Auto merge of #137215 - onur-ozkan:rustc-tool-build-stages, r=jieyouxu,Kobzol

stabilize stage management for rustc tools

#135990 got out of control due to excessive complexity. This PR aims to achieve the same goal with a simpler approach, likely through multiple smaller PRs. I will keep the other one read-only and open as a reference for future work.

This work stabilizes the staging logic for `ToolRustc` programs, so you no longer need to handle build and target compilers separately in steps. Previously, most tools didn't do this correctly, which was causing the compiler to be built twice (e.g., `x test cargo --stage 1` would compile the stage 2 compiler before, but now it only compiles the stage 1 compiler).

I also tried to document how we should write `ToolRustc` steps as they are quite different and require more attention than other tools.

Next goal is to stabilize how stages are handled for the rustc itself. Currently, `x build --stage 1` builds the stage 1 compiler which is fine, but `x build compiler --stage 1` builds stage 2 compiler.

~~for now, r? ghost~~

Author: bors

src/bootstrap/defaults/config.tools.toml

@@ -8,6 +8,8 @@ incremental = true
 download-rustc = "if-unchanged"
 
 [build]
+# cargo and clippy tests don't pass on stage 1
+test-stage = 2
 # Document with the in-tree rustdoc by default, since `download-rustc` makes it quick to compile.
 doc-stage = 2
 # Contributors working on tools will probably expect compiler docs to be generated, so they can figure out how to use the API.

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

@@ -1983,13 +1983,14 @@ impl Step for Assemble {
         let maybe_install_llvm_bitcode_linker = |compiler| {
             if builder.config.llvm_bitcode_linker_enabled {
                 trace!("llvm-bitcode-linker enabled, installing");
-                let src_path = builder.ensure(crate::core::build_steps::tool::LlvmBitcodeLinker {
-                    compiler,
-                    target: target_compiler.host,
-                    extra_features: vec![],
-                });
+                let llvm_bitcode_linker =
+                    builder.ensure(crate::core::build_steps::tool::LlvmBitcodeLinker {
+                        compiler,
+                        target: target_compiler.host,
+                        extra_features: vec![],
+                    });
                 let tool_exe = exe("llvm-bitcode-linker", target_compiler.host);
-                builder.copy_link(&src_path, &libdir_bin.join(tool_exe));
+                builder.copy_link(&llvm_bitcode_linker.tool_path, &libdir_bin.join(tool_exe));
             }
         };
 
@@ -2181,14 +2182,13 @@ impl Step for Assemble {
         // logic to create the final binary. This is used by the
         // `wasm32-wasip2` target of Rust.
         if builder.tool_enabled("wasm-component-ld") {
-            let wasm_component_ld_exe =
-                builder.ensure(crate::core::build_steps::tool::WasmComponentLd {
-                    compiler: build_compiler,
-                    target: target_compiler.host,
-                });
+            let wasm_component = builder.ensure(crate::core::build_steps::tool::WasmComponentLd {
+                compiler: build_compiler,
+                target: target_compiler.host,
+            });
             builder.copy_link(
-                &wasm_component_ld_exe,
-                &libdir_bin.join(wasm_component_ld_exe.file_name().unwrap()),
+                &wasm_component.tool_path,
+                &libdir_bin.join(wasm_component.tool_path.file_name().unwrap()),
             );
         }
 

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

@@ -430,7 +430,7 @@ impl Step for Rustc {
                 },
                 builder.kind,
             ) {
-                builder.install(&ra_proc_macro_srv, &image.join("libexec"), 0o755);
+                builder.install(&ra_proc_macro_srv.tool_path, &image.join("libexec"), 0o755);
             }
 
             let libdir_relative = builder.libdir_relative(compiler);
@@ -1145,7 +1145,7 @@ impl Step for Cargo {
         let mut tarball = Tarball::new(builder, "cargo", &target.triple);
         tarball.set_overlay(OverlayKind::Cargo);
 
