Teaches Rust's build system to vendor std's dependencies into the
rust-src component.

This was originally landed in https://github.com/rust-lang/rust/pull/78790
but was backed out for causing some breakage for distro maintainers who
need to build Rust itself in a vendored/offline context. It doesn't actually
fetch anything interesting from crates.io, just the magic fake std/core crates
that exist to make the build work right. Those crates *are* vendored but
their contents are ignored in favour of the actual stdlib.

For firefox's purposes, these patches still work fine, and are necessary
to make -Zbuild-std work in a vendored environment.

diff --git a/src/bootstrap/dist.rs b/src/bootstrap/dist.rs
index 3cb0eccd324..a3b8154c024 100644
--- a/src/bootstrap/dist.rs
+++ b/src/bootstrap/dist.rs
@@ -905,6 +905,30 @@ fn run(self, builder: &Builder<'_>) -> GeneratedTarball {
             builder.copy(&builder.src.join(file), &dst_src.join(file));
         }
 
+        // libtest includes std and everything else, so vendoring it
+        // creates exactly what's needed for `cargo -Zbuild-std` or any
+        // other analysis of the stdlib's source. Cargo also needs help
+        // finding the lock, so we copy it to libtest temporarily.
+        //
+        // Note that this requires std to only have one version of each
+        // crate. e.g. two versions of getopts won't be patchable.
+        let dst_libtest = dst_src.join("library/test");
+        let dst_vendor = dst_src.join("vendor");
+        let root_lock = dst_src.join("Cargo.lock");
+        let temp_lock = dst_libtest.join("Cargo.lock");
+
+        // `cargo vendor` will delete everything from the lockfile that
+        // isn't used by libtest, so we need to not use any links!
+        builder.really_copy(&root_lock, &temp_lock);
+
+        let mut cmd = Command::new(&builder.initial_cargo);
+        cmd.arg("vendor").arg(dst_vendor).current_dir(&dst_libtest);
+        builder.info("Dist src");
+        let _time = timeit(builder);
+        builder.run(&mut cmd);
+
+        builder.remove(&temp_lock);
+
         tarball.generate()
     }
 }
diff --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index 3ed53452309..1fc0d887748 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -1437,6 +1437,27 @@ fn read_stamp_file(&self, stamp: &Path) -> Vec<(PathBuf, DependencyType)> {
         paths
     }
 
+    /// Copies a file from `src` to `dst` and doesn't use links, so
+    /// that the copy can be modified without affecting the original.
+    pub fn really_copy(&self, src: &Path, dst: &Path) {
+        if self.config.dry_run() {
+            return;
+        }
+        self.verbose_than(1, &format!("Copy {:?} to {:?}", src, dst));
+        if src == dst {
+            return;
+        }
+        let _ = fs::remove_file(&dst);
+        let metadata = t!(src.symlink_metadata());
+        if let Err(e) = fs::copy(src, dst) {
+            panic!("failed to copy `{}` to `{}`: {}", src.display(), dst.display(), e)
+        }
+        t!(fs::set_permissions(dst, metadata.permissions()));
+        let atime = FileTime::from_last_access_time(&metadata);
+        let mtime = FileTime::from_last_modification_time(&metadata);
+        t!(filetime::set_file_times(dst, atime, mtime));
+    }
+
     /// Copies a file from `src` to `dst`
     pub fn copy(&self, src: &Path, dst: &Path) {
         self.copy_internal(src, dst, false);