use precompiled rustdoc with CI rustc

When CI rustc is enabled and rustdoc sources are unchanged, we can use
the precompiled rustdoc from the CI rustc's sysroot. This speeds up
bootstrapping quite a lot by avoiding unnecessary rustdoc compilation.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

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

@@ -9,7 +9,7 @@ use crate::core::builder::{Builder, Cargo as CargoCommand, RunConfig, ShouldRun,
 use crate::core::config::TargetSelection;
 use crate::utils::channel::GitInfo;
 use crate::utils::exec::{command, BootstrapCommand};
-use crate::utils::helpers::{add_dylib_path, exe, t};
+use crate::utils::helpers::{add_dylib_path, exe, get_closest_merge_base_commit, git, t};
 use crate::Compiler;
 use crate::Mode;
 use crate::{gha, Kind};
@@ -554,6 +554,57 @@ impl Step for Rustdoc {
         }
         let target = target_compiler.host;
 
+        let bin_rustdoc = || {
+            let sysroot = builder.sysroot(target_compiler);
+            let bindir = sysroot.join("bin");
+            t!(fs::create_dir_all(&bindir));
+            let bin_rustdoc = bindir.join(exe("rustdoc", target_compiler.host));
+            let _ = fs::remove_file(&bin_rustdoc);
+            bin_rustdoc
+        };
+
+        // If CI rustc is enabled and we haven't modified the rustdoc sources,
+        // use the precompiled rustdoc from CI rustc's sysroot to speed up bootstrapping.
+        if builder.download_rustc()
+            && target_compiler.stage > 0
+            && builder.rust_info().is_managed_git_subrepository()
+        {
+            let commit = get_closest_merge_base_commit(
+                Some(&builder.config.src),
+                &builder.config.git_config(),
+                &builder.config.stage0_metadata.config.git_merge_commit_email,
+                &[],
+            )
+            .unwrap();
+
+            let librustdoc_src = builder.config.src.join("src/librustdoc");
+            let rustdoc_src = builder.config.src.join("src/tools/rustdoc");
+
+            // FIXME: The change detection logic here is quite similar to `Config::download_ci_rustc_commit`.
+            // It would be better to unify them.
+            let has_changes = !git(Some(&builder.config.src))
+                .allow_failure()
+                .run_always()
+                .args(["diff-index", "--quiet", &commit])
+                .arg("--")
+                .arg(librustdoc_src)
+                .arg(rustdoc_src)
+                .run(builder)
+                .is_success();
+
+            if !has_changes {
+                let precompiled_rustdoc = builder
+                    .config
+                    .ci_rustc_dir()
+                    .join("bin")
+                    .join(exe("rustdoc", target_compiler.host));
+
+                let bin_rustdoc = bin_rustdoc();
+                builder.copy_link(&precompiled_rustdoc, &bin_rustdoc);
+                return bin_rustdoc;
+            }
+        }
+
         let build_compiler = if builder.download_rustc() && target_compiler.stage == 1 {
             // We already have the stage 1 compiler, we don't need to cut the stage.
             builder.compiler(target_compiler.stage, builder.config.build)
@@ -614,11 +665,7 @@ impl Step for Rustdoc {
 
         // don't create a stage0-sysroot/bin directory.
         if target_compiler.stage > 0 {
-            let sysroot = builder.sysroot(target_compiler);
-            let bindir = sysroot.join("bin");
-            t!(fs::create_dir_all(&bindir));
-            let bin_rustdoc = bindir.join(exe("rustdoc", target_compiler.host));
-            let _ = fs::remove_file(&bin_rustdoc);
+            let bin_rustdoc = bin_rustdoc();
             builder.copy_link(&tool_rustdoc, &bin_rustdoc);
             bin_rustdoc
         } else {