Enable building/disting standard library in stage 0

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

@@ -167,8 +167,13 @@ impl Step for Std {
     fn run(self, builder: &Builder<'_>) -> Self::Output {
         let target = self.target;
 
-        // We already have std ready to be used for stage 0.
-        if self.build_compiler.stage == 0 {
+        // In most cases, we already have the std ready to be used for stage 0.
+        // However, if we are doing a local rebuild (so the build compiler can compile the standard
+        // library even on stage 0), and we're cross-compiling (so the stage0 standard library for
+        // *target* is not available), we still allow the stdlib to be built here.
+        if self.build_compiler.stage == 0
+            && !(builder.local_rebuild && target != builder.host_target)
+        {
             let compiler = self.build_compiler;
             builder.ensure(StdLink::from_std(self, compiler));
 

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

@@ -1435,22 +1435,30 @@ impl<'a> Builder<'a> {
         // FIXME: make the `Std` step return some type-level "proof" that std was indeed built,
         // and then require passing that to all Cargo invocations that we do.
 
-        // The "stage 0" std is always precompiled and comes with the stage0 compiler, so we have
-        // special logic for it, to avoid creating needless and confusing Std steps that don't
+        // The "stage 0" std is almost always precompiled and comes with the stage0 compiler, so we
+        // have special logic for it, to avoid creating needless and confusing Std steps that don't
         // actually build anything.
+        // We only allow building the stage0 stdlib if we do a local rebuild, so the stage0 compiler
+        // actually comes from in-tree sources, and we're cross-compiling, so the stage0 for the
+        // given `target` is not available.
         if compiler.stage == 0 {
             if target != compiler.host {
-                panic!(
-                    r"It is not possible to build the standard library for `{target}` using the stage0 compiler.
+                if self.local_rebuild {
+                    self.ensure(Std::new(compiler, target))
+                } else {
+                    panic!(
+                        r"It is not possible to build the standard library for `{target}` using the stage0 compiler.
 You have to build a stage1 compiler for `{}` first, and then use it to build a standard library for `{target}`.
+Alternatively, you can set `build.local-rebuild=true` and use a stage0 compiler built from in-tree sources.
 ",
-                    compiler.host
-                )
+                        compiler.host
+                    )
+                }
+            } else {
+                // We still need to link the prebuilt standard library into the ephemeral stage0 sysroot
+                self.ensure(StdLink::from_std(Std::new(compiler, target), compiler));
+                None
             }
-
-            // We still need to link the prebuilt standard library into the ephemeral stage0 sysroot
-            self.ensure(StdLink::from_std(Std::new(compiler, target), compiler));
-            None
         } else {
             // This step both compiles the std and links it into the compiler's sysroot.
             // Yes, it's quite magical and side-effecty.. would be nice to refactor later.

src/bootstrap/src/core/builder/tests.rs

@@ -853,6 +853,18 @@ mod snapshot {
         ctx.config("build").path("library").stage(0).run();
     }
 
+    #[test]
+    fn build_library_stage_0_local_rebuild() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx.config("build")
+                .path("library")
+                .stage(0)
+                .targets(&[TEST_TRIPLE_1])
+                .args(&["--set", "build.local-rebuild=true"])
+                .render_steps(), @"[build] rustc 0 <host> -> std 0 <target1>");
+    }
+
     #[test]
     fn build_library_stage_1() {
         let ctx = TestCtx::new();
@@ -1696,6 +1708,22 @@ mod snapshot {
         ");
     }
 
+    #[test]
+    fn dist_library_stage_0_local_rebuild() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx.config("dist")
+                .path("rust-std")
+                .stage(0)
+                .targets(&[TEST_TRIPLE_1])
+                .args(&["--set", "build.local-rebuild=true"])
+                .render_steps(), @r"
+        [build] rustc 0 <host> -> std 0 <target1>
+        [build] rustc 0 <host> -> RustInstaller 1 <host>
+        [dist] rustc 0 <host> -> std 0 <target1>
+        ");
+    }
+
     #[test]
     fn check_compiler_no_explicit_stage() {
         let ctx = TestCtx::new();

src/bootstrap/src/core/config/config.rs

@@ -969,31 +969,38 @@ impl Config {
             | Subcommand::Vendor { .. } => flags_stage.unwrap_or(0),
         };
 
-        // Now check that the selected stage makes sense, and if not, print a warning and end
+        let local_rebuild = build_local_rebuild.unwrap_or(false);
+
+        let check_stage0 = |kind: &str| {
+            if local_rebuild {
+                eprintln!("WARNING: running {kind} in stage 0. This might not work as expected.");
+            } else {
+                eprintln!(
+                    "ERROR: cannot {kind} anything on stage 0. Use at least stage 1 or set build.local-rebuild=true and use a stage0 compiler built from in-tree sources."
+                );
+                exit!(1);
+            }
+        };
+
+        // Now check that the selected stage makes sense, and if not, print an error and end
         match (stage, &flags_cmd) {
             (0, Subcommand::Build { .. }) => {
-                eprintln!("ERROR: cannot build anything on stage 0. Use at least stage 1.");
-                exit!(1);
+                check_stage0("build");
             }
             (0, Subcommand::Check { .. }) => {
-                eprintln!("ERROR: cannot check anything on stage 0. Use at least stage 1.");
-                exit!(1);
+                check_stage0("check");
             }
             (0, Subcommand::Doc { .. }) => {
-                eprintln!("ERROR: cannot document anything on stage 0. Use at least stage 1.");
-                exit!(1);
+                check_stage0("doc");
             }
             (0, Subcommand::Clippy { .. }) => {
-                eprintln!("ERROR: cannot run clippy on stage 0. Use at least stage 1.");
-                exit!(1);
+                check_stage0("clippy");
             }
             (0, Subcommand::Dist) => {
-                eprintln!("ERROR: cannot dist anything on stage 0. Use at least stage 1.");
-                exit!(1);
+                check_stage0("dist");
             }
             (0, Subcommand::Install) => {
-                eprintln!("ERROR: cannot install anything on stage 0. Use at least stage 1.");
-                exit!(1);
+                check_stage0("install");
             }
             _ => {}
         }
@@ -1234,7 +1241,7 @@ impl Config {
             llvm_use_libcxx: llvm_use_libcxx.unwrap_or(false),
             llvm_use_linker,
             llvm_version_suffix,
-            local_rebuild: build_local_rebuild.unwrap_or(false),
+            local_rebuild,
             locked_deps: build_locked_deps.unwrap_or(false),
             low_priority: build_low_priority.unwrap_or(false),
             mandir: install_mandir.map(PathBuf::from),

src/bootstrap/src/utils/change_tracker.rs

@@ -526,4 +526,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "The `optimized-compiler-builtins` option now accepts a path to an existing compiler-rt builtins library.",
     },
+    ChangeInfo {
+        change_id: 145876,
+        severity: ChangeSeverity::Info,
+        summary: "It is now possible to `check/build/dist` the standard stage 0 library if you use a stage0 rustc built from in-tree sources. This is useful for quickly cross-compiling the standard library. You have to enable build.local-rebuild for this to work.",
+    },
 ];