-        tarball.add_file(cargo, "bin", 0o755);
+        tarball.add_file(cargo.tool_path, "bin", 0o755);
         tarball.add_file(etc.join("_cargo"), "share/zsh/site-functions", 0o644);
         tarball.add_renamed_file(etc.join("cargo.bashcomp.sh"), "etc/bash_completion.d", "cargo");
         tarball.add_dir(etc.join("man"), "share/man/man1");
@@ -1191,7 +1191,7 @@ impl Step for Rls {
         let mut tarball = Tarball::new(builder, "rls", &target.triple);
         tarball.set_overlay(OverlayKind::Rls);
         tarball.is_preview(true);
-        tarball.add_file(rls, "bin", 0o755);
+        tarball.add_file(rls.tool_path, "bin", 0o755);
         tarball.add_legal_and_readme_to("share/doc/rls");
         Some(tarball.generate())
     }
@@ -1233,7 +1233,7 @@ impl Step for RustAnalyzer {
         let mut tarball = Tarball::new(builder, "rust-analyzer", &target.triple);
         tarball.set_overlay(OverlayKind::RustAnalyzer);
         tarball.is_preview(true);
-        tarball.add_file(rust_analyzer, "bin", 0o755);
+        tarball.add_file(rust_analyzer.tool_path, "bin", 0o755);
         tarball.add_legal_and_readme_to("share/doc/rust-analyzer");
         Some(tarball.generate())
     }
@@ -1279,8 +1279,8 @@ impl Step for Clippy {
         let mut tarball = Tarball::new(builder, "clippy", &target.triple);
         tarball.set_overlay(OverlayKind::Clippy);
         tarball.is_preview(true);
-        tarball.add_file(clippy, "bin", 0o755);
-        tarball.add_file(cargoclippy, "bin", 0o755);
+        tarball.add_file(clippy.tool_path, "bin", 0o755);
+        tarball.add_file(cargoclippy.tool_path, "bin", 0o755);
         tarball.add_legal_and_readme_to("share/doc/clippy");
         Some(tarball.generate())
     }
@@ -1329,8 +1329,8 @@ impl Step for Miri {
         let mut tarball = Tarball::new(builder, "miri", &target.triple);
         tarball.set_overlay(OverlayKind::Miri);
         tarball.is_preview(true);
-        tarball.add_file(miri, "bin", 0o755);
-        tarball.add_file(cargomiri, "bin", 0o755);
+        tarball.add_file(miri.tool_path, "bin", 0o755);
+        tarball.add_file(cargomiri.tool_path, "bin", 0o755);
         tarball.add_legal_and_readme_to("share/doc/miri");
         Some(tarball.generate())
     }
@@ -1460,8 +1460,8 @@ impl Step for Rustfmt {
         let mut tarball = Tarball::new(builder, "rustfmt", &target.triple);
         tarball.set_overlay(OverlayKind::Rustfmt);
         tarball.is_preview(true);
-        tarball.add_file(rustfmt, "bin", 0o755);
-        tarball.add_file(cargofmt, "bin", 0o755);
+        tarball.add_file(rustfmt.tool_path, "bin", 0o755);
+        tarball.add_file(cargofmt.tool_path, "bin", 0o755);
         tarball.add_legal_and_readme_to("share/doc/rustfmt");
         Some(tarball.generate())
     }
@@ -2283,7 +2283,7 @@ impl Step for LlvmBitcodeLinker {
         tarball.set_overlay(OverlayKind::LlvmBitcodeLinker);
         tarball.is_preview(true);
 
-        tarball.add_file(llbc_linker, self_contained_bin_dir, 0o755);
+        tarball.add_file(llbc_linker.tool_path, self_contained_bin_dir, 0o755);
 
         Some(tarball.generate())
     }

src/bootstrap/src/core/build_steps/perf.rs

@@ -166,7 +166,7 @@ Consider setting `rust.debuginfo-level = 1` in `config.toml`."#);
     let results_dir = rustc_perf_dir.join("results");
     builder.create_dir(&results_dir);
 
-    let mut cmd = command(collector);
+    let mut cmd = command(collector.tool_path);
 
     // We need to set the working directory to `src/tools/rustc-perf`, so that it can find the directory
     // with compile-time benchmarks.

