Auto merge of #145902 - Kobzol:dist-docs-build-compiler, r=jieyouxu

Avoid more rustc rebuilds in cross-compilation scenarios

This is a continuation of #145874.

It adds a `compiler_for_std` function, which is a slimmed down version of `compiler_for`, which is much simpler, and designed to be used only for the standard library.

The build, dist and doc steps somtimes work with a stage2 std for a given target. That currently requires building a stage2 host compiler. However, if we uplift the stage1 libstd anyway, that is wasteful, in particular when we are cross-compiling.

The last two commits progressively make the stage 2 host rustc build avoidance more and more aggressive. I think that if we decide that it is fine to ship stage1 libstd everywhere, then it makes sense to go all the way.

When we ship stuff, we always build it with the stage 1 compiler (e.g. we ship stage 2 rustc which is built with stage 1 rustc). Libstd is the only component where stage N is built with the stage N compiler. So I think that shipping stage 1 libstd is "enough", and we could thus optimize what gets built on CI.

r? `@jieyouxu`

Author: bors

src/bootstrap/src/core/build_steps/compile.rs

@@ -99,28 +99,12 @@ impl Std {
         deps
     }
 
-    /// Returns true if the standard library will be uplifted from stage 1 for the given
-    /// `build_compiler` (which determines the stdlib stage) and `target`.
+    /// Returns true if the standard library should be uplifted from stage 1.
     ///
-    /// Uplifting is enabled if we're building a stage2+ libstd, full bootstrap is
-    /// disabled and we have a stage1 libstd already compiled for the given target.
-    pub fn should_be_uplifted_from_stage_1(
-        builder: &Builder<'_>,
-        stage: u32,
-        target: TargetSelection,
-    ) -> bool {
-        stage > 1
-            && !builder.config.full_bootstrap
-            // This estimates if a stage1 libstd exists for the given target. If we're not
-            // cross-compiling, it should definitely exist by the time we're building a stage2
-            // libstd.
-            // Or if we are cross-compiling, and we are building a cross-compiled rustc, then that
-            // rustc needs to link to a cross-compiled libstd, so again we should have a stage1
-            // libstd for the given target prepared.
-            // Even if we guess wrong in the cross-compiled case, the worst that should happen is
-            // that we build a fresh stage1 libstd below, and then we immediately uplift it, so we
-            // don't pay the libstd build cost twice.
-            && (target == builder.host_target || builder.config.hosts.contains(&target))
+    /// Uplifting is enabled if we're building a stage2+ libstd and full bootstrap is
+    /// disabled.
+    pub fn should_be_uplifted_from_stage_1(builder: &Builder<'_>, stage: u32) -> bool {
+        stage > 1 && !builder.config.full_bootstrap
     }
 }
 
@@ -150,7 +134,9 @@ impl Step for Std {
         trace!(force_recompile);
 
         run.builder.ensure(Std {
-            build_compiler: run.builder.compiler(run.builder.top_stage, run.build_triple()),
+            // Note: we don't use compiler_for_std here, so that `x build library --stage 2`
+            // builds a stage2 rustc.
+            build_compiler: run.builder.compiler(run.builder.top_stage, builder.host_target),
             target: run.target,
             crates,
             force_recompile,
@@ -226,7 +212,7 @@ impl Step for Std {
         // Stage of the stdlib that we're building
         let stage = build_compiler.stage;
 
-        if Self::should_be_uplifted_from_stage_1(builder, build_compiler.stage, target) {
+        if Self::should_be_uplifted_from_stage_1(builder, build_compiler.stage) {
             let build_compiler_for_std_to_uplift = builder.compiler(1, builder.host_target);
             let stage_1_stamp = builder.std(build_compiler_for_std_to_uplift, target);
 

src/bootstrap/src/core/build_steps/dist.rs

@@ -96,6 +96,8 @@ impl Step for Docs {
     }
 }
 
+/// Builds the `rust-docs-json` installer component.
+/// It contains the documentation of the standard library in JSON format.
 #[derive(Debug, Clone, Hash, PartialEq, Eq)]
 pub struct JsonDocs {
     build_compiler: Compiler,
@@ -113,12 +115,11 @@ impl Step for JsonDocs {
 
     fn make_run(run: RunConfig<'_>) {
         run.builder.ensure(JsonDocs {
-            build_compiler: run.builder.compiler(run.builder.top_stage, run.builder.host_target),
+            build_compiler: run.builder.compiler_for_std(run.builder.top_stage),
             target: run.target,
         });
     }
 
