Avoid more rustc rebuilds in cross-compilation scenarios

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.