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.

config.toml.example

@@ -372,7 +372,9 @@ changelog-seen = 2
 # Whether to download the stage 1 and 2 compilers from CI.
 # This is mostly useful for tools; if you have changes to `compiler/` they will be ignored.
 #
-# FIXME: currently, this also uses the downloaded compiler for stage0, but that causes unnecessary rebuilds.
+# You can set this to "if-unchanged" to only download if `compiler/` has not been modified.
+#
+# FIXME(#82739): currently, this also uses the downloaded compiler for stage0, but that causes unnecessary rebuilds.
 #download-rustc = false
 
 # Number of codegen units to use for each compiler invocation. A value of 0

src/bootstrap/bootstrap.py

@@ -638,8 +638,10 @@ class RustBuild(object):
     # Return the stage1 compiler to download, if any.
     def maybe_download_rustc(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"]
@@ -654,6 +656,8 @@ 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/")
 
         return commit
@@ -1158,6 +1162,8 @@ 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)
 
 

src/bootstrap/config.rs

@@ -510,7 +510,8 @@ struct Rust {
     new_symbol_mangling: Option<bool>,
     profile_generate: Option<String>,
     profile_use: Option<String>,
-    download_rustc: Option<bool>,
+    // ignored; this is set from an env var set by bootstrap.py
+    download_rustc: Option<StringOrBool>,
 }
 
 /// TOML representation of how each build target is configured.
@@ -897,7 +898,7 @@ pub fn parse(args: &[String]) -> Config {
             config.rust_codegen_units_std = rust.codegen_units_std.map(threads_from_config);
             config.rust_profile_use = flags.rust_profile_use.or(rust.profile_use);
             config.rust_profile_generate = flags.rust_profile_generate.or(rust.profile_generate);
-            config.download_rustc = rust.download_rustc.unwrap_or(false);
+            config.download_rustc = env::var("BOOTSTRAP_DOWNLOAD_RUSTC").as_deref() == Ok("1");
         } else {
             config.rust_profile_use = flags.rust_profile_use;
             config.rust_profile_generate = flags.rust_profile_generate;