-    /// Builds the `rust-docs-json` installer component.
     fn run(self, builder: &Builder<'_>) -> Option<GeneratedTarball> {
         let target = self.target;
         let directory = builder.ensure(crate::core::build_steps::doc::Std::from_build_compiler(
@@ -135,6 +136,10 @@ impl Step for JsonDocs {
         tarball.add_bulk_dir(directory, dest);
         Some(tarball.generate())
     }
+
+    fn metadata(&self) -> Option<StepMetadata> {
+        Some(StepMetadata::dist("json-docs", self.target).built_by(self.build_compiler))
+    }
 }
 
 /// Builds the `rustc-docs` installer component.
@@ -764,22 +769,7 @@ pub struct Std {
 
 impl Std {
     pub fn new(builder: &Builder<'_>, target: TargetSelection) -> Self {
-        // This is an important optimization mainly for CI.
-        // Normally, to build stage N libstd, we need stage N rustc.
-        // However, if we know that we will uplift libstd from stage 1 anyway, building the stage N
-        // rustc can be wasteful.
-        // In particular, if we do a cross-compiling dist stage 2 build from T1 to T2, we need:
-        // - stage 2 libstd for T2 (uplifted from stage 1, where it was built by T1 rustc)
-        // - stage 2 rustc for T2
-        // However, without this optimization, we would also build stage 2 rustc for **T1**, which
-        // is completely wasteful.
-        let build_compiler =
-            if compile::Std::should_be_uplifted_from_stage_1(builder, builder.top_stage, target) {
-                builder.compiler(1, builder.host_target)
-            } else {
-                builder.compiler(builder.top_stage, builder.host_target)
-            };
-        Std { build_compiler, target }
+        Std { build_compiler: builder.compiler_for_std(builder.top_stage), target }
     }
 }
 

src/bootstrap/src/core/build_steps/doc.rs

@@ -616,7 +616,7 @@ impl Step for Std {
             return;
         }
         run.builder.ensure(Std {
-            build_compiler: run.builder.compiler(run.builder.top_stage, run.builder.host_target),
+            build_compiler: run.builder.compiler_for_std(run.builder.top_stage),
             target: run.target,
             format: if run.builder.config.cmd.json() {
                 DocumentationFormat::Json

src/bootstrap/src/core/builder/mod.rs

@@ -1360,6 +1360,30 @@ impl<'a> Builder<'a> {
         self.ensure(compile::Assemble { target_compiler: Compiler::new(stage, host) })
     }
 
+    /// This function can be used to provide a build compiler for building
+    /// the standard library, in order to avoid unnecessary rustc builds in case where std uplifting
+    /// would happen anyway.
+    ///
+    /// This is an important optimization mainly for CI.
+    ///
+    /// Normally, to build stage N libstd, we need stage N rustc.
+    /// However, if we know that we will uplift libstd from stage 1 anyway, building the stage N
+    /// rustc can be wasteful.
+    /// In particular, if we do a cross-compiling dist stage 2 build from target1 to target2,
+    /// we need:
+    /// - stage 2 libstd for target2 (uplifted from stage 1, where it was built by target1 rustc)
+    /// - stage 2 rustc for target2
+    ///
+    /// However, without this optimization, we would also build stage 2 rustc for **target1**,
+    /// which is completely wasteful.
+    pub fn compiler_for_std(&self, stage: u32) -> Compiler {
+        if compile::Std::should_be_uplifted_from_stage_1(self, stage) {
+            self.compiler(1, self.host_target)
+        } else {
+            self.compiler(stage, self.host_target)
+        }
+    }
+
     /// Similar to `compiler`, except handles the full-bootstrap option to
     /// silently use the stage1 compiler instead of a stage2 compiler if one is
     /// requested.