Rollup merge of #83368 - jyn514:download-if-unchanged, r=Mark-Simulacrum

Add `download-rustc = "if-unchanged"`

This allows keeping the setting to a fixed value without having to
toggle it when you want to work on the compiler instead of on tools.

This sets `BOOTSTRAP_DOWNLOAD_RUSTC` in bootstrap.py so rustbuild doesn't have to try and replicate its logic.

Helps with https://github.com/rust-lang/rust/issues/81930.

r? `@Mark-Simulacrum` cc `@camelid`

src/bootstrap/bootstrap.py

@@ -644,8 +644,10 @@ class RustBuild(object):
     # If `download-rustc` is set, download the most recent commit with CI artifacts
     def maybe_download_ci_toolchain(self):
         # If `download-rustc` is not set, default to rebuilding.
-        if self.get_toml("download-rustc", section="rust") != "true":
+        download_rustc = self.get_toml("download-rustc", section="rust")
+        if download_rustc is None or download_rustc == "false":
             return None
+        assert download_rustc == "true" or download_rustc == "if-unchanged", download_rustc
 
         # Handle running from a directory other than the top level
         rev_parse = ["git", "rev-parse", "--show-toplevel"]
@@ -660,6 +662,8 @@ class RustBuild(object):
         # Warn if there were changes to the compiler since the ancestor commit.
         status = subprocess.call(["git", "diff-index", "--quiet", commit, "--", compiler])
         if status != 0:
+            if download_rustc == "if-unchanged":
+                return None
             print("warning: `download-rustc` is enabled, but there are changes to compiler/")
 
         if self.verbose:
@@ -1175,6 +1179,8 @@ def bootstrap(help_triggered):
     env["RUSTC_BOOTSTRAP"] = '1'
     if toml_path:
         env["BOOTSTRAP_CONFIG"] = toml_path
+    if build.rustc_commit is not None:
+        env["BOOTSTRAP_DOWNLOAD_RUSTC"] = '1'
     run(args, env=env, verbose=build.verbose)