src/bootstrap/src/core/build_steps/run.rs

@@ -126,11 +126,7 @@ impl Step for Miri {
 
         // This compiler runs on the host, we'll just use it for the target.
         let target_compiler = builder.compiler(stage, host);
-        // Similar to `compile::Assemble`, build with the previous stage's compiler. Otherwise
-        // we'd have stageN/bin/rustc and stageN/bin/rustdoc be effectively different stage
-        // compilers, which isn't what we want. Rustdoc should be linked in the same way as the
-        // rustc compiler it's paired with, so it must be built with the previous stage compiler.
-        let host_compiler = builder.compiler(stage - 1, host);
+        let host_compiler = tool::get_tool_rustc_compiler(builder, target_compiler);
 
         // Get a target sysroot for Miri.
         let miri_sysroot = test::Miri::build_miri_sysroot(builder, target_compiler, target);

src/bootstrap/src/core/build_steps/test.rs

@@ -263,7 +263,7 @@ impl Step for Cargotest {
 
         let _time = helpers::timeit(builder);
         let mut cmd = builder.tool_cmd(Tool::CargoTest);
-        cmd.arg(&cargo)
+        cmd.arg(&cargo.tool_path)
             .arg(&out_dir)
             .args(builder.config.test_args())
             .env("RUSTC", builder.rustc(compiler))
@@ -298,9 +298,16 @@ impl Step for Cargo {
 
     /// Runs `cargo test` for `cargo` packaged with Rust.
     fn run(self, builder: &Builder<'_>) {
+        if self.stage < 2 {
+            eprintln!("WARNING: cargo tests on stage {} may not behave well.", self.stage);
+            eprintln!("HELP: consider using stage 2");
+        }
+
         let compiler = builder.compiler(self.stage, self.host);
 
-        builder.ensure(tool::Cargo { compiler, target: self.host });
+        let cargo = builder.ensure(tool::Cargo { compiler, target: self.host });
+        let compiler = cargo.build_compiler;
+
         let cargo = tool::prepare_tool_cargo(
             builder,
             compiler,
@@ -367,6 +374,7 @@ impl Step for RustAnalyzer {
         let stage = self.stage;
         let host = self.host;
         let compiler = builder.compiler(stage, host);
+        let compiler = tool::get_tool_rustc_compiler(builder, compiler);
 
         // We don't need to build the whole Rust Analyzer for the proc-macro-srv test suite,
         // but we do need the standard library to be present.
@@ -427,7 +435,8 @@ impl Step for Rustfmt {
         let host = self.host;
         let compiler = builder.compiler(stage, host);
 
-        builder.ensure(tool::Rustfmt { compiler, target: self.host });
+        let tool_result = builder.ensure(tool::Rustfmt { compiler, target: self.host });
+        let compiler = tool_result.build_compiler;
 
         let mut cargo = tool::prepare_tool_cargo(
             builder,
@@ -522,16 +531,11 @@ impl Step for Miri {
 
         // This compiler runs on the host, we'll just use it for the target.
         let target_compiler = builder.compiler(stage, host);
-        // Similar to `compile::Assemble`, build with the previous stage's compiler. Otherwise
-        // we'd have stageN/bin/rustc and stageN/bin/rustdoc be effectively different stage
-        // compilers, which isn't what we want. Rustdoc should be linked in the same way as the
-        // rustc compiler it's paired with, so it must be built with the previous stage compiler.
-        let host_compiler = builder.compiler(stage - 1, host);
 
         // Build our tools.
-        let miri = builder.ensure(tool::Miri { compiler: host_compiler, target: host });
+        let miri = builder.ensure(tool::Miri { compiler: target_compiler, target: host });
         // the ui tests also assume cargo-miri has been built
-        builder.ensure(tool::CargoMiri { compiler: host_compiler, target: host });
+        builder.ensure(tool::CargoMiri { compiler: target_compiler, target: host });
 
