Unrolled build for rust-lang#127974

Rollup merge of rust-lang#127974 - onur-ozkan:force-std-builds, r=Mark-Simulacrum

force compiling std from source if 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`.

Ref. rust-lang#127322 (comment).
Blocker for rust-lang#122709.

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

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,
  });