simplify force-recompile logic for "library"

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

Author: onur-ozkan

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

@@ -15,7 +15,6 @@ use std::path::{Path, PathBuf};
 use std::process::Stdio;
 use std::{env, fs, str};
 
-use build_helper::git::get_closest_merge_commit;
 use serde_derive::Deserialize;
 
 use crate::core::build_steps::tool::SourceType;
@@ -27,7 +26,7 @@ use crate::core::builder::{
 use crate::core::config::{DebuginfoLevel, LlvmLibunwind, RustcLto, TargetSelection};
 use crate::utils::exec::command;
 use crate::utils::helpers::{
-    self, exe, get_clang_cl_resource_dir, is_debug_info, is_dylib, symlink_dir, t, up_to_date,
+    exe, get_clang_cl_resource_dir, is_debug_info, is_dylib, symlink_dir, t, up_to_date,
 };
 use crate::{CLang, Compiler, DependencyType, GitRepo, LLVM_TOOLS, Mode};
 
@@ -125,23 +124,9 @@ impl Step for Std {
         // Force compilation of the standard library from source if the `library` is modified. This allows
         // library team to compile the standard library without needing to compile the compiler with
         // the `rust.download-rustc=true` option.
-        let force_recompile =
-            if builder.rust_info().is_managed_git_subrepository() && builder.download_rustc() {
-                let closest_merge_commit =
-                    get_closest_merge_commit(Some(&builder.src), &builder.config.git_config(), &[])
-                        .unwrap();
-
-                // Check if `library` has changes (returns false otherwise)
-                !t!(helpers::git(Some(&builder.src))
-                    .args(["diff-index", "--quiet", &closest_merge_commit])
-                    .arg("--")
-                    .arg("library")
-                    .as_command_mut()
-                    .status())
-                .success()
-            } else {
-                false
-            };
+        let force_recompile = builder.rust_info().is_managed_git_subrepository()
+            && builder.download_rustc()
+            && builder.config.last_modified_commit(&["library"], "download-rustc", true).is_none();
 
         run.builder.ensure(Std {
             compiler: run.builder.compiler(run.builder.top_stage, run.build_triple()),