc_serialize/src/lib.rs

@@ -3,6 +3,7 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
 #![allow(rustc::internal)]
+#![cfg_attr(bootstrap, feature(ptr_sub_ptr))]
 #![cfg_attr(test, feature(test))]
 #![doc(
     html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",

compiler/rustc_serialize/src/opaque.rs

@@ -280,13 +280,27 @@ impl<'a> MemDecoder<'a> {
     #[inline]
     pub fn len(&self) -> usize {
         // SAFETY: This recovers the length of the original slice, only using members we never modify.
-        unsafe { self.end.offset_from_unsigned(self.start) }
+        #[cfg(bootstrap)]
+        unsafe {
+            return self.end.sub_ptr(self.start);
+        }
+        #[cfg(not(bootstrap))]
+        unsafe {
+            self.end.offset_from_unsigned(self.start)
+        }
     }
 
     #[inline]
     pub fn remaining(&self) -> usize {
         // SAFETY: This type guarantees current <= end.
-        unsafe { self.end.offset_from_unsigned(self.current) }
+        #[cfg(bootstrap)]
+        unsafe {
+            return self.end.sub_ptr(self.current);
+        }
+        #[cfg(not(bootstrap))]
+        unsafe {
+            self.end.offset_from_unsigned(self.current)
+        }
     }
 
     #[cold]
@@ -400,7 +414,14 @@ impl<'a> Decoder for MemDecoder<'a> {
     #[inline]
     fn position(&self) -> usize {
         // SAFETY: This type guarantees start <= current
-        unsafe { self.current.offset_from_unsigned(self.start) }
+        #[cfg(bootstrap)]
+        unsafe {
+            return self.current.sub_ptr(self.start);
+        }
+        #[cfg(not(bootstrap))]
+        unsafe {
+            self.current.offset_from_unsigned(self.start)
+        }
     }
 }
 

compiler/rustc_span/src/analyze_source_file.rs

@@ -83,16 +83,30 @@ cfg_match! {
 
                 // For character in the chunk, see if its byte value is < 0, which
                 // indicates that it's part of a UTF-8 char.
+                #[cfg(bootstrap)]
+                let multibyte_test = unsafe { _mm_cmplt_epi8(chunk, _mm_set1_epi8(0)) };
+                #[cfg(not(bootstrap))]
                 let multibyte_test = _mm_cmplt_epi8(chunk, _mm_set1_epi8(0));
+
                 // Create a bit mask from the comparison results.
+                #[cfg(bootstrap)]
+                let multibyte_mask = unsafe { _mm_movemask_epi8(multibyte_test) };
+                #[cfg(not(bootstrap))]
                 let multibyte_mask = _mm_movemask_epi8(multibyte_test);
 
                 // If the bit mask is all zero, we only have ASCII chars here:
                 if multibyte_mask == 0 {
                     assert!(intra_chunk_offset == 0);
 
                     // Check for newlines in the chunk
+                    #[cfg(bootstrap)]
+                    let newlines_test = unsafe { _mm_cmpeq_epi8(chunk, _mm_set1_epi8(b'\n' as i8)) };
+                    #[cfg(not(bootstrap))]
                     let newlines_test = _mm_cmpeq_epi8(chunk, _mm_set1_epi8(b'\n' as i8));
+
+                    #[cfg(bootstrap)]
+                    let mut newlines_mask = unsafe { _mm_movemask_epi8(newlines_test) };
+                    #[cfg(not(bootstrap))]
                     let mut newlines_mask = _mm_movemask_epi8(newlines_test);
 
                     let output_offset = RelativeBytePos::from_usize(chunk_index * CHUNK_SIZE + 1);

compiler/rustc_trait_selection/src/lib.rs

@@ -14,6 +14,7 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
+#![cfg_attr(bootstrap, feature(extract_if))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]

library/alloc/src/collections/btree/set_val.rs

@@ -9,7 +9,7 @@ pub(super) struct SetValZST;
 /// Returns `true` only for type `SetValZST`, `false` for all other types (blanket implementation).
 /// `TypeId` requires a `'static` lifetime, use of this trait avoids that restriction.
 ///
-/// [`TypeId`]: std::any::TypeId
+/// [`TypeId`]: core::any::TypeId
 pub(super) trait IsSetVal {
     fn is_set_val() -> bool;
 }

library/core/src/clone.rs

@@ -216,7 +216,7 @@ pub macro Clone($item:item) {
 /// Use closures allow captured values to be automatically used.
 /// This is similar to have a closure that you would call `.use` over each captured value.
 #[unstable(feature = "ergonomic_clones", issue = "132290")]
-#[cfg_attr(not(bootstrap), lang = "use_cloned")]
+#[lang = "use_cloned"]
 pub trait UseCloned: Clone {
     // Empty.
 }