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.
 }

library/core/src/fmt/mod.rs

@@ -1546,7 +1546,7 @@ unsafe fn run(fmt: &mut Formatter<'_>, arg: &rt::Placeholder, args: &[rt::Argume
 #[cfg(bootstrap)]
 unsafe fn getcount(args: &[rt::Argument<'_>], cnt: &rt::Count) -> Option<u16> {
     match *cnt {
-        rt::Count::Is(n) => Some(n as u16),
+        rt::Count::Is(n) => Some(n),
         rt::Count::Implied => None,
         rt::Count::Param(i) => {
             debug_assert!(i < args.len());

library/core/src/fmt/rt.rs

@@ -51,10 +51,6 @@ pub enum Alignment {
 #[derive(Copy, Clone)]
 pub enum Count {
     /// Specified with a literal number, stores the value
-    #[cfg(bootstrap)]
-    Is(usize),
-    /// Specified with a literal number, stores the value
-    #[cfg(not(bootstrap))]
     Is(u16),
     /// Specified using `$` and `*` syntaxes, stores the index into `args`
     Param(usize),

library/core/src/intrinsics/mod.rs

@@ -2307,41 +2307,17 @@ pub unsafe fn truncf128(x: f128) -> f128;
 /// [`f16::round_ties_even`](../../std/primitive.f16.html#method.round_ties_even)
 #[rustc_intrinsic]
 #[rustc_nounwind]
-#[cfg(not(bootstrap))]
 pub fn round_ties_even_f16(x: f16) -> f16;
 
-/// To be removed on next bootstrap bump.
-#[cfg(bootstrap)]
-pub fn round_ties_even_f16(x: f16) -> f16 {
-    #[rustc_intrinsic]
-    #[rustc_nounwind]
-    unsafe fn rintf16(x: f16) -> f16;
-
-    // SAFETY: this intrinsic isn't actually unsafe
-    unsafe { rintf16(x) }
-}
-
 /// Returns the nearest integer to an `f32`. Rounds half-way cases to the number with an even
 /// least significant digit.
 ///
 /// The stabilized version of this intrinsic is
 /// [`f32::round_ties_even`](../../std/primitive.f32.html#method.round_ties_even)
 #[rustc_intrinsic]
 #[rustc_nounwind]
-#[cfg(not(bootstrap))]
 pub fn round_ties_even_f32(x: f32) -> f32;
 
-/// To be removed on next bootstrap bump.
-#[cfg(bootstrap)]
-pub fn round_ties_even_f32(x: f32) -> f32 {
-    #[rustc_intrinsic]
-    #[rustc_nounwind]
-    unsafe fn rintf32(x: f32) -> f32;
-
-    // SAFETY: this intrinsic isn't actually unsafe
-    unsafe { rintf32(x) }
-}
-
 /// Provided for compatibility with stdarch. DO NOT USE.
 #[inline(always)]
 pub unsafe fn rintf32(x: f32) -> f32 {
@@ -2355,20 +2331,8 @@ pub unsafe fn rintf32(x: f32) -> f32 {
 /// [`f64::round_ties_even`](../../std/primitive.f64.html#method.round_ties_even)
 #[rustc_intrinsic]
 #[rustc_nounwind]
-#[cfg(not(bootstrap))]
 pub fn round_ties_even_f64(x: f64) -> f64;
 
-/// To be removed on next bootstrap bump.
-#[cfg(bootstrap)]
-pub fn round_ties_even_f64(x: f64) -> f64 {
-    #[rustc_intrinsic]
-    #[rustc_nounwind]
-    unsafe fn rintf64(x: f64) -> f64;
-
-    // SAFETY: this intrinsic isn't actually unsafe
-    unsafe { rintf64(x) }
-}
-
 /// Provided for compatibility with stdarch. DO NOT USE.
 #[inline(always)]
 pub unsafe fn rintf64(x: f64) -> f64 {
@@ -2382,20 +2346,8 @@ pub unsafe fn rintf64(x: f64) -> f64 {
 /// [`f128::round_ties_even`](../../std/primitive.f128.html#method.round_ties_even)
 #[rustc_intrinsic]
 #[rustc_nounwind]
-#[cfg(not(bootstrap))]
 pub fn round_ties_even_f128(x: f128) -> f128;
 
-/// To be removed on next bootstrap bump.
-#[cfg(bootstrap)]
-pub fn round_ties_even_f128(x: f128) -> f128 {
-    #[rustc_intrinsic]
-    #[rustc_nounwind]
-    unsafe fn rintf128(x: f128) -> f128;
-
-    // SAFETY: this intrinsic isn't actually unsafe
-    unsafe { rintf128(x) }
-}
-
 /// Returns the nearest integer to an `f16`. Rounds half-way cases away from zero.
 ///
 /// The stabilized version of this intrinsic is

library/core/src/macros/mod.rs

@@ -1753,7 +1753,6 @@ pub(crate) mod builtin {
         reason = "`type_alias_impl_trait` has open design concerns"
     )]
     #[rustc_builtin_macro]
-    #[cfg(not(bootstrap))]
     pub macro define_opaque($($tt:tt)*) {
         /* compiler built-in */
     }

library/core/src/pat.rs

@@ -25,15 +25,15 @@ macro_rules! pattern_type {
 )]
 pub trait RangePattern {
     /// Trait version of the inherent `MIN` assoc const.
-    #[cfg_attr(not(bootstrap), lang = "RangeMin")]
+    #[lang = "RangeMin"]
     const MIN: Self;
 
