Refactor change detection for rustdoc and download-rustc

Author: liwagu

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

@@ -10,7 +10,7 @@ use crate::core::builder::{Builder, Cargo as CargoCommand, RunConfig, ShouldRun,
 use crate::core::config::TargetSelection;
 use crate::utils::channel::GitInfo;
 use crate::utils::exec::{BootstrapCommand, command};
-use crate::utils::helpers::{add_dylib_path, exe, git, t};
+use crate::utils::helpers::{add_dylib_path, exe, t};
 use crate::{Compiler, Kind, Mode, gha};
 
 #[derive(Debug, Clone, Hash, PartialEq, Eq)]
@@ -602,20 +602,11 @@ impl Step for Rustdoc {
             )
             .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);
-
+            let dirs = vec![
+                builder.config.src.join("src/librustdoc"),
+                builder.config.src.join("src/tools/rustdoc"),
+            ];
+            let has_changes = builder.config.check_for_changes(&dirs, &commit);
             if !has_changes {
                 let precompiled_rustdoc = builder
                     .config

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

@@ -2903,6 +2903,20 @@ impl Config {
 
         Some(commit.to_string())
     }
+
+    /// Check for changes in specified directories since a given commit.
+    /// Returns true if changes exist, false if no changes
+    pub fn check_for_changes(&self, dirs: &[PathBuf], commit: &str) -> bool {
+        let mut git = helpers::git(Some(&self.src));
+        git.args(["diff-index", "--quiet", commit]);
+        if !dirs.is_empty() {
+            git.arg("--");
+            for dir in dirs {
+                git.arg(dir);
+            }
+        }
+        !t!(git.as_command_mut().status()).success()
+    }
 }
 
 /// Compares the current `Llvm` options against those in the CI LLVM builder and detects any incompatible options.