         // We also need sysroots, for Miri and for the host (the latter for build scripts).
         // This is for the tests so everything is done with the target compiler.
@@ -542,7 +546,8 @@ impl Step for Miri {
         // Miri has its own "target dir" for ui test dependencies. Make sure it gets cleared when
         // the sysroot gets rebuilt, to avoid "found possibly newer version of crate `std`" errors.
         if !builder.config.dry_run() {
-            let ui_test_dep_dir = builder.stage_out(host_compiler, Mode::ToolStd).join("miri_ui");
+            let ui_test_dep_dir =
+                builder.stage_out(miri.build_compiler, Mode::ToolStd).join("miri_ui");
             // The mtime of `miri_sysroot` changes when the sysroot gets rebuilt (also see
             // <https://github.com/RalfJung/rustc-build-sysroot/commit/10ebcf60b80fe2c3dc765af0ff19fdc0da4b7466>).
             // We can hence use that directly as a signal to clear the ui test dir.
@@ -553,7 +558,7 @@ impl Step for Miri {
         // This is with the Miri crate, so it uses the host compiler.
         let mut cargo = tool::prepare_tool_cargo(
             builder,
-            host_compiler,
+            miri.build_compiler,
             Mode::ToolRustc,
             host,
             Kind::Test,
@@ -571,7 +576,7 @@ impl Step for Miri {
         // miri tests need to know about the stage sysroot
         cargo.env("MIRI_SYSROOT", &miri_sysroot);
         cargo.env("MIRI_HOST_SYSROOT", &host_sysroot);
-        cargo.env("MIRI", &miri);
+        cargo.env("MIRI", &miri.tool_path);
 
         // Set the target.
         cargo.env("MIRI_TEST_TARGET", target.rustc_target_arg());
@@ -743,7 +748,13 @@ impl Step for Clippy {
         let host = self.host;
         let compiler = builder.compiler(stage, host);
 
-        builder.ensure(tool::Clippy { compiler, target: self.host });
+        if stage < 2 {
+            eprintln!("WARNING: clippy tests on stage {stage} may not behave well.");
+            eprintln!("HELP: consider using stage 2");
+        }
+
+        let tool_result = builder.ensure(tool::Clippy { compiler, target: self.host });
+        let compiler = tool_result.build_compiler;
         let mut cargo = tool::prepare_tool_cargo(
             builder,
             compiler,
@@ -1728,18 +1739,7 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
                 // If we're using `--stage 0`, we should provide the bootstrap cargo.
                 builder.initial_cargo.clone()
             } else {
-                // We need to properly build cargo using the suitable stage compiler.
-
-                let compiler = builder.download_rustc().then_some(compiler).unwrap_or_else(||
-                    // HACK: currently tool stages are off-by-one compared to compiler stages, i.e. if
-                    // you give `tool::Cargo` a stage 1 rustc, it will cause stage 2 rustc to be built
-                    // and produce a cargo built with stage 2 rustc. To fix this, we need to chop off
-                    // the compiler stage by 1 to align with expected `./x test run-make --stage N`
-                    // behavior, i.e. we need to pass `N - 1` compiler stage to cargo. See also Miri
-                    // which does a similar hack.
-                    builder.compiler(builder.top_stage - 1, compiler.host));
-
-                builder.ensure(tool::Cargo { compiler, target: compiler.host })
+                builder.ensure(tool::Cargo { compiler, target: compiler.host }).tool_path
             };
 
             cmd.arg("--cargo-path").arg(cargo_path);
@@ -1760,9 +1760,10 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
             // Use the beta compiler for jsondocck
             let json_compiler = compiler.with_stage(0);
             cmd.arg("--jsondocck-path")
-                .arg(builder.ensure(tool::JsonDocCk { compiler: json_compiler, target }));
-            cmd.arg("--jsondoclint-path")
-                .arg(builder.ensure(tool::JsonDocLint { compiler: json_compiler, target }));
+                .arg(builder.ensure(tool::JsonDocCk { compiler: json_compiler, target }).tool_path);
+            cmd.arg("--jsondoclint-path").arg(
+                builder.ensure(tool::JsonDocLint { compiler: json_compiler, target }).tool_path,
+            );
         }
 
         if matches!(mode, "coverage-map" | "coverage-run") {
@@ -2999,12 +3000,15 @@ impl Step for RemoteCopyLibs {
 
         builder.info(&format!("REMOTE copy libs to emulator ({target})"));
 
-        let server = builder.ensure(tool::RemoteTestServer { compiler, target });
+        let remote_test_server = builder.ensure(tool::RemoteTestServer { compiler, target });
 
