From bbeff8c7862ad70fada412502324ecd41d1dea14 Mon Sep 17 00:00:00 2001
From: onur-ozkan <work@onurozkan.dev>
Date: Tue, 30 Jul 2024 16:55:45 +0300
Subject: [PATCH 1/2] set `force_recompile: true` if library is modified

This allows the standard library to be compiled even with `download-rustc` enabled.
Which means it's no longer a requirement to compile `rustc` in order to compile `std`.

Signed-off-by: onur-ozkan <work@onurozkan.dev>
---
 src/bootstrap/src/core/build_steps/compile.rs | 39 +++++++++++++++----
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/src/bootstrap/src/core/build_steps/compile.rs b/src/bootstrap/src/core/build_steps/compile.rs
index 268e89c7f6011..c09180e542ff6 100644
--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/src/bootstrap/src/core/build_steps/compile.rs
@@ -26,7 +26,8 @@ use crate::core::builder::{
 use crate::core::config::{DebuginfoLevel, LlvmLibunwind, RustcLto, TargetSelection};
 use crate::utils::exec::command;
 use crate::utils::helpers::{
-    exe, get_clang_cl_resource_dir, is_debug_info, is_dylib, symlink_dir, t, up_to_date,
+    self, exe, get_clang_cl_resource_dir, get_closest_merge_base_commit, is_debug_info, is_dylib,
+    symlink_dir, t, up_to_date,
 };
 use crate::{CLang, Compiler, DependencyType, GitRepo, Mode, LLVM_TOOLS};
 
@@ -114,21 +115,43 @@ impl Step for Std {
     const DEFAULT: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        // When downloading stage1, the standard library has already been copied to the sysroot, so
-        // there's no need to rebuild it.
-        let builder = run.builder;
-        run.crate_or_deps("sysroot")
-            .path("library")
-            .lazy_default_condition(Box::new(|| !builder.download_rustc()))
+        run.crate_or_deps("sysroot").path("library")
     }
 
     fn make_run(run: RunConfig<'_>) {
         let crates = std_crates_for_run_make(&run);
+        let builder = run.builder;
+
+        // 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_base_commit(
+                    Some(&builder.src),
+                    &builder.config.git_config(),
+                    &builder.config.stage0_metadata.config.git_merge_commit_email,
+                    &[],
+                )
+                .unwrap();
+
+                // Check if `library` has changes (returns false otherwise)
+                !t!(helpers::git(Some(&builder.src))
+                    .args(["diff-index", "--quiet", &closest_merge_commit])
+                    .arg("--")
+                    .arg(builder.src.join("library"))
+                    .as_command_mut()
+                    .status())
+                .success()
+            } else {
+                false
+            };
+
         run.builder.ensure(Std {
             compiler: run.builder.compiler(run.builder.top_stage, run.build_triple()),
             target: run.target,
             crates,
-            force_recompile: false,
+            force_recompile,
             extra_rust_args: &[],
             is_for_mir_opt_tests: false,
         });

From 6fcc630e56263d7d17a37a77202717f1c83c5b2a Mon Sep 17 00:00:00 2001
From: onur-ozkan <work@onurozkan.dev>
Date: Tue, 30 Jul 2024 16:56:47 +0300
Subject: [PATCH 2/2] update download-rustc documentation

Signed-off-by: onur-ozkan <work@onurozkan.dev>
---
 config.example.toml | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/config.example.toml b/config.example.toml
index 45faa66ec114f..1a7bdc4737fbf 100644
--- a/config.example.toml
+++ b/config.example.toml
@@ -472,7 +472,8 @@
 # This is mostly useful for tools; if you have changes to `compiler/` or `library/` they will be ignored.
 #
 # Set this to "if-unchanged" to only download if the compiler and standard library have not been modified.
-# Set this to `true` to download unconditionally (useful if e.g. you are only changing doc-comments).
+# Set this to `true` to download unconditionally. This is useful if you are working on tools, doc-comments,
+# or library (you will be able to build the standard library without needing to build the compiler).
 #download-rustc = false
 
 # Number of codegen units to use for each compiler invocation. A value of 0