handle stage0 cargo and rustc separately

This change allows setting either `build.cargo` or `build.rustc` without requiring
both to be set simultaneously, which was not possible previously.

To try it, set `build.rustc` without setting `build.cargo`, and try to bootstrap on clean build.

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

Author: onur-ozkan

src/bootstrap/bootstrap.py

@@ -533,7 +533,7 @@ def download_toolchain(self):
         bin_root = self.bin_root()
 
         key = self.stage0_compiler.date
-        if self.rustc().startswith(bin_root) and \
+        if (self.rustc().startswith(bin_root) or self.cargo().startswith(bin_root)) and \
                 (not os.path.exists(self.rustc()) or
                  self.program_out_of_date(self.rustc_stamp(), key)):
             if os.path.exists(bin_root):
@@ -568,11 +568,16 @@ def download_toolchain(self):
 
             toolchain_suffix = "{}-{}{}".format(rustc_channel, self.build, tarball_suffix)
 
-            tarballs_to_download = [
-                ("rust-std-{}".format(toolchain_suffix), "rust-std-{}".format(self.build)),
-                ("rustc-{}".format(toolchain_suffix), "rustc"),
-                ("cargo-{}".format(toolchain_suffix), "cargo"),
-            ]
+            tarballs_to_download = []
+
+            if self.rustc().startswith(bin_root):
+                tarballs_to_download.append(
+                    ("rust-std-{}".format(toolchain_suffix), "rust-std-{}".format(self.build))
+                )
+                tarballs_to_download.append(("rustc-{}".format(toolchain_suffix), "rustc"))
+
+            if self.cargo().startswith(bin_root):
+                tarballs_to_download.append(("cargo-{}".format(toolchain_suffix), "cargo"))
 
             tarballs_download_info = [
                 DownloadInfo(