         // Spawn the emulator and wait for it to come online
         let tool = builder.tool_exe(Tool::RemoteTestClient);
         let mut cmd = command(&tool);
-        cmd.arg("spawn-emulator").arg(target.triple).arg(&server).arg(builder.tempdir());
+        cmd.arg("spawn-emulator")
+            .arg(target.triple)
+            .arg(&remote_test_server.tool_path)
+            .arg(builder.tempdir());
         if let Some(rootfs) = builder.qemu_rootfs(target) {
             cmd.arg(rootfs);
         }

src/bootstrap/src/core/build_steps/tool.rs

@@ -1392,7 +1392,7 @@ impl<'a> Builder<'a> {
     }
 
     pub fn rustdoc(&self, compiler: Compiler) -> PathBuf {
-        self.ensure(tool::Rustdoc { compiler })
+        self.ensure(tool::Rustdoc { compiler }).tool_path
     }
 
     pub fn cargo_clippy_cmd(&self, run_compiler: Compiler) -> BootstrapCommand {
@@ -1408,38 +1408,35 @@ impl<'a> Builder<'a> {
             return cmd;
         }
 
-        let build_compiler = self.compiler(run_compiler.stage - 1, self.build.build);
-        self.ensure(tool::Clippy { compiler: build_compiler, target: self.build.build });
+        let _ = self.ensure(tool::Clippy { compiler: run_compiler, target: self.build.build });
         let cargo_clippy =
-            self.ensure(tool::CargoClippy { compiler: build_compiler, target: self.build.build });
+            self.ensure(tool::CargoClippy { compiler: run_compiler, target: self.build.build });
         let mut dylib_path = helpers::dylib_path();
         dylib_path.insert(0, self.sysroot(run_compiler).join("lib"));
 
-        let mut cmd = command(cargo_clippy);
+        let mut cmd = command(cargo_clippy.tool_path);
         cmd.env(helpers::dylib_path_var(), env::join_paths(&dylib_path).unwrap());
         cmd.env("CARGO", &self.initial_cargo);
         cmd
     }
 
     pub fn cargo_miri_cmd(&self, run_compiler: Compiler) -> BootstrapCommand {
         assert!(run_compiler.stage > 0, "miri can not be invoked at stage 0");
-        let build_compiler = self.compiler(run_compiler.stage - 1, self.build.build);
-
         // Prepare the tools
-        let miri = self.ensure(tool::Miri { compiler: build_compiler, target: self.build.build });
+        let miri = self.ensure(tool::Miri { compiler: run_compiler, target: self.build.build });
         let cargo_miri =
-            self.ensure(tool::CargoMiri { compiler: build_compiler, target: self.build.build });
+            self.ensure(tool::CargoMiri { compiler: run_compiler, target: self.build.build });
         // Invoke cargo-miri, make sure it can find miri and cargo.
-        let mut cmd = command(cargo_miri);
-        cmd.env("MIRI", &miri);
+        let mut cmd = command(cargo_miri.tool_path);
+        cmd.env("MIRI", &miri.tool_path);
         cmd.env("CARGO", &self.initial_cargo);
-        // Need to add the `run_compiler` libs. Those are the libs produces *by* `build_compiler`,
-        // so they match the Miri we just built. However this means they are actually living one
-        // stage up, i.e. we are running `stage0-tools-bin/miri` with the libraries in `stage1/lib`.
-        // This is an unfortunate off-by-1 caused (possibly) by the fact that Miri doesn't have an
-        // "assemble" step like rustc does that would cross the stage boundary. We can't use
-        // `add_rustc_lib_path` as that's a NOP on Windows but we do need these libraries added to
-        // the PATH due to the stage mismatch.
+        // Need to add the `run_compiler` libs. Those are the libs produces *by* `build_compiler`
+        // in `tool::ToolBuild` step, so they match the Miri we just built. However this means they
+        // are actually living one stage up, i.e. we are running `stage0-tools-bin/miri` with the
+        // libraries in `stage1/lib`. This is an unfortunate off-by-1 caused (possibly) by the fact
+        // that Miri doesn't have an "assemble" step like rustc does that would cross the stage boundary.
+        // We can't use `add_rustc_lib_path` as that's a NOP on Windows but we do need these libraries
+        // added to the PATH due to the stage mismatch.
         // Also see https://github.com/rust-lang/rust/pull/123192#issuecomment-2028901503.
         add_dylib_path(self.rustc_lib_paths(run_compiler), &mut cmd);
         cmd

