Change:
Running the tests with changing the rustccodegenllvm and running the tests again with will ignore all the tests that have passed during the first run.
It also seems like changes to don't trigger the following stages of the compiler itself to rebuild.

Discussion:
let mut cargo = Command::new(&self.initial_cargo); let out_dir = self.stage_out(compiler, mode); <del> // command specific path, we call clear_if_dirty with this let mut my_out = match cmd { "build" => self.cargo_out(compiler, mode, target), // This is the intended out directory for crate documentation. "doc" | "rustdoc" =>  self.crate_doc_out(target), _ => self.stage_out(compiler, mode), }; // This is for the original compiler, but if we're forced to use stage 1, then // std/test/rustc stamps won't exist in stage 2, so we need to get those from stage 1, since // we copy the libs forward. let cmp = self.compiler_for(compiler.stage, compiler.host, target); let libstd_stamp = match cmd { "check" | "clippy" | "fix" => check::libstd_stamp(self, cmp, target), _ => compile::libstd_stamp(self, cmp, target), }; let libtest_stamp = match cmd { "check" | "clippy" | "fix" => check::libtest_stamp(self, cmp, target), _ => compile::libtest_stamp(self, cmp, target), }; let librustc_stamp = match cmd { "check" | "clippy" | "fix" => check::librustc_stamp(self, cmp, target), _ => compile::librustc_stamp(self, cmp, target), }; </del> <ins> // Codegen backends are not yet tracked by -Zbinary-dep-depinfo, // so we need to explicitly clear out if they've been updated. for backend in self.codegen_backends(compiler) { self.clear_if_dirty(&out_dir, &backend); } </ins> if cmd == "doc" || cmd == "rustdoc" { <del> if mode == Mode::Rustc || mode == Mode::ToolRustc || mode == Mode::Codegen { </del> <ins> let my_out = match mode { </ins> // This is the intended out directory for compiler documentation. <del> my_out = self.compiler_doc_out(target); } </del> <ins> Mode::Rustc | Mode::ToolRustc | Mode::Codegen => self.compiler_doc_out(target), _ => self.crate_doc_out(target), }; </ins> let rustdoc = self.rustdoc(compiler); self.clear_if_dirty(&my_out, &rustdoc); <del> } else if cmd != "test" { match mode { Mode::Std => { self.clear_if_dirty(&my_out, &self.rustc(compiler)); for backend in self.codegen_backends(compiler) { self.clear_if_dirty(&my_out, &backend); } }, Mode::Test => { self.clear_if_dirty(&my_out, &libstd_stamp); }, Mode::Rustc => { self.clear_if_dirty(&my_out, &self.rustc(compiler)); self.clear_if_dirty(&my_out, &libstd_stamp); self.clear_if_dirty(&my_out, &libtest_stamp); }, Mode::Codegen => { self.clear_if_dirty(&my_out, &librustc_stamp); }, Mode::ToolBootstrap => { }, Mode::ToolStd => { self.clear_if_dirty(&my_out, &libstd_stamp); }, Mode::ToolTest => { self.clear_if_dirty(&my_out, &libstd_stamp); self.clear_if_dirty(&my_out, &libtest_stamp); }, Mode::ToolRustc => { self.clear_if_dirty(&my_out, &libstd_stamp); self.clear_if_dirty(&my_out, &libtest_stamp); self.clear_if_dirty(&my_out, &librustc_stamp); }, } </del> } cargo

}, } <ins> // This tells Cargo (and in turn, rustc) to output more complete // dependency information.  Most importantly for rustbuild, this // includes sysroot artifacts, like libstd, which means that we don't // need to track those in rustbuild (an error prone process!). This // feature is currently unstable as there may be some bugs and such, but // it represents a big improvement in rustbuild's reliability on // rebuilds, so we're using it here. // // For some additional context, see #63470 (the PR originally adding // this), as well as #63012 which is the tracking issue for this // feature on the rustc side. cargo.arg("-Zbinary-dep-depinfo"); </ins> cargo.arg("-j").arg(self.jobs().to_string()); // Remove make-related flags to ensure Cargo can correctly set things up cargo.env_remove("MAKEFLAGS");

let libdir = builder.sysroot_libdir(compiler, target); let hostdir = builder.sysroot_libdir(compiler, compiler.host); add_to_sysroot(&builder, &libdir, &hostdir, &rustdoc_stamp(builder, compiler, target)); <del> builder.cargo(compiler, Mode::ToolRustc, target, "clean"); </del> } }

use std::process::{Command, Stdio, exit}; use std::str; <del> use build_helper::{output, mtime, t, up_to_date}; </del> <ins> use build_helper::{output, t, up_to_date}; </ins> use filetime::FileTime; use serde::Deserialize; use serde_json;

// for reason why the sanitizers are not built in stage0. copy_apple_sanitizer_dylibs(builder, &builder.native_dir(target), "osx", &libdir); } <del> builder.cargo(target_compiler, Mode::ToolStd, target, "clean"); </del> } }

&builder.sysroot_libdir(target_compiler, compiler.host), &libtest_stamp(builder, compiler, target) ); <del> builder.cargo(target_compiler, Mode::ToolTest, target, "clean"); </del> } }

&builder.sysroot_libdir(target_compiler, compiler.host), &librustc_stamp(builder, compiler, target) ); <del> builder.cargo(target_compiler, Mode::ToolRustc, target, "clean"); </del> } }

deps.push((path_to_add.into(), false)); } <del> // Now we want to update the contents of the stamp file, if necessary. First // we read off the previous contents along with its mtime. If our new // contents (the list of files to copy) is different or if any dep's mtime // is newer then we rewrite the stamp file. </del> deps.sort(); <del> let stamp_contents = fs::read(stamp); let stamp_mtime = mtime(&stamp); </del> let mut new_contents = Vec::new(); <del> let mut max = None; let mut max_path = None; </del> for (dep, proc_macro) in deps.iter() { <del> let mtime = mtime(dep); if Some(mtime) > max { max = Some(mtime); max_path = Some(dep.clone()); } </del> new_contents.extend(if *proc_macro { b"h" } else { b"t" }); new_contents.extend(dep.to_str().unwrap().as_bytes()); new_contents.extend(b"0"); } <del> let max = max.unwrap(); let max_path = max_path.unwrap(); let contents_equal = stamp_contents .map(|contents| contents == new_contents) .unwrap_or_default(); if contents_equal && max <= stamp_mtime { builder.verbose(&format!("not updating {:?}; contents equal and {:?} <= {:?}", stamp, max, stamp_mtime)); return deps.into_iter().map(|(d, _)| d).collect() } if max > stamp_mtime { builder.verbose(&format!("updating {:?} as {:?} changed", stamp, max_path)); } else { builder.verbose(&format!("updating {:?} as deps changed", stamp)); } </del> t!(fs::write(&stamp, &new_contents)); deps.into_iter().map(|(d, _)| d).collect() }