     /// Trait version of the inherent `MIN` assoc const.
-    #[cfg_attr(not(bootstrap), lang = "RangeMax")]
+    #[lang = "RangeMax"]
     const MAX: Self;
 
     /// A compile-time helper to subtract 1 for exclusive ranges.
-    #[cfg_attr(not(bootstrap), lang = "RangeSub")]
+    #[lang = "RangeSub"]
     #[track_caller]
     fn sub_one(self) -> Self;
 }

library/core/src/prelude/v1.rs

@@ -117,5 +117,4 @@ pub use crate::macros::builtin::deref;
     issue = "63063",
     reason = "`type_alias_impl_trait` has open design concerns"
 )]
-#[cfg(not(bootstrap))]
 pub use crate::macros::builtin::define_opaque;

library/std/src/backtrace.rs

@@ -432,7 +432,7 @@ mod helper {
     use super::*;
     pub(super) type LazyResolve = impl (FnOnce() -> Capture) + Send + Sync + UnwindSafe;
 
-    #[cfg_attr(not(bootstrap), define_opaque(LazyResolve))]
+    #[define_opaque(LazyResolve)]
     pub(super) fn lazy_resolve(mut capture: Capture) -> LazyResolve {
         move || {
             // Use the global backtrace lock to synchronize this as it's a

library/std/src/prelude/v1.rs

@@ -109,7 +109,6 @@ pub use core::prelude::v1::deref;
     issue = "63063",
     reason = "`type_alias_impl_trait` has open design concerns"
 )]
-#[cfg(not(bootstrap))]
 pub use core::prelude::v1::define_opaque;
 
 // The file so far is equivalent to core/src/prelude/v1.rs. It is duplicated

src/bootstrap/defaults/bootstrap.library.toml

@@ -1,17 +1,18 @@
 # These defaults are meant for contributors to the standard library and documentation.
 [build]
-# When building the standard library, you almost never want to build the compiler itself.
-build-stage = 0
-test-stage = 0
-bench-stage = 0
+build-stage = 1
+test-stage = 1
+bench-stage = 1
 
 [rust]
 # This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
 incremental = true
 # Make the compiler and standard library faster to build, at the expense of a ~20% runtime slowdown.
 lto = "off"
-# Download rustc by default for library profile if compiler-affecting
-# directories are not modified. For CI this is disabled.
+# When building the standard library, you almost never want to build the compiler itself.
+#
+# If compiler-affecting directories are not modified, use precompiled rustc to speed up
+# library development by skipping compiler builds.
 download-rustc = "if-unchanged"
 
 [llvm]

src/bootstrap/src/core/build_steps/check.rs

@@ -1,5 +1,6 @@
 //! Implementation of compiling the compiler and standard library, in "check"-based modes.
 
+use crate::core::build_steps::compile;
 use crate::core::build_steps::compile::{
     add_to_sysroot, run_cargo, rustc_cargo, rustc_cargo_env, std_cargo, std_crates_for_run_make,
 };
@@ -45,10 +46,12 @@ impl Step for Std {
     const DEFAULT: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
+        let stage = run.builder.top_stage;
         run.crate_or_deps("sysroot")
             .crate_or_deps("coretests")
             .crate_or_deps("alloctests")
             .path("library")
+            .default_condition(stage != 0)
     }
 
     fn make_run(run: RunConfig<'_>) {
@@ -62,6 +65,12 @@ impl Step for Std {
         let target = self.target;
         let compiler = builder.compiler(builder.top_stage, builder.config.build);
 
+        if builder.top_stage == 0 {
+            // Reuse the stage0 libstd
+            builder.ensure(compile::Std::new(compiler, target));
+            return;
+        }
+
         let mut cargo = builder::Cargo::new(
             builder,
             compiler,

src/bootstrap/src/core/build_steps/clippy.rs

@@ -207,16 +207,18 @@ impl Step for Rustc {
         let compiler = builder.compiler(builder.top_stage, builder.config.build);
         let target = self.target;
 
-        if compiler.stage != 0 {
-            // If we're not in stage 0, then we won't have a std from the beta
-            // compiler around. That means we need to make sure there's one in
-            // the sysroot for the compiler to find. Otherwise, we're going to
-            // fail when building crates that need to generate code (e.g., build
-            // scripts and their dependencies).
-            builder.ensure(compile::Std::new(compiler, compiler.host));
-            builder.ensure(compile::Std::new(compiler, target));
-        } else {
-            builder.ensure(check::Std::new(target).build_kind(Some(Kind::Check)));
+        if !builder.download_rustc() {
+            if compiler.stage != 0 {
+                // If we're not in stage 0, then we won't have a std from the beta
+                // compiler around. That means we need to make sure there's one in
+                // the sysroot for the compiler to find. Otherwise, we're going to
+                // fail when building crates that need to generate code (e.g., build
+                // scripts and their dependencies).
+                builder.ensure(compile::Std::new(compiler, compiler.host));
+                builder.ensure(compile::Std::new(compiler, target));
+            } else {
+                builder.ensure(check::Std::new(target).build_kind(Some(Kind::Check)));
+            }
         }
 
         let mut cargo = builder::Cargo::new(
@@ -286,7 +288,9 @@ macro_rules! lint_any {
                 let compiler = builder.compiler(builder.top_stage, builder.config.build);
                 let target = self.target;
 
-                builder.ensure(check::Rustc::new(target, builder).build_kind(Some(Kind::Check)));
+                if !builder.download_rustc() {
+                    builder.ensure(check::Rustc::new(target, builder).build_kind(Some(Kind::Check)));
+                };
 
                 let cargo = prepare_tool_cargo(
                     builder,