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

@@ -525,6 +525,7 @@ mod dist {
             first(cache.all::<compile::Rustc>()),
             &[
                 rustc!(TEST_TRIPLE_1 => TEST_TRIPLE_1, stage = 0),
+                rustc!(TEST_TRIPLE_1 => TEST_TRIPLE_2, stage = 0),
                 rustc!(TEST_TRIPLE_1 => TEST_TRIPLE_2, stage = 1),
             ]
         );
@@ -1084,3 +1085,33 @@ fn test_is_builder_target() {
         assert!(!builder.is_builder_target(target2));
     }
 }
+
+#[test]
+fn test_get_tool_rustc_compiler() {
+    let mut config = configure("build", &[], &[]);
+    config.download_rustc_commit = None;
+    let build = Build::new(config);
+    let builder = Builder::new(&build);
+
+    let target_triple_1 = TargetSelection::from_user(TEST_TRIPLE_1);
+
+    let compiler = Compiler { stage: 2, host: target_triple_1 };
+    let expected = Compiler { stage: 1, host: target_triple_1 };
+    let actual = tool::get_tool_rustc_compiler(&builder, compiler);
+    assert_eq!(expected, actual);
+
+    let compiler = Compiler { stage: 1, host: target_triple_1 };
+    let expected = Compiler { stage: 0, host: target_triple_1 };
+    let actual = tool::get_tool_rustc_compiler(&builder, compiler);
+    assert_eq!(expected, actual);
+
+    let mut config = configure("build", &[], &[]);
+    config.download_rustc_commit = Some("".to_owned());
+    let build = Build::new(config);
+    let builder = Builder::new(&build);
+
+    let compiler = Compiler { stage: 1, host: target_triple_1 };
+    let expected = Compiler { stage: 1, host: target_triple_1 };
+    let actual = tool::get_tool_rustc_compiler(&builder, compiler);
+    assert_eq!(expected, actual);
+}

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

@@ -355,4 +355,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "It is now possible to configure `jemalloc` for each target",
     },
+    ChangeInfo {
+        change_id: 137215,
+        severity: ChangeSeverity::Info,
+        summary: "Added `build.test-stage = 2` to 'tools' profile defaults",
+    },
 ];

src/bootstrap/src/utils/change_tracker.rs

@@ -75,6 +75,7 @@
 - [Prologue](./building/bootstrapping/intro.md)
 - [What Bootstrapping does](./building/bootstrapping/what-bootstrapping-does.md)
 - [How Bootstrap does it](./building/bootstrapping/how-bootstrap-does-it.md)
+- [Writing tools in Bootstrap](./building/bootstrapping/writing-tools-in-bootstrap.md)
 - [Debugging bootstrap](./building/bootstrapping/debugging-bootstrap.md)
 
 # High-level Compiler Architecture

src/doc/rustc-dev-guide/src/SUMMARY.md

@@ -0,0 +1,23 @@
+# Writing tools in Bootstrap
+
+There are three types of tools you can write in bootstrap:
+
+- **`Mode::ToolBootstrap`**
+  Use this for tools that don’t need anything from the in-tree compiler and can run with the stage0 `rustc`.
+  The output is placed in the "stage0-bootstrap-tools" directory. This mode is for general-purpose tools built
+  entirely with the stage0 compiler, including target libraries and only works for stage 0.
+
+- **`Mode::ToolStd`**
+  Use this for tools that rely on the locally built std. The output goes into the "stageN-tools" directory.
+  This mode is rarely used, mainly for `compiletest` which requires `libtest`.
+
+- **`Mode::ToolRustc`**
+  Use this for tools that depend on both the locally built `rustc` and the target `std`. This is more complex than
+  the other modes because the tool must be built with the same compiler used for `rustc` and placed in the "stageN-tools"
+  directory. When you choose `Mode::ToolRustc`, `ToolBuild` implementation takes care of this automatically.
+  If you need to use the builder’s compiler for something specific, you can get it from `ToolBuildResult`, which is
+  returned by the tool's [`Step`].
+
+Regardless of the tool type you must return `ToolBuildResult` from the tool’s [`Step`] implementation and use `ToolBuild` inside it.
+
+[`Step`]: https://doc.rust-lang.org/nightly/nightly-rustc/bootstrap/core/builder/